Battery Bank Calculator
What size battery bank do you need? Enter your load and how long it must run to get the capacity in kWh and amp-hours, plus how many batteries to connect in series and parallel. It suits off-grid homes, cabins, boats, vans, and backup systems.
Your battery bank
You need about 6.6 kWh of batteries, or 274 Ah at 24 V, to supply 4 kWh of usable energy.
Assumptions used
- Layout
- 2 in series, 3 in parallel
- Installed capacity
- 7.2 kWh
- Temperature allowance
- 100%
- Load current
- 21 A
Preliminary estimate. Must be verified by a qualified professional.
How the batteries connect
2 in series makes 24 V. 3 strings in parallel makes 300 Ah. This shows the idea, not a wiring plan.
What if?
Save this result, change any setting, and compare.
How this is calculated
Energy you need
Multiply the average load by the hours it must run. Use your real average, because most appliances do not run at full power all the time.
Energy (kWh) = load (W) × hours ÷ 1,000
Capacity to buy
A battery cannot give all of its rated energy. We allow for depth of discharge, inverter and battery losses, cold, and ageing, so the bank still does the job in later years.
Battery size = energy ÷ (depth of discharge × efficiency × temperature × ageing)
Series and parallel
Series connections add voltage. Parallel connections add amp-hours. We work out the layout that reaches your system voltage and capacity with the battery you choose.
Amp-hours = battery size (Wh) ÷ system voltage
Worked example: 500 W for 8 hours
That load uses 4 kWh. With 80% depth of discharge, 95% efficiency, and 80% capacity kept at end of life, the bank needs 6.6 kWh, or about 274 Ah at 24 V.
Using 12 V 100 Ah batteries gives 2 in series and 3 in parallel, which is 6 batteries.
How accurate is this?
The sizing formula is standard, but the default depth of discharge, efficiency, and temperature values are planning figures. Your battery's datasheet takes priority. A qualified installer should confirm the final design.
Common mistakes
- Using a battery's full rated capacity as usable energy
- Adding up nameplate power instead of the average load
- Connecting old and new batteries together
- Ignoring cold, which reduces available capacity
Limits of this tool
- This is for planning, not an electrical design
- It does not choose fuses, cables, or a battery management system
- Follow the battery maker's instructions for connecting units together
What to do next
- Check how long specific appliances will run with the battery backup calculator
- Estimate the solar you need to recharge it with the system size calculator
Frequently asked questions
How do I calculate battery bank size?
Multiply load in watts by hours, then divide by depth of discharge, efficiency, and any temperature and ageing allowance.
How many batteries do I need for a 5 kW load?
It depends on how long it must run. Five hours at a steady 5 kW needs 25 kWh of usable energy, which is about 41 kWh of lithium batteries after this tool's default allowances.
Should I use 12 V, 24 V, or 48 V?
Higher voltage lowers the current for the same power, so cables can be thinner. Larger systems commonly use 48 V.
What depth of discharge should I use?
About 80% is a common planning figure for lithium iron phosphate, and about 50% for lead-acid. Check your own battery's datasheet.