Glossary

Amp-hour

Unit Ah · Also Ah

What is Amp-hour?

A measure of an amount of charge: a current of 5 amps for 4 hours equals 20 amp-hours. Capacity and consumption on board are usually stated in this unit.

Ah = current in A × time in h

What matters about Amp-hour in practice?

Amp-hours can only be compared within one voltage level. 100 Ah at 24 volts contain twice as much energy as 100 Ah at 12 volts — to compare across voltage levels, calculate in watt-hours.

Only part of the rated capacity in amp-hours is usable: about half for lead-acid, considerably more for lithium. The difference is called depth of discharge.

How do you convert amp-hours into watt-hours?

Watt-hours = amp-hours × voltage. A 100 Ah LiFePO4 battery at 12.8 V therefore stores 1,280 Wh. The other way round: a 45 W load running 10 hours a day needs 450 Wh, which at 12 V is about 37.5 Ah.

How do you work out daily consumption in Ah?

  • For each load: current in amps times running time in hours per day.
  • If only the power is known: watts ÷ system voltage = amps.
  • Do not forget standby currents: 0.2 A around the clock is almost 5 Ah a day.

How many amp-hours does my battery need?

Daily consumption times days without recharging, divided by the usable depth of discharge. The battery capacity calculator takes you through it step by step.

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