How long does a 100Ah battery last?

That depends on your load. A 100 Ah lithium battery at 12 V gives about 1080 Wh usable at 90 % discharge, so roughly ten hours with a 100 W load.

Why do I get less in practice?

Inverter losses, monitoring, cable losses, temperature and battery age all reduce the yield, so keep some margin.

How much capacity do I need?

Derive that from your daily consumption and the days without charging; that is what the battery capacity calculator is for.

Calculator

Battery runtime calculator

Enter your battery capacity, the system voltage and the power of your load. You see how long the battery sustains it and which assumptions that uses.

Free, no account needed
12V, 24V, 48V and 230V
Result while you type
Carries over into your diagram

Battery capacity Ah

System voltage

Load power WUsable share of the battery %Around 50 % for lead or AGM, 80 to 90 % for LiFePO4.Inverter efficiency %Only relevant for a 230V load through the inverter.

Estimated runtime

9 h 43 min

Energy in the battery: 1,200 Wh
Usable: 1,080 Wh
Current from the bank: 9.3 A

Based on constant consumption; in practice loads switch on and off.

The result is guidance based on your input. Manufacturer instructions and local regulations always take precedence; you remain responsible for the installation.

What determines runtime

Amp-hours mean little without voltage: 100 Ah at 12 V is roughly 1200 Wh. Not all of that is usable — lead batteries are usually drained to about half, lithium to around 90 % — and a 230V load loses more through the inverter.

Ah × V = Wh

Convert to watt-hours first; only then can you compare batteries at different voltages.

Usable share

Enter 50 % for lead or AGM and 80 to 90 % for LiFePO4, depending on how deeply you discharge.

Inverter losses

A 230V load runs through the inverter; assume about 90 % efficiency and add its own consumption.

From calculation to complete diagram

Designing a complete electrical system? In WireElectric you place the battery, the charge sources and the loads in one diagram, so you can see how the chosen capacity fits the rest of the installation.

You can start drawing without an account.

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WireElectric editor showing a 12V campervan diagram: LiFePO4 battery, MEGA fuse, busbars, MultiPlus-II and SmartSolar MPPT with cables between the terminals.

Questions about battery runtime

How long does a 100Ah battery last?: That depends on your load. A 100 Ah lithium battery at 12 V gives about 1080 Wh usable at 90 % discharge, so roughly ten hours with a 100 W load.
Why do I get less in practice?: Inverter losses, monitoring, cable losses, temperature and battery age all reduce the yield, so keep some margin.
How much capacity do I need?: Derive that from your daily consumption and the days without charging; that is what the battery capacity calculator is for.

Other calculators

Every calculator answers one question about your installation. WireElectric then runs the same calculations for every connection in your complete diagram.