How do I convert Ah to Wh?

Capacity in Ah times the nominal voltage gives the energy in Wh. A 100 Ah battery at 12.8 V holds roughly 1280 Wh.

How deep can I discharge?

With lead and AGM you keep about 50 % in reserve for lifespan, with LiFePO4 you can use 80 to 90 %. Follow the manufacturer's figures.

Why do I get less in practice?

Inverter loss, monitoring standby, cable loss, temperature and battery age all reduce the yield. Keep margin.

How much battery do I need?

Add up the daily consumption of your loads in Wh, divide by the usable share and multiply by the days you want to bridge without charging.

Calculator

Battery capacity calculator

Enter your daily consumption in watt-hours, the system voltage and the days you want to bridge without charging. You see the amp-hours your bank needs.

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

Daily consumption WhAdd up what your loads use in 24 hours.

System voltage

Days without charging Usable share of the battery %Around 50 % for lead or AGM, 80 to 90 % for LiFePO4.

Capacity needed

222 Ah

Energy needed: 2,400 Wh
Usable share: 90 %
Voltage: 12 V

This covers only the energy you want to draw; charging, temperature and ageing call for extra headroom.

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

From amp hours to usable energy

Amp hours mean nothing without voltage: 100 Ah at 12 V is 1200 Wh, at 24 V twice that. Then some of it goes away. A lead battery is only discharged to about half in practice, lithium to around 90 %, and an inverter loses about ten percent making 230V.

Ah × V = Wh

Always convert to watt hours; that lets you compare batteries at different voltages fairly.

Usable share

Use 50 % for lead or AGM and 80 to 90 % for LiFePO4, depending on how deep you want to discharge.

Inverter loss

An AC load runs through the inverter; count on roughly 90 % efficiency plus its own standby draw.

From calculation to complete diagram

Designing a complete electrical system? In WireElectric you place the battery, charging sources and 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 capacity

How do I convert Ah to Wh?: Capacity in Ah times the nominal voltage gives the energy in Wh. A 100 Ah battery at 12.8 V holds roughly 1280 Wh.
How deep can I discharge?: With lead and AGM you keep about 50 % in reserve for lifespan, with LiFePO4 you can use 80 to 90 %. Follow the manufacturer's figures.
Why do I get less in practice?: Inverter loss, monitoring standby, cable loss, temperature and battery age all reduce the yield. Keep margin.
How much battery do I need?: Add up the daily consumption of your loads in Wh, divide by the usable share and multiply by the days you want to bridge without charging.

Other calculators

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