Battery bank calculator

What battery you need to run your devices for a day or more without charging.

DC/ACAhPU
Letters on the drawing match the letters next to the inputs.
kWh
days
V
Ah
W
More settings
%
Battery capacity3.7 kWh
Capacity at system voltage77 Ah

Shopping list

  • Batteries1 pcs · 48 V 100 Ah
  • Inverter, at least3,000 W

How we got there

Energy you use
3 kWh
Usable share of capacity
90 %
Capacity of the batteries you buy
4.8 kWh
Battery current at the largest load
46 A

Results are estimates. Always check the consumption printed on the product you buy.

Calculated on planicalc.com · https://planicalc.com/en/battery-bank-calculator/

How it is calculated

You use only part of a battery: lithium iron phosphate (LiFePO4) about 90 %, lead-acid (AGM, gel) about 50 % if you want it to last. The inverter loses about 10 % more. So the energy you need per day, multiplied by the days without charging, is divided by the usable share and the inverter efficiency. In a cold room a battery gives about 20 % less, so the calculator adds that. Capacity in amp-hours is watt-hours divided by the system voltage.

FormulaCapacity (kWh) = energy per day × days ÷ (usable share × inverter efficiency)Capacity (Ah) = capacity (Wh) ÷ system voltageInverter ≥ largest load × 1.25Battery current = largest load ÷ (voltage × efficiency)

Example

A cabin needs 3 kWh a day. For one day with lithium batteries at 48 V and a 90 % inverter: 3 ÷ (0.9 × 0.9) = 3.7 kWh, or 77 Ah at 48 V, so one 48 V 100 Ah battery (4.8 kWh). With AGM at 24 V the same day needs 6.7 kWh: six 12 V 100 Ah blocks, two in series and three strings in parallel.

Tips from practice

  • Lithium (LiFePO4) batteries must not be charged below 0 °C unless they have built-in heating. In an unheated cabin or garage choose heated batteries or keep them in a warm room.
  • Do not mix old and new batteries or different capacities in one bank: the weakest one sets the life of the whole bank.
  • A higher system voltage means a lower current and thinner, cheaper cables. Above about 2 kW, 24 V or 48 V is usually the better choice.
  • Put a fuse or DC breaker close to the battery positive terminal. Check the cable size with the DC cable calculator.

Questions and answers

How many kWh battery do I need for my house?

Add up the energy your devices use per day, or take it from your bill: yearly kWh ÷ 365. For one day of backup with lithium batteries, buy about 1.25 times that.

How do I convert Ah to kWh?

Multiply amp-hours by the voltage and divide by 1,000. A 12 V 100 Ah battery holds 1.2 kWh, a 48 V 100 Ah battery 4.8 kWh.

Lithium or lead-acid?

Lithium iron phosphate costs more per kWh but you can use about 90 % of it, it lasts 3,000–6,000 cycles and weighs about a third. Lead-acid is cheaper to buy but gives only half its capacity and lasts 500–1,000 cycles.

Found a mistake in a formula? Write to [email protected]