How to use the battery bank calculator
- Choose to check a bank you plan or to find the batteries needed for a target.
- Enter the voltage and the capacity in amp hours of one battery.
- For a planned bank, enter the batteries in series and the strings in parallel. For a target, enter the bank voltage and capacity you need.
- Open More options to enter a load, the share of capacity you will use and the inverter efficiency, then read the bank and the run time.
Formula
bank voltage = battery voltage x batteries in series
bank amp hours = battery amp hours x strings in parallel
stored energy in Wh = bank voltage x bank amp hours
run time in hours = stored energy x usable share x efficiency / load in watts
For a target, series = target voltage / battery voltage and parallel = target amp hours / battery amp hours, each rounded up.
Four 12 volt 100 amp hour batteries
Two in series and two strings in parallel make a 24 volt, 200 amp hour bank storing 4,800 watt hours. At 50% usable capacity that is 2,400 watt hours, and with 90% inverter efficiency a 300 watt load runs 2,400 x 0.9 / 300 = 7.2 hours.
Twelve 6 volt 225 amp hour batteries wired 4 in series and 3 strings in parallel make 24 volts and 675 amp hours, or 16,200 watt hours. Four 3.2 volt 100 amp hour cells in series make 12.8 volts and 1,280 watt hours. For a 48 volt, 350 amp hour target from 12 volt, 100 amp hour batteries you need 4 in series and 4 strings, which is 16 batteries.
Series, parallel and safety
Series wiring adds voltage, and parallel wiring adds capacity. Use batteries of the same type, size and age in one bank, because a weaker one drags the others down. Fuse each string, size the wire for the current and follow the wiring and charging instructions of the battery maker. A battery bank can deliver enough current to cause burns and fires, so get help from a qualified person if you are unsure.
How much of the capacity you can use
Batteries last longer when you do not drain them completely. The share you can use depends on the chemistry and the maker, so the 50% on the page is only an example. An inverter that makes alternating current loses some energy as heat, and the efficiency is another figure from the maker.
Related electrical tools
Apply Ohm's Law, size a generator with the Generator Wattage Calculator, estimate run time of a device with the Battery Life Calculator, and add up loads in the Circuit Load Calculator.
Assumptions and limits
- All batteries are identical, and each string has the same number of batteries.
- Capacity is the rated amp hours at the stated voltage.
- The usable share and efficiency are numbers you choose.
- Charging, temperature, aging and the fusing of the wiring are not modeled.
Frequently asked questions
How do batteries in series and parallel add up?
Series adds the voltages and keeps the amp hours, and parallel adds the amp hours and keeps the voltage. Two series by two parallel 12 volt 100 amp hour batteries make 24 volts and 200 amp hours.
How do I calculate watt hours from a battery bank?
Multiply the bank voltage by the bank amp hours. A 24 volt, 200 amp hour bank stores 4,800 watt hours.
How long will the bank run a load?
Multiply the stored energy by the share you can use and by the efficiency, then divide by the load in watts. A 300 watt load on the example bank runs 7.2 hours.
How many batteries do I need for a 48 volt bank?
Divide 48 by the battery voltage for the number in series, then add strings in parallel for capacity. With 12 volt batteries that is 4 in series per string.