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Battery Runtime

Battery Size and Capacity Calculator

Tell us the load and how long it needs to run, and we will work out the battery capacity in amp-hours and watt-hours, and how many batteries to wire in series and parallel.

Updated · Free, no sign-up · Runs in your browser

Quick answer

Battery size (Wh) = load (W) × hours ÷ efficiency ÷ usable share. To run 300 W for 8 hours through an inverter you need about 257 Ah of 12 V LiFePO4 (3.1 kWh) or about 458 Ah of AGM lead-acid.

Find the battery size you need

W
hours
System voltage
How the load is powered

Battery you plan to buy

Ah
V
Advanced settings
%
%

Battery capacity you need

257 Ah

at 12 V, or 3.08 kWh of rated capacity

With your batteries

3 × 100 Ah 12 V

1 in series × 3 in parallel = 3.6 kWh, runs about 9 h 26 min

Average draw at the battery
333 W
Energy you will use
2.67 kWh

Rounded up to whole batteries. Add 10–20% if the batteries will sit in the cold or you expect to add loads later.

How to size a battery

  1. Add up the load. List what will run at the same time and add the watts. For things that cycle, like fridges, use the average draw.
  2. Decide the runtime in hours: the longest outage you want to cover, or the time between recharges.
  3. Pick a battery type. It sets how much of the rated capacity you can use: about 90% for LiFePO4, 50% for lead-acid if you want it to last.
  4. Pick the system voltage and the battery you plan to buy. The calculator rounds up to whole batteries and shows how to wire them.

The battery sizing formula

Capacity (Wh) = load (W) × hours ÷ efficiency ÷ usable share
Capacity (Ah) = capacity (Wh) ÷ system voltage

For lead-acid the calculator also corrects for Peukert’s law: at high loads lead-acid delivers less than its rating, so you need more capacity than the simple formula gives. For long runtimes at light loads the correction is negligible. See how battery runtime is calculated for the full working.

Battery size chart (12 V)

Rated capacity needed at 12 V through a 90% efficient inverter.

Load × hoursEnergy usedLiFePO4AGM
100 W × 8 h800 Wh86 Ah153 Ah
300 W × 4 h1,200 Wh133 Ah250 Ah
300 W × 8 h2,400 Wh257 Ah458 Ah
500 W × 5 h2,500 Wh273 Ah507 Ah
1000 W × 3 h3,000 Wh336 Ah650 Ah

For whole-home or off-grid systems that run on solar, size by daily energy and days without sun instead, with the solar battery calculator. To check how long a battery you already have will last, use the battery runtime calculator. To compare batteries rated in Ah with ones rated in Wh or kWh, use the Ah to Wh converter.

Frequently asked questions

What size battery do I need?

Multiply the load in watts by the hours you need, divide by the inverter efficiency (about 0.9) and by the usable share of the battery (0.9 for LiFePO4, 0.5 for lead-acid). That gives the rated watt-hours; divide by the system voltage for amp-hours. Running 300 W for 8 hours on a 12 V LiFePO4 system needs about 3,080 Wh, or roughly 255 Ah.

How many batteries do I need to run a 1000 watt load?

For 3 hours on a 12 V system with lithium batteries, about 336 Ah, which is 4 × 12 V 100 Ah batteries in parallel. With AGM batteries used to 50% you need about 650 Ah. At this size a 24 V or 48 V system is a better choice, because it halves or quarters the current.

Should I choose 12V, 24V or 48V?

Under about 1,000 W, 12 V is simple and works with most RV and marine gear. From 1,000 to 3,000 W, 24 V halves the current, so cables can be thinner. Above 3,000 W, and for home and solar systems, 48 V is the standard. The energy you need in Wh is the same at any voltage; only the Ah and the wiring change.

How do I wire batteries to get the voltage and capacity I need?

Batteries in series add their voltages: two 12 V 100 Ah batteries in series make 24 V 100 Ah. Batteries in parallel add their capacities: two 12 V 100 Ah batteries in parallel make 12 V 200 Ah. Use identical batteries of the same age in any bank, and follow the battery maker’s limits on how many can be connected.

Should I add extra capacity?

Yes, 10–20% is sensible. Batteries lose capacity as they age and in cold weather, and loads tend to grow. Sizing for the end of the battery’s life means you still get the runtime you planned in a few years.