fieldguidephotovoltaicsystems.pdf

Effective May 1, 2013 – The incentive for residential and commercial solar PV will change from a rate of $3.00 per watt, up to a maximum of $6,000 per customer to $1.50 per watt, up to a maximum of $6,000 per customer.

3

THE NORTHWESTERN ENERGY ™FIELD GUIDE TO

PHOTOVOLTAIC SYSTEMS

Research and Production: Chris Dorsi and John Krigger

Illustrations: Steve Hogan

Project Management: John Campbell

Copyright 2010 • Saturn Resource Management, Inc. and NorthWestern Energy • All rights reserved

Printed in the U.S.

Many Montana homeowners have found good reason to install photovotaic electric systems in their homes.

Some want to reduce their use of fossil fuels and so protect the environment, many hope to improve our nation’s energy

independence, and others are ready to invest in their family’s energy future by creating a non-polluting power source right at home.

Whatever your interest in photovoltaic power, right now is a great time to do some research and learn if it’s for you.

This manual was prepared for NorthWestern Energy by Saturn Resource Management, Inc.

The

NorthWestern Energy

Field Guide to Photovoltaic Systems • Copyright 2010 5

I

NTRODUCTION

TO

P

HOTOVOLTAIC

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OWER

Photovoltaic systems produce electricity from sun-light ctrical power can be used to operate all the lights, appliances, and electronics that nor-mally consume electricity from the power grid. Photovoltaic systems have been used in homes and business for years, but are enjoying a resurgence in popularity due to falling prices for photovoltaic panels, technical advances in inverter technology, and increasing utility costs.

Some homeowners and business managers install photovoltaic (PV) systems because their buildings are located in remote areas beyond the reach of power lines. In these

o grid

applications, the PV system may include batteries or a generator to pro-vide electricity when the sun isn’t shining. One advantage of these o -grid PV systems is that they allow property owners to avoid the cost of extend-ing electric lines to their site. But many consumers who are located within the service territory of an electric utility still invest in PV power. In these cases, the PV system is o en connected to the elec-tric grid. se

grid-tied

systems feed excess PV power into the grid when the system’s production is greater than occupants’ consumption, as during

sunny weather. ey draw power from the grid when the system’s production is less that the con-sumption, such as at night and during cloudy weather.

Grid-tied systems are usu-ally

net-metered

, with elec-tric meters that record electricity going both into and out of the home. Utility companies such as North-Western Energy o er net metering agreements that credit customers for power fed back into the electric grid by quali ed PV sys-tems.

P

HOTOVOLTAIC

E

CONOMICS

Photovoltaic systems require a substantial eco-nomic investment. A typi-cal 2 to 3 kilowatt system in Montana costs $15,000 to $25,000 before any incentives are applied. At these costs, you’ll pay about 25¢ per kilowatt-hour of PV electricity produced over the life of the panel, compared to about 10¢ per kilowatt-hour for electricity purchased from NorthWestern Energy.

Fortunately, there are some very attractive incen-tives available for PV systems. NorthWestern Energy provides an incentive of $3.00 per watt, up to a maximum of $6,000, for residential systems installed by .

Montana Department of Environmental Qualityadministers a low-interest loan program that can be used for renewable energy systems. And both the Montana Department of Revenue and the InternalRevenue Service provide income tax credits for some PV installations.

P H O T O V O L T A I CA R R A Y

I N V E R T E R

E L E C T R I C A LP A N E L

M E T E R

Y O U R

A P P L I A N C E SL I G H T S &

E L E C T R I C A LG R I D

To learn how you can tie a

photovoltaic system into the

electrical grid, see Can I Connect My

System to the Utility Grid? on page 19.

$ave

Introduction to Photovoltaic Power6

�ese incentives help bring the cost of PV systems within the reach many building owners. Also, the �nancial viability of your PV system may improve in the future as electric costs increase.

H

OME

E

FFICIENCY

I

MPROVEMENTS

Since PV systems are relatively expensive and have a long payback, you should always spend your �rst home improvement dollars on simple conservation measures. �ese might include installing compact �uorescent lamps, upgrading the insulation in your attic, sealing air leaks, or replacing older ine�cient appliances. �ese measures will reduce your elec-tric load at a lower cost than installing a PV system, and some of them will also improve the comfort of your home. Once you’ve reduced your electrical load as much as possible with these basic improve-ments, it makes sense to consider a long-term investment in a PV system.

Economically-sized PV systems do not pro-duce enough power to support large electric loads such as electric space heat. If you have an electrically heated home, a better invest-ment would be in switching your heat source from electricity to natural gas, propane, or solar thermal heat.

A W

ORD

F

ROM

N

ORTH

W

ESTERN

E

NERGY

Photovoltaic technology embodies two important characteristics:

distributed generation

and

renewable energy

. Distributed generation move us away from reliance on large central generating stations. It can include PV systems, wind generators, small hydro-electric plants, and even small fossil-fueled plants such as natural gas turbines. �ese far-�ung facili-

ties gain e�ciency by being located close to the sources of energy.

Renewable energy sources help us develop a sus-tainable energy future with less reliance on increas-ingly scarce fossil fuels such as oil, coal, and natural gas. For customers who want to reduce their home’s environmental impact, PV power can be an impor-tant component of a personal energy plan. A mod-estly-sized PV system as described in this booklet can produce 2,500 kilowatt-hours of electricity per year, eliminating carbon dioxide emissions equal to driving about 10,000 miles in a typical automobile. Every PV system we install brings Montana and the U.S. closer to a sustainable energy future, and reduces our dependence on domestic and foreign sources of energy.

We encourage you to do some research to learn whether a PV system makes sense for your family or business. We hope you �nd this booklet useful in making that decision.

To learn more about the economics of PV power, and �nancial

incentives for installing PV

systems, see How Much Do

Photovoltaic Systems Cost? on

page 15.

0 0

“Be sure to improve your home’s e�ciency before you investment in a photovoltaic system.”

The

NorthWestern Energy


H

OW

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OES

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ORK

?

Solar cells

are the small-est building blocks of photovoltaic systems. �ese silicon-based semi-conductors utilize the reaction of sunlight within the cell to create an electrical current. Each cell produces a few watts of

direct current

(DC) electricity.

Photovoltaic modules

include groups of solar cells that are electrically interconnected and mounted in lightweight frames. Modules are usu-ally covered with glass and backed with a polymer sheet that protects the cells from the elements. �ese modules are the basic PV components sold by most manufacturers. Typical modules produced for residential and light commercial installations produce 50 to 200 watts.

Solar arrays

are the groups of these modules you see installed on the roofs of buildings or on ground-mounted racks. �e electrical output of an array, usually measured in

watts

, is determined by how many modules it contains and the wattage of the individual modules.

T

YPES

OF

S

OLAR

C

ELLS

Crystalline

solar cells are the most common type used in today’s PV systems. �ey are sliced into thin wafers a few inches in diameter and several thousandths of an inch thick. You’ll see these indi-vidual cells if you look closely at most solar panels. Among the various types of solar cells, crystalline cells convert sunlight to electricity the most e�-ciently, reducing the area needed for a given output. But their manufacturing cost is also the highest, requiring designers to balance the advantages of smaller area versus cost.

�in �lm

materials are made by depositing a thin layer of PV material onto glass, plastic, or metal foil. �e �lm is usually less than one ten-thousandth of an inch thick. �ough thin �lm materials are less e�cient at converting sunlight to electricity, and so require more area to produce the same output, they tend to be cheaper per square foot than crystalline cells. �in �lm materials can also be integrated into building materials such as roof shingles.

I

NVERTERS

AND

C

ONTROLLERS

�e direct current produced by solar cells can be used for some applications. Solar-powered calcula-tors and battery chargers, for example, consume DC power, and several manufactures make DC refrigerators and other equipment for o�-grid DC systems. But almost all the lights, appliances, and equipment in modern homes use the

alternating current

(AC) that is supplied by the electrical grid.

SUNL IGHT

Simple PV Solar Cell: Sunlight dislodges electrons in silicon-based semi-conductors, producing direct current electricity.

Crystalline Solar Cells: This most common type of solar cell is sliced into wafers that are interconnected and assembled into modules.

Solar Array Space Requirements

Crystalline Silicone Cells

require 100 to 150 square feet per kilowatt of output, so a typical 2-kilowatt system will cover 200 to 300 square feet.

Thin Film Cells

require 170 to 300 square feet per kilowatt of output, so a typical 2-kilowatt system will cover 340 to 600 square feet.

These areas are for the solar array only. Be sure to leave addi-tional room for access and maintenance.

How Does Photovoltaic Power Work?8

Inverters

are electronic devices that convert DC power into AC power. If you intend to use your PV electricity to power your home’s existing equip-ment, your system will require an inverter. An inverter is also required if you plan to connect your PV system to the utility grid. The majority of PV systems installed today utilize an inverter.

Inverters are specified by their capacity in kilowatts (to meet or exceed the peak output of the array), and according to their output voltage (usually 120-240 volts AC). Inverters can convert input voltages of up to 600 volts from the PV array down to the 120-240 volts output needed by most equipment.

Most inverters also include relays, circuit breakers, and/or charge controllers that manage the task of safely connecting your solar array to your home’s existing electrical system, battery bank, and/or util-ity grid. They usually include a DC ground-fault circuit interrupter (GFCI) that disconnects the PV array if wiring defects are detected.

B

ACKUP

S

YSTEMS

Photovoltaic systems generate power only when the sun strikes the array. Backup systems provide an uninterrupted supply of solar electricity at night, during overcast weather, or during power failures.

If you plan to install such a

standalone

or

off-grid

system, you’ll probably want to install a back-up system that includes batteries, a gas-powered gener-ator, or both. You’ll also need an inverter that can

manage the charging of your batteries and can start your generator when it’s needed.

If you plan to install a

grid-tied system

, you may not need a backup system, since you can usually rely on the grid to provide power when your PV system produces less power than you need.

B

ATTERIES

Open-cell lead-acid batteries are most commonly used for PV backup systems because they provide the best ratio of storage capacity per dollar of cost. These are similar to the open-cell batteries used in automotive applications. But PV batteries are designed for deep-cycle applications, rather than the brief load experienced when starting a vehicle, and so aren’t damaged by the deep draw-down common to back-up systems. Open-cell batteries do require periodic maintenance to replace water lost to evaporation. Sealed batteries, though more

Inverter/Controller: This inverter/controller converts direct current to alternating current, controls the charging of a bank of batteries, and manages a net metering connection with the electrical grid.

To learn how you can tie a

photovoltaic system into the

electrical grid, see Can I Connect My

System to the Utility Grid? on page 19.

Wiring Configuration: Modules are combined in series to produce a higher voltage. In this drawing, groups of two 60-volt modules are wired in a series string to produce 120 volts.

Three strings of modules are combined in parallel to produce higher amperage. The three strings of 12-amp modules shown are wired in parallel to produce 36 amps.

Most installers assemble strings of modules that produce up to 600 volts, since higher voltages at a given power allow the use of smaller wire gauges. The inverter manages the task of reducing the voltage to the 120-240 volt standard used by most equipment.

60v12a

-

+

60v12a

60v12a

60v12a

60v12a

60v12a

120 volts 36 amps

The

NorthWestern Energy


costly, are used where low-maintenance equipment is needed as in remote locations.

�e installation and maintenance of a battery bank will add $2,000 to $3,000 to the cost of your PV sys-tem. If you need to power only a stock-tank pump or o�ce lights, for example, you may be able to uti-lize the daytime-only power produced by a system with no batteries. But most PV systems include either a battery bank, a back-up generator, or a con-nection to the utility grid.

If you need the greatest reliability in electrical ser-vice, as when operating critical electronic equip-ment such as emergency response communications or health care systems, you can install batteries in conjunction with a grid-tied system to create a

uninterruptible power supply (UPS)

.

Battery Bank: The open-cell batteries shown here are con�gured to match the 240 volts delivered by the grid. The cabinet is ventilated by a low voltage fan that moves potentially explosive hydrogen gas to the outdoors.

Ventilating fan

Airtight cabinet

Other Types of Solar Power

Some solar systems produce heat rather than elec-tricity. These

solar thermal

systems can heat either water or air.

Active

solar systems utilize pumps to circulate water or other �uids through solar panels where heat is collected and carried into the home. These systems usually provide domestic hot water, and they are often installed as a back-up to a standard electric or gas water heater.

Passive

solar systems capture the sun’s energy when it enters the home’s windows, releasing heat when it is needed at night. Every home bene�ts from some passive solar energy, but the most advanced passive homes utilize a lot of south-fac-ing glass, and strategically placed thermal mass to capture and store heat.

These solar thermal systems can reduce your con-sumption of utility-provided electricity, just like PV power, but they are not described in this booklet.

Will Photovoltaic Power Work on My Site?10

W

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OWER

W

ORK

ON

M

Y SITE?

Some sites perform better than others for producing photovoltaic power. �e size and orientation of your home, the presence of shade, and possible zoning restric-tions all a�ect the viability of PV systems.

Photovoltaic systems are most commonly installed on the roofs of homes, garages, carports, greenhouses, and other struc-tures. But they can also be installed vertically against a wall of your home, as part of an awning, or near the ground as a free-standing structure.

A preliminary site assessment will help you identify whether there are major obstacles to overcome. �e information you’ll gather in this process will be use-ful whether you plan to do the work yourself or hire a professional installer. And much of what you’ll learn in this assessment, such as the e�ect of shad-ing on your home, will inform you about the viabil-ity of other solar thermal technologies such as solar water heating and passive space heating.

ACCESS TO THE SUN

Your PV system will produce the most power when it is exposed to a solar window that is as long as pos-sible. A perfect solar window with no shadows all day allows 100 percent of the potential solar energy to strike your array. Any shading, as from trees or an adjoining building, will reduce your system’s output from what it would be at this optimum exposure.

For the best performance your site should have an un-obstructed solar window in the middle of the day. But a little shade is acceptable, especially if it crosses your array early or late in the day. An hour of shade at 8:00 am will reduce your system’s output a lot less than an hour of shade at noon.

Shade has a large e�ect on the PV output because of the series/parallel con�guration in which solar cells

are combined in the modules. A small strip of shade along the side of one module can reduce its output to zero since the shaded cells create resistance to the �ow of current from the other cells in that module. In some con�gurations, an array that is shaded over 10 percent of its area could lose 80 percent of its output for as long as the shade persists.

Shading issues are usually the worst in the winter when the sun’s altitude is lowest and shadows are the longest. You may be able to tolerate the shadow of an adjoining building, for example, if it only passes over one corner of your array for a few months in winter.

HOW TO EVALUATE YOUR SOLAR WINDOW

�e �rst step in assessing your site is to determine the direction of solar south. �is is the location of the sun at solar noon, and the point on the compass at which the sun is at its highest altitude above the horizon. It’s from solar south that a PV system is exposed to the most solar energy, since the sun’s high path passes through the least atmosphere.

If you hire a professional �rm to install your sys-tem, they should assess your site’s potential as part of their installation. But you should do some pre-liminary research, too, by simply following the sun’s path in the course of a day as it passes over your potential site. If you expect to install your PV array

East

West

South

Solar Window: The ideal site for a PV system has an unobstructed view of the south sky, with no shade falling on the array from at least 9:00 am until 3:00 pm.

The NorthWestern Energy Field Guide to Photovoltaic Systems • Copyright 2010 11

on your roof, do this inspection from the roof if you are comfortable with heights and with climbing lad-ders.

Note any trees, buildings, or other obstructions that could cast shadows. Identify the sun’s location at noon standard time, or at 1:00 pm during daylight savings time, since shade during the middle of the day will cause the greatest reduction in solar poten-tial. Consider where the sun will pass during other seasons, since the sun’s path will be lower in the sky during winter.

SOLUTIONS TO SHADE

If the site where you’ll install your array receives some shade, you have three options to account for its e�ects:

Eliminate the shade, as by trimming the tree or moving the chimney.

Move the array elsewhere, such as to a garage, patio cover, or barn.

Increase the size of the array to make up for the impaired production.

GETTING ORIENTED WITH A COMPASS OR GPS

To improve the accuracy of your assessment, you can use a simple compass or GPS unit to identify solar south. Learn how to adjust your unit for mag-netic declination, since the earth’s magnetic poles are not the same as the geographic or solar poles.

Solar south varies from magnetic south by 12 to 15 degrees in Montana.

With your compass adjusted to account for the declination, you can iden-tify solar south with more accuracy. Look due east (90 degrees le� from solar south) and due west (90 degrees right from solar south) to see where the sun will rise and set on the March 20th spring equinox and the September 19th fall equinox. Remember that during the long days of summer the location of both sunrise and sunset will be farther to the north, and that during winter they’ll be farther to the south.

Identify any potential obstructions in the sun’s path. Consider the future, and whether trees or bushes will grow larger, and whether you or the neighbors will erect any new structures. Perform this assess-ment for all your potential sites, paying closest attention to any obstructions that could cast mid-day shade.

MEASURING YOUR SOLAR WINDOW WITH A SOLAR PATHFINDER®

�e Solar Path�nder® allows you to assess any potential obstructions on your site with a high level of accuracy. �e Path�nder’s plastic dome re�ects the objects surrounding your site onto a latitude-speci�c chart onto which you can trace the sun’s path.

By tabulating the areas on the chart that are obscured by shade-producing objects, you can determine the per-centage of possible sunlight your site will receive over the course of the year. �is is very useful for comparing various

Tracking the Sun

Azimuth describes the sun’s location on a horizontal plane that runs east and west. The sun’s azimuth changes from morning to night as it rises in the east, crosses solar south, and sets in the west.

Altitude describes the sun’s location in a vertical plane above the horizon. The sun’s altitude above the horizon varies throughout the day as it rises from the horizon, passes high in the sky, and descends again to the horizon. Its altitude also var-ies throughout the year. In the northern hemi-sphere, you’ll see it at its highest altitude on June 21, the summer solstice. Its lowest altitude will be on December 21, the winter solstice.

You can �nd the magnetic

declination at your site by visiting the

National Geophysical Data

Center website listed in Resources

on page 22.

Solar Path�nder: This simple tool allows you to calculate the hour-by-hour e�ects of shade on your site.


locations within your site such as one end of your roof versus the other, or roof mount versus ground mount to determine where you’ll get the best per-formance.

�e best sites will have no shade, or a 100 percent solar window. If your site has a solar window with less than an 80 percent solar fraction, you should consider locating your array elsewhere.

ROOFTOP VERSUS GROUND-MOUNT INSTALLATION

Your analysis of the sun’s path across your site will give you a better idea of the best place to locate your array of PV modules.

For many building owners, the roof is the preferred site for a PV array because it’s out of the way, close to the existing electrical system, and above many objects that cast shade. Most roo�op systems are mounted on racks that stand above the roof. �ese racks can hold modules at either the same slope as the roof surface, or at a steeper angle that optimizes the exposure to incoming solar energy. Roo�op sys-tems can also utilize solar “shingles,” thin-�lm modules that take the place of standard roo�ng shingles.

Ground-mounted systems have the advantage of easy and safe access, and, on some sites, more avail-able room.

Rooftop Installation Considerations

Consider these issues if you plan to mount a PV array on your roof:

• What is the condition of your current roof? If your roof will need to be replaced within the next �ve or ten years, consider replacing the roofing before installing the array.

• What type of roo�ng material is installed on your building? Wood and tile roofs can be di�cult to seal, for example, where the array’s support rack penetrates the roof.

• Are there chimneys and other utility pene-trations that break up the space? If you’ll install the modules around obstructions, consider if those objects will cast shade on the array.

• Which face of your roof has the best expo-sure? You’ll get the best performance when your array faces solar south, though you may be able to utilize east or west faces if you can accept diminished output.

• How much room will PV installers and other tradesmen need to work around the array? Be sure workers have a safe walkway at the edges of the roof.

• Do you plan to install new equipment in the future on your roof, such as a satellite dish or air conditioner? You should avoid �lling your entire roof with a PV system.

• Do local zoning or building regulations limit the height or appearance of buildings? You can limit these impacts by installing your array close to the pitch of your roof, or by choosing a roof face that is out of view.

You can learn more about assessing

your site’s potential by visiting the Solar Path�nder website listed in Resources

on page 22.

Rooftop Solar Array: This solar array is installed to match the pitch of the roof. This simpli�es the installation, compared to installing the modules on a sloped rack, and it minimizes wind load on the mounting system. The trade-o�, on this low-slope 4-in-12 pitch roof, will be a lower output in the winter when the sun is low in the sky.


Ground-Mount Installation Considerations

If you mount a PV array on the ground, you should consider a di�erent set of issues:

• Do local ordinances or zoning regulations control how close to the edge of your prop-erty you can build a structure?

• Will the array be subject to vandalism?

• Are there any underground utility lines that could be damaged when excavating for the footings or when running underground electrical lines to the array?

• Do you have plans to expand roads, gar-dens, or structures into the area where you’ll install the array?

• How much will the supporting framework, concrete foundation, and extra service wire add to the installation?

OPTIONS IN MOUNTING RACKS

Your choice in mounting racks will determine how closely your array is aligned with the sun through-out the day and across the seasons. Your array will produce the most power if it faces the sun directly at all times. �is is di�cult in practice since the sun crosses the sky from east to west each day, and the

sun’s path varies in distance above the horizon throughout the year.

Your mounting rack will also e�ect the outward appearance of the array. A �xed rack will be the least obtrusive, especially if it’s mounted parallel to the roof surface. Tracking arrays take up additional space, and so are usually used for ground-mount installations.

Fixed Racks

Fixed racks are the cheapest, strongest, and most common choice, especially for roo�op installations where space is at a premium. �ey hold the mod-ules in an unchangeable orientation, usually facing an azimuth of 0 degrees, or due south. �eir tilt, or altitude, can be set parallel to the roof surface on which they’re mounted for the easiest installation. But on low-pitch roofs they are o�en raised above the roof angle to optimize output.

Manually Adjustable Racks

Manually adjustable racks can be changed through-out the year to follow the sun’s changing altitude. �eir azimuth is usually �xed at due south. If adjusted faithfully–perhaps 4 times per year–they can increase the annual system output by 10 to 12 percent when compared to �xed rack systems. �is bene�t must be weighed against the adjustable rack’s increased cost and space requirements.

Single-Axis Tracking Systems

Single-axis tracking systems rotate east to west to generally follow the sun’s daily path. �eir altitude remains �xed throughout the year. �eir motion is driven by either a small motor that is guided by a sun-seeking photocell, or by a passive gas-charge system in which a canister of refrigerant is heated by the sun and expands or contracts to move the array. Single-axis tracking systems can increase annual output by up to 25 percent over �xed racks, and their cost is $1,000 to $1,500 higher.

Dual-Axis Tracking Systems

Dual-axis tracking systems utilize the same systems to track both the sun’s altitude and azimuth

Ground Mounted Solar Array: This solar array takes advantage of a large south-facing yard. It also solves the orientation issue of the home (in background) with roof planes that don’t face solar south.


throughout the day and year. �ese most compli-cated systems can produce up to 30 percent more energy than �xed rack systems, and their cost is $1,500 to $2,000 higher.

CHOOSING AN ORIENTATION AND TILT ANGLE

Despite the lower energy production of �xed rack systems, many installers choose them for their sim-plicity, minimal maintenance, and low pro�le. �e trade-o� may be the need to invest in a slightly larger array to get the same year-round production as a tracking array.

�e optimum orientation will usually be solar south, or at an azimuth of 0 degrees. With roof-mount installations this is easiest to accomplish if there is an existing roof plane that already faces solar south. With ground-mount installations it’s usually possible to rotate the array to face solar south.

It’s not critical that your array faces exactly solar south. If your array faces 30 degrees away from solar south, its year-round production will still be 90 percent of what it would be at an optimum solar south orientation.

You will have some choice in setting a tilt angle. �e best angle for year-round production is to match the site’s latitude since this tends to face the sun’s average altitude over the course of a year. A steeper angle will favor winter production, when the sun stays close to the horizon, and a �atter angle will favor summer when the sun passes high overhead. But most installers choose to match the latitude if possible.

At Montana’s latitude, a PV array installed on a roof with a pitch of 18 degrees (4-in-12 slope) will pro-duce about 8 percent less energy than a system installed at a 47 degree tilt to match the latitude.

Fixed Rack: This �xed rack is aligned perpendicular to the building’s roof line. This orientation is 15 degrees away from solar south, reducing the average output 1 to 2 percent when compared to a solar south orientation. The tilt angle is set at 47 degrees to match the site’s latitude. Waterproof �exible conduit connects each module’s junction box to the combiner box at the far end of the array.

Junction box

Combiner box

Tilt Angles : The ideal tilt angle for PV arrays in Montana is close to 45 degrees. But most modern roofs are designed with a lower pitch. If you plan to install a PV array on a low-pitch roof, your site analysis should compare the bene�ts installing a mounting rack that optimizes output–but doesn’t follow the roof pitch–versus the lower cost and slim pro�le of an array that is parallel to the roof.

12-in-12 pitch45 degrees




HOW MUCH DO PHOTOVOLTAIC SYSTEMS COST?

of your investment in photovoltaic power will depend primarily upon the rated electrical production of your syste peak output is usually measured in watts. Residential systems can range from one to ve thousand watts (one to ve kilowatts), while commercial systems range up to thirty kilowatts or more.

YOUR INITIAL INVESTMENT

pical installed cost for PV systems in Montana, including labor and materials, varies from $7.50 to $8.50 per watt. For a typical 2-kilowatt (2 kw) system, the total cost is usually about $15,000 tbase cost is for grid-tied systems with no battery storage, generator, or other backup system. Your cost will vary depending on the ease of access to your site, whether you choose to install a backup system, your choice of mounting racks, whether you install PV shingles or other architectural options, and other factors.

EXPECTED POWER PRODUCTION

ctrical power generated by a typical 2-kilo-watt system in central Montana is usually about 2,500 kilowatt-hours per year. At current residential rates of 10¢ per kilowatt-hour (kwh), this system would produce enough electricity to replace about $250 worth of utility-provided power per year.

average residential NorthWestern Energy cus-tomer in Montana consumes about 9,000 kilowatt-hours of electricity per year. At current rates, the yearly cost for that electricity would be about $900 per year us this example 2-kilowatt system can be expected to produce about 30 percent of a typical family’s needs. To produce 100 percent of this fam-

ily’s consumption, you’d need a PV system of 6 1/2 to 7 kilowatts, at a cost of $40,00 to $50,000.

EQUIPMENT LIFESPAN

Modern PV systems have proven to be very long-lastin solar modules make up about half the cost of most systems, and can be expected to have a life of 25 years or more, though their output tends to deteriorate about one percent per year. Inverters, which make up less than a quarter the cost of most systems, should last just as long, though like all electronic devices they may sometimes fail prema-turely. e remainder of the system–mounting racks, wiring, and disconnect equipment–should last as long as the building.

NORTHWESTERN ENERGY INCENTIVE PROGRAM

NorthWestern Energy provides generous nancial incentives for PV systems that are installed on our customers’ homes in Montan mum incentive at present is for $3.00 per watt of installed output, up to a maximum o its the incentive to systems of up to 2 kilowatts, though customers may choose to install additional capacity without incentive payments.

se incentives have supported the installation of PV systems on at least 30 Montana schools, 175 homes and small business, and dozens of other public buildings to date.

STATE TAX INCENTIVES

Montana Department of Revenue may provide a tax credit of up to $500 against your state income tax to help defray the cost of a new PV systeAlternative Energy Systems Credit can be used by Montana residents who install a qualifying system in their primary home redit must rst be claimed in the year you install the system, but any unused credits can be carried over for up to four successive tax years. credit can also be used for

$ave

The res used in this economic discussion are current as of 2006.

How Much Do Photovoltaic Systems Cost?16

other alternative energy systems such as wind, geo-thermal, or hydroelectric systems.

MONTANA DEQ LOW INTEREST LOANS

�e Montana Department of Environmental Qual-ity o�ers a low-interest loan program that can be used to help �nance the installation of PV systems. �e Alternative Energy Revolving Loan Program provides loans of up to $40,000 for installing alter-native energy systems in residences and small busi-nesses. �e loans must be used for net-metered systems that generate energy for your own use. �e current interest rate is 5 percent annually, and the loans must be repaid within ten years.

FEDERAL TAX INCENTIVES

�e Internal Revenue Ser-vice (IRS) provides tax credits for the installation of some PV systems. �e Home Energy E�ciency Improvement Tax Credits provide a credit for up to 30 percent of the cost of some PV equipment. �e credit can’t exceed $2,000, and the systems must be installed on the taxpayer’s princi-pal residence in the United States.

To learn more about �nancial incentives

for installing PV systems, see the

Incentive Programs listed in Resources

on page 22.

Firehouse PV System: The Baxendale Volunteer Fire Department, near Helena, installed this grid-tied 2,400 watt PV system with battery backup to reduce ongoing utility expense and provide reliable power to critical equipment during power outages. NorthWestern Energy has provided �nancial incentives for installation of PV systems on several �re stations in Montana. This �xed array faces solar south and has a tilt angle of 47 degrees.


HOW MUCH POWER WILL MY SYSTEM PRODUCE?

You can use the methods described here to perform an estimate of your system’s output. �is will help you assess its �nancial viability, and will allow you to com-pare design options such as tilt angle, compass orienta-tion, and array size to see how they a�ect output.

DESCRIBING PV OUTPUT

�e capacity of photovoltaic systems is usually stated as peak output. �is measurement, in watts or kilowatts, describes the maximum power pro-duced by the PV array under ideal cloudless condi-tions. Typical residential systems have a peak output of 1,000 to 5,000 watts (1 to 5 kilowatts), while light commercial systems can be up to 30 kilowatts or more. Your system’s inverter, wiring, and disconnect equipment must always be sized to safely operate at this peak output. Financial incen-tives o�ered by utility companies are based upon the system’s peak output.

Your system’s predicted annual output describes how much energy you can expect it to produce over time. �is electrical energy is measured in kilowatt-hours, and is an important measure of a system’s �nancial viability. �e annual output of energy from any PV system will always be just a fraction of its potential peak output because of varying day-length, cloud cover, and normal losses within the system.

PREDICTING PV OUTPUT

One of the easiest ways to predict your system’s annual output is with an online tool called PVWatts. �e PVWatts website, hosted by the National Renewable Energy Lab, includes a simple calculator that allows you to predict how much energy your planned system will produce, as mea-

sured in either kilowatt-hours of electricity or in dollars worth of savings.

�e PVWatts program can produce an approximate estimate of your system’s output with just two fac-tors: your location, and the peak output of your sys-tem’s modules.

�e rough calculation you’ll make with this infor-mation will give you an idea of your site’s PV poten-tial. But as you learn more about your system, you can �ne tune your PVWatts cal-culation by including these other factors:

Whether your array tracks the sun’s path.

�e direction your collectors will face in relation to solar south.

�e tilt of your collectors in relation to your latitude.

�e losses inherent in your system’s inverter and wiring.

�e cost of grid-provided electricity your system will replace.

ADVANCED OUTPUT CALCULATIONS

You can perform a more detailed analysis of your system’s potential output using data collected and hosted by the National Renewable Energy Lab. �e Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors, or Redbook, includes all the data included in the PVWATTS calculator, but it also allows you to calculate output on a daily basis, and to view side-by-side tables that illustrate the e�ect of various combinations of orientation, tilt angle, and types of mounting racks.

You can learn more about the

PVWatts calculator in Resources on

page 22.

You can learn where to �nd the Redbook data in Resources on

page 22.

How Much Power Will My System Produce?18

A Comparison of Montana Cities Using PVWatts

Billings Kalispell Lewistown Missoula NotesSi

te D

escr

iptio

n

Latitude 45 degrees N 48 degrees N 47 degrees N 47 degrees N

The distance in degrees from the Equator; determines the optimum array tilt for year round production.

Elevation 3,569 feet 2,566 feet 4,147 feet 3,189 feetHeight above sea level; higher locations receive more solar insolation.

Syst

DC rating 2 kilowatts 2 kilowatts 2 kilowatts 2 kilowatts

This is the maximum output stated by the manufacturer of the modules. Assumed here to be 2 kw, the maximum size that would receive a full NWE incentive under 2006 guidelines.

DC derate factor .77 .77 .77 .77

Derate factors account for normal system losses, and vary from 0.7 to 0.8.

AC rating 1.5 kilowatts 1.5 kilowatts 1.5 kilowatts 1.5 kilowatts

Calculated as the DC rating (maximum array output) multiplied by the derate factor above.

Array tilt 45 degrees 48 degrees 47 degrees 47 degreesAssumed to be the same as each city’s latitude.

Array azimuth 180 degrees 180 degrees 180 degrees 180 degreesAssumed to be solar south, or 180 degrees compass bearing.

Ener

gy O

utpu

t

Annual output of PV system

2,678 kilowatt

hours

2,222kilowatt

hours

2,546 kilowatt

hours

2,263 kilowatt

hours

The variation shown here is the result of

erences among the towns. The typical NorthWestern energy customer uses 9,000 kilowatt hours per year.

Average cost of grid-supplied electricity

10¢ perkilowatt hour

10¢ per kilowatt hour



Average cost for residential customers as of 2010.

Annual value of PV output $268 $222 $255 $226

The reduction you can expect to see in your electric utility bill at current rates.


CAN I CONNECT MY SYSTEM TO THE UTILITY GRID?

Many utility companies allow their customers to connect properly installed photovoltaic systems onto the existing electrical grid. ese grid-tied sys-tems can feed excess PV power back into the power grid any time the customer uses less power than the PV system prod pically during the day when the sun is out and the occupants aren’t home or have minimal demands. customer can then draw power back from the utility grid when they consume more than the PV system produces, such as during cloudy weather, at night, or when the system is out of operation.

Grid-tied systems do not require batteries, making them the most economical and popular type of PV system, though batteries can be included to provide service if the electric grid fails. O -grid systems usually have battery back-up to provide power when the PV system can’t meet the customer’s needs.

NET METERING AGREEMENTS

If you have a grid-tied PV system, you may be eligi-ble for a net-metering agreement with your utility company. Net metering allows you to bene t from the excess power your system feeds into the electric grid. Your utility company will not pay you for excess power you feed into the system, but they will credit your account at retail rates for up to the total amount of your consumption. With net-metering,

you can e ectively “bank” your power for later use without the use of batteries.

Under a net-metering agreement, you must install a compliant generation system that is compatible with the electric grid se are the steps you’ll go through in setting up net metering:

Inform your electric utility company that you intend to install a PV power system.

Project Coordinator will deter-mine what type of metering arrangement is appropriate, and mail you the documents.

Review, sign and return the Net Metering Agreement.

Install your PV system, and have it inspected by a state or municipal electrical inspector.

A er your system has been installed and inspected, con-tact the Project Coordinator and arrange to have a new meter installed.

utility company will install a new meter within two weeks to a month.

INTERCONNECTION AGREEMENTS

An interconnection agreement de nes the terms of your arrangement with the utility company. It spec-i es the voltage, frequency, and other characteris-tics of the power your system feeds into the grid, and it requires that you install your system accord-ing to industry standards that protect your family or employees from electrical hazard agree-ment speci es how your system will avoid back-feeding into the grid during outages–to protect utility personnel–and it allows the utility company access to your PV system’s switching equipment so it can be disconnected from the grid in an emer-gency.

Net meter: The net meter plugs into the standard meter base on your electrical panel. It measures curr ow both into and out of your home, and so registers your net electrical consumption.

NET METER

Learn more about net metering

agreements by visiting the

NorthWestern Energy website

listed in Resources on page 22.

Who Installs Photovoltaic Systems?20

WHO INSTALLS PHOTOVOLTAIC SYSTEMS?

Almost all photovoltaic sys-tems are installed by electrical contractors. Your understand-ing of PV basics will help you choose a competent contractor and effectively supervise his work. This is the best approach unless you have a lot of exper-tise with complicated electrical installations.

FINDING A CONTRACTOR

Do some research before you hire a contractor to install your system. Ask for a written proposals from contractors who bid your job. Be sure that each is bidding on the same size and type of system. If the bids don’t specify similar details, you may need to go back and ask for adjustments so you can make an accurate comparison.

Don’t buy on price alone. Evaluate your contractor based on his entire proposal, and be sure to ask these questions:

✔ How long has the contractor been in busi-ness? Is he bonded and insured?

✔ Has the contractor installed PV systems before? Have you contacted previous cus-tomers?

✔ Does the contractor have a valid electrical contractors license? Does he have any industry PV certifications?

✔ Has the contractor provided a rough esti-mate of the power the system will produce?

✔ Will the contractor apply for the proper permits?

✔ Will the contractor provide an installation warranty that goes beyond the equipment manufacturer’s warranty?

GETTING THE PROPER PERMITS

You’ll probably need a building permit to install a PV system. If you work with a professional installer, he will usually include this as part of his contract. If you do the work yourself, you should contact your municipal inspector (if you live in town), or a state electrical inspector (in unincorporated areas).

If you are among the first in your area to install a PV system, your inspector may not be familiar with the technology. Be patient as he learns about the unique details of PV systems. The more you under-stand about the equipment and its installation, the easier this process will go.

Building codes protect both your safety and your home’s value. If you plan to install a grid-tied system, your utility company will expect a permit and final inspec-tion certificate before they’ll allow you to hook up your system.

IDENTIFYING INSTALLATION HAZARDS

Any electrical system presents a very real hazard of electrocution. But PV systems pose unusual haz-ards that require special caution. Be sure you understand these hazards, or hire someone who does:

✔ DC voltages between the array and the inverter often run as high as 500 volts. And current can come from any of three sources: the PV array, the batteries or gen-erator (if present), and the electrical grid. These multiple sources of power require specialized disconnect equipment to avoid electrical shocks or burns.

✔ Battery banks produce explosive hydrogen gas that must be vented to the outdoors, and the acid in wet-cell batteries is

To find local firms that install PV

systems, contact the professional

organizations listed in Resources on

page 22.


extremely corrosive. You should always use eye protection when working with PV bat-tery banks, and con�rm that gases are vented to the outdoors.

Where PV systems are installed on roofs, the danger of falling is always present. You should always use proper sca�olding, lad-ders, and fall-protection harnesses to reduce this hazard.

If you do plan to install your own PV system, be sure to do some research, ask owners of other PV systems what they have learned in the process, and consider attending an installers’ workshop.

To learn more about the technical aspects

of installing a PV system, or to check

the schedule for upcoming

workshops, contact Solar Energy

International, listed in Resources on page 22.

Exterior Meter Base: This electrical panel houses the main disconnect switches for the building, the overhead service entrance conductors from the electrical grid, grounding equipment, and a net meter that measures the �ow of energy into and out of the home.

PV system disconnect

House and garage

Net meter

240-volt service entrance conduit – from electrical grid

Grounding conductors and lightning protection

disconnects

Resources22

RESOURCES

INCENTIVE PROGRAMS

NORTHWESTERN ENERGY

You can �nd informa-tion here about photo-voltaic electric and other renewable energy systems. You can also view and download information about net-metering and interconnection agreements.

Renewable Energy Coordinator: 406-497-3516www.northwesternenergy.com

MONTANA DEPARTMENT OF ENVIRONMENTAL QUALITY

Montana DEQ provides a lot of good general infor-mation about renewable energy and building e�-ciency.

�ey also manage the Alternative Energy Revolving Loan Program to provide low-interest loans of up to $40,000 on photovoltaics and other renewables.

Loan Program phone: 406-841-5243http://deq.mt.gov/energy/Renewable/index.asp

MONTANA DEPARTMENT OF REVENUE

Resident individuals may claim an income tax credit of up to $500 per year for installing photovol-taic systems in their principal residence. Any unused credit may be carried over for up to four years a�er the �rst year it is claimed. Ask for Form ENRG-B.

Phone: 1-866-859-2254www.mt.gov/revenue

TECHNICAL TOOLS

NATIONAL RENEWABLE ENERGY LAB (NREL)

�e Redbook tabulates the Peak Sun-Hours for locations throughout North America. Useful as part of the calculation of output at a given site for PV arrays.http://rredc.nrel.gov/solar/pubs/redbook/

PVWATTS

PVWatts is an online tool developed by the National Renewable Energy Lab that allows you to calculate the predicted electrical energy produced by grid-tied PV systems.http://rredc.nrel.gov/solar/calculators/PVWATTS

FINDSOLAR.COM

�is website is also sponsored �e American Solar Energy Society and Department of Energy. It includes an interactive tool for sizing your PV system and a database of PV contractors.www.�ndsolar.com

NATIONAL GEOPHYSICAL DATA CENTER

�e page titled Declination allows you to enter your zip code to retrieve your latitude (useful for deter-mining the height of the sun at your site) and mag-netic declination (needed to calibrate magnetic compasses so you can �nd true south).www.ngdc.noaa.gov/geomag/geomag.shtml


INFORMATION PROVIDERS

SOLAR ENERGY INTERNATIONAL

SEI publishes some of the best installation manuals for PV system ey also sponsor workshops around the country on PV and other renewable energy sources.

www.solarenergy.org

CEC Listing of Approved Equipment

A list of equipment that quali es for California rebates. In other areas, it’s still a good resource for comparing equipment among manufacturers.www.consumerenergycenter.org/erprebate/equip-ment.html

A Guide to PV System Design and Installation

an advanced installer’s guide for those planning to install their own systems.www.energy.ca.gov/reports/2001-09-04_500-01-020.PDF

MANUFACTURERS

SOLAR DEPOT

One of the largest distributors of PV and solar thermal equipment in the U.S.

Phone: 707-766-7727www.solardepot.com

BP SOLAR

One of the largest manufacturers of PV modules in the world, their website includes case studies and an extensive economic analysis of PV technology.www.bpsolar.com

SOLAR PATHFINDER

ey manufacture and sell the Solar Path nder, a useful tool for ne-tun-ing your site analysis.

Phone: 317-501-2529www.solarpa .com

NORTH AMERICAN BOARD OF CERTIFIED ENERGY PRACTITIONERS

NABCEP provides training and certi cation for photo-voltaic installers.

Phone: 518-889-8126www.nabcep.org

Montana Renewable Energy Association

Montana Renewable Energy Association includes a directory of Montana

Montana renewable energy systems, and useful informational resources.

Phone: 406-214-9405www.montanarenewables.org

CALIFORNIA ENERGY CENTER (CEC)

oversees the biggest PV mar-ket in the Uwebsite is loaded with useful information.www.consumerenergycenter.org

fieldguidephotovoltaicsystems.pdf

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