Battery runtime calculator
Enter the battery and the load, get how long it runs — or the battery you need.
50% and 90% are examples — use your battery's and inverter's datasheets. No inverter (DC load): 100%.
How long will a battery last?
Runtime (hours) = battery watt-hours × usable depth of discharge × inverter efficiency ÷ load in watts. Watt-hours are amp-hours × battery volts. Example: a 12 V, 100 Ah battery holds 1,200 Wh; using 50% of it through a 90% efficient inverter gives 1,200 × 0.5 × 0.9 = 540 Wh, which runs a 100 W load for 5.4 hours — 5 h 24 min.
What depth of discharge should I use?
The one in your battery's datasheet — it depends on the chemistry and the maker. Trojan Battery recommends discharging its flooded lead-acid batteries 20 to 50% for the longest life, although they can be cycled to 80%. Battle Born rates its 100 Ah LiFePO4 battery for 100% depth of discharge. The 50% in the calculator is only an example.
What inverter efficiency should I enter?
The efficiency on your inverter's or UPS's datasheet; the 90% in the calculator is an example. If the load runs straight from the battery with no inverter — a 12 V fridge or LED strip — enter 100%. For a load given in amps, the calculator multiplies by its voltage: 5 A at 120 V is 600 W.
How big a battery do I need?
Switch to "Battery size": watts × hours ÷ (depth of discharge × inverter efficiency) gives the watt-hours, and dividing by the battery voltage gives amp-hours. 100 W for 5.4 hours at 50% and 90% needs 1,200 Wh — 100 Ah at 12 V. Enter one battery's capacity to see how many you need.
Why does my battery run out sooner than this?
Rated capacity assumes a particular discharge rate — Trojan uses the 20-hour rate — and a battery drained faster delivers less, especially lead-acid. Cold and age reduce it too. This calculator uses the rated capacity as is, so treat its result as a best case for heavy loads.
How do I combine several batteries?
Connected in parallel, batteries of the same voltage add their amp-hours and the voltage stays the same. Connected in series, the voltages add and the amp-hours stay the same. Watt-hours always add, so the energy of a bank is the sum of its batteries.
Does the inverter use power when nothing is plugged in?
Yes. Inverters draw some power just to run, and the datasheet usually lists it as no-load or idle consumption. For a small load that left-over drain can be a noticeable share of the total, so add it to the load figure.
Worked example
A 12 V, 200 Ah battery stores 12 × 200 = 2,400 Wh. At a depth of discharge of 50% and an inverter efficiency of 90%, the usable energy is 2,400 × 0.50 × 0.90 = 1,080 Wh. A 60 W load then runs for 18 h 0 min, a 150 W load for 7 h 12 min, and a 400 W load for 2 h 42 min. Runtime is inversely proportional to the load, so doubling the load halves the hours.
The other direction: to run a 150 W load for 8 hours with the same assumptions you need 150 × 8 ÷ (0.50 × 0.90) = 2,667 Wh, which is 222 Ah at 12 V.
Common mistakes
- Comparing amp-hours of different voltages. A 24 V, 100 Ah battery holds twice the energy of a 12 V, 100 Ah one. Compare watt-hours.
- Using the plug-in power rating instead of the real load. A 1,000 W appliance only draws that while it heats. Use the average or measured watts, and allow for the start-up surge of motors on the inverter side.
Sources: Trojan Battery Company, FAQs — depth of discharge and the 20-hour rate; Battle Born Batteries, 100Ah 12V battery manual (BB10012). An estimate for planning, not a guarantee of runtime. How we check our calculators.