Skip to main content

Enter the battery capacity and voltage, and the power of what you connect. The usable share of the battery and the inverter losses are included, because you never get the full label capacity out of a battery.

Enter your values

Amp-hours on the battery label. For a power station in Wh, divide by the voltage.
Where do I find this?

The capacity is printed on the battery in Ah (amp-hours). A power station often states Wh: divide Wh by the voltage to get Ah (1200 Wh ÷ 12 V = 100 Ah).

For several batteries in parallel, add the capacities; in series, add the voltages.

Nominal voltage: 12 V for a car or caravan battery, 24 or 48 V for larger systems.
Where do I find this?

The voltage is printed on the battery. Use the nominal value (12, 24, 48 …), not the momentary reading.

Total power of what you run, in watts (1 kW = 1000 W).
Where do I find this?

Add up the power of the devices from their labels. If the device only shows amps and volts, multiply them.

A plug-in energy meter shows the real consumption.

Lead-acid batteries should not be emptied: about 50 %. Lithium (LiFePO4) batteries allow 80 to 100 %.
Where do I find this?

The battery data sheet states the recommended depth of discharge. Deep discharges shorten the life of a lead-acid battery. 80 is an example.

Only for AC devices on an inverter, often 85 to 95 %. Use 100 for DC devices.
Where do I find this?

Devices that run on 12 V DC directly need no inverter: use 100. The inverter's data sheet gives its efficiency.

Result

Enter your values to see the result.

Runtime
– hours
In minutes
– min
Usable energy
– Wh

Want this tracked for you? UtilityControl logs your meters and costs automatically.

How it is calculated

  • Stored energy (Wh) = capacity (Ah) × voltage (V)
  • Usable energy = stored energy × usable share × inverter efficiency
  • Runtime (h) = usable energy ÷ load (W)

Example

A 100 Ah, 12 V battery stores 1,200 Wh. With 80 % usable and a 90 % inverter that is 864 Wh, which runs a 200 W load for 864 ÷ 200 = 4.3 hours.

What this estimate leaves out

Cold weather, old batteries and high loads reduce the real runtime, and some batteries lose capacity at high discharge rates. Keep a margin.

Written and reviewed by QLines, Cologne · Last reviewed

FAQ

Frequently Asked Questions

How long will a battery last?

Divide the usable energy in watt-hours by the load in watts. A 100 Ah, 12 V battery stores 1,200 Wh; with 80 % usable and a 90 % inverter it runs a 200 W load for about 4.3 hours.

How do I convert amp-hours to watt-hours?

Multiply the amp-hours by the battery voltage: 100 Ah × 12 V = 1,200 Wh.

Why not use the full capacity?

Emptying a lead-acid battery completely shortens its life, so only about half is used. Lithium batteries allow a deeper discharge.

Does the inverter matter?

Yes. An inverter that turns 12 V DC into 230 V AC loses part of the energy as heat, typically 5 to 15 %.

More free calculators

kW to kWh Calculator

Convert kilowatts to kilowatt-hours with the hours of use, or kWh back to kW.

Watts to Amps Calculator

Convert watts to amps or amps to watts for DC, single-phase and three-phase circuits.

Track your meters instead of calculating by hand

UtilityControl keeps your readings, usage and costs for every meter and location, and it starts free.

See UtilityControl Pricing