Enter a battery voltage and its amp-hour rating to convert to watt-hours and kilowatt-hours, the true measure of the energy stored and the figure used for off-grid sizing.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Calculator
Energy
1,200 Wh
Energy
1.20 kWh
Formula
Watt-hours = voltage × amp-hours. Kilowatt-hours = watt-hours / 1000. To go the other way, amp-hours = watt-hours / voltage.
Worked example
A 12 V battery rated at 100 Ah stores 12 × 100 = 1,200 Wh, which is 1.2 kWh. The same 1,200 Wh at 24 V would be 1200 / 24 = 50 Ah, so a higher-voltage battery needs fewer amp-hours for the same energy.
Watt-hours are the currency of energy sizing: one watt-hour is one watt drawn for one hour, and summing the watt-hours of every load over a day gives the single number that drives array, battery and generator sizing. This calculator lets you build that total load by load, which is where accuracy is won or lost — the always-on parasitic draws, such as a router, a fridge cycling or standby lights, often outweigh the big appliances used only briefly. Measure with a plug-in meter where you can, because nameplate ratings are maximums rather than typical draw. Once you trust this figure, the rest of the off-grid chain follows directly from it.
Frequently asked questions
Why convert amp-hours to watt-hours?
Amp-hours alone do not tell you how much energy a battery holds, because energy also depends on voltage. A 100 Ah battery at 12 V stores half the energy of a 100 Ah battery at 24 V, even though the amp-hour number is identical. Watt-hours combine charge and voltage into a single energy figure, which is what your loads consume and what solar sizing is based on. Converting to watt-hours lets you compare batteries of different voltages fairly and plan how long they will last.
How do I work out usable watt-hours?
Multiply the nominal watt-hours by the depth of discharge you can safely use. A 1,200 Wh lead-acid battery at 50 percent gives about 600 usable watt-hours, while a 1,200 Wh lithium battery at 80 percent gives about 960. This calculator shows nominal energy; for planning, apply your chemistry depth of discharge to get the figure that actually covers your loads. Inverter and wiring losses then trim a little more off what reaches your devices.
What is the difference between Wh and kWh?
They measure the same thing at different scales: a kilowatt-hour is simply 1,000 watt-hours. Small batteries and individual devices are usually described in watt-hours, while home energy bills and larger battery banks use kilowatt-hours. To convert, divide watt-hours by 1,000 to get kilowatt-hours, or multiply the other way. A 1,200 Wh battery is 1.2 kWh, and a 5 kWh home battery is 5,000 Wh.
How long will a battery run a device?
Divide the usable watt-hours by the device power in watts. A 1,200 Wh battery at 80 percent usable, about 960 Wh, running a 100 W load lasts roughly 9.6 hours before recharge, minus inverter losses. Higher loads drain it faster in direct proportion. For a dedicated runtime estimate that also accounts for depth of discharge, use the battery runtime calculator, which performs this division for you.
Does temperature affect stored energy?
Yes. Cold reduces the capacity you can actually withdraw, particularly from lead-acid batteries, where freezing temperatures can cut available energy substantially. Lithium handles cold better for discharge but should not be charged below freezing without a heater or protection circuit. The amp-hour rating on the label is given at a standard temperature, usually around 25 degrees Celsius, so plan for less in cold conditions and keep batteries insulated where possible.
Source: Energy definition: watt-hours = volts × amp-hours; kilowatt-hours = watt-hours / 1000. · All sources