Sizing the Battery Bank

All of the following dictate the battery bank capacity you’re looking for:

✓ The efficiency of the inverter

✓ The number of days you expect the battery bank to last without recharging

✓ The batteries’ operating temperature and voltage

✓ How much of the battery bank your client is willing to use

✓ The voltage at which you want the battery to operate

1. Determine the average daily AC watt-hours (or kilowatt-hours) con-

sumption level.

5.78 kWh

2. Divide the watt-hours value from Step 1 by the estimated inverter

efficiency.

5.78 kWh ÷ 0.9 = 6.42 kWh

3. Add any energy consumption from DC loads to the watt-hours value in

Step 2.

i f the client has three 20 W DC

lights that she runs for two hours each day, the total DC energy con-

sumption is 3 lights × 20 W × 2 hours = 120 Wh, or 0.12 kWh. The total

energy consumption is therefore 6.42 kWh + 0.12 kWh = 6.54 kWh.

4. Multiply the energy value from Step 3 by the desired days of

autonomy.

6.54 kWh × 3 days = 19.62 kWh

5. Divide the value calculated in Step 4 by the temperature compensa-

tion value provided by the battery manufacturer

19.62 kWh ÷ 0.9 = 21.8 kWh.

6. Divide the value from Step 5 by the allowable depth of discharge.

21.8 kWh ÷ 0.75 = 29.1 kWh.

7. Divide the value from Step 6 by your desired nominal voltage for the

battery bank.

29.1 kWh ÷ 48 V = 0.606 kAh, or 606 Ah.

Wire sizing will depend on the load currents and the distances of the loads from the source.

PV module considered – MBPV 125

Battery considered – LMS400

Depth of discharge – 0.8

Days of autonomy – 2

Array output efficiency – 85%

nverter efficiency – 90%

Battery efficiency – 85%

Battery Design

• Battery output required – 300Ah, 48 V

• At 80% depth of discharge, capacity required – 375Ah

• Capacity required with 2 days autonomy – 720Ah at 48V

• Selected battery – Exide LMS750, 2V, 750Ah

• Number of batteries in series – 24 nos.

• Number of batteries in parallel – 1 no.

• Total number of batteries = 24 x 1 = 24 nos.

PV Array Design

• PV array output required – 16.5 kWh, 48V

• Array output considering losses of 15% - 19.4 kWh

• Considering solar insolation of 4.98 hours array capacity – 3.89 kWp

• Module selection – MBPV125 (130Wp, 28.5Vmp, 4.5 Imp)

• Number of modules is series = 48/28.5 = 2

Number of modules in parallel = 70/4.5 = 16

Inverter Capacity

• Inverter capacity required – 2 kVA, 1 phase, 48V/230V

Charge Controller

• Rating for charge controller – 70 Amps, 48 V.

Overall System Design

Designing Stand alone Solar PV Power Plant

Case study 1

3.5.1. Case Description

A small village in a remote area has been facing a lot of problems because of the frequent load shedding. The villagers are well educated and are interested in having a 2kW standalone solar photovoltaic power plant to cater to their daily most critical domestic electricity needs for 5 hours a day.

Typical System Design

Assumptions in System Design

• Solar PV system is considered.

• Latitude considered for site (Amravati city) – 21.0°N

• Longitude considered for site (Amravati city) – 77.8°E

• Daily solar insolation at 25° slope – 5.54 kWh/m2/day

• PV module considered – MBPV 125

• Battery considered – LMS650

• Depth of discharge – 0.8

• Days of autonomy – 2

• Array output efficiency – 85%

• Inverter efficiency – 90%

• Charge controller efficiency – 95%

• Battery efficiency – 85%

Battery Design

• Battery output required – 250Ah, 48 V

• At 80% depth of discharge, capacity required – 315Ah

• Capacity required with 2 days autonomy – 630Ah at 48V

• Selected battery – Exide LMS650, 2V, 650Ah

• Number of batteries in series – 24 nos.

• Number of batteries in parallel – 1 no.

• Total number of batteries = 24 x 1 = 24 nos.

PV Array Design

• PV array output required – 13.78 kWh, 48V

• Array output considering losses of 15% - 16.21 kWh

• Considering solar insolation of 5.54 hours array capacity – 3.00 kWp

• Module selection – MBPV125 (130Wp, 28.5Vmp, 4.5 Imp)

• Number of modules is series = 48/28.5 = 2

• Number of modules in parallel = 60/4.5 = 14

• Total number of modules = 14 x 2 =28 nos.

Inverter Capacity

• Inverter capacity required – 2 kVA, 1 phase, 48V/230V

Charge Controller

• Rating for charge controller – 60 Amps, 48 V