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Amp Hour Calculator

An amp hour calculator converts a battery's amp-hour rating into watt-hours, and estimates how long it will run a given load — the two questions behind sizing any battery bank for solar, an RV, a boat or backup power. Amp-hours alone do not tell you energy; you need the voltage too.

Your measurements

Result

Enter the capacity, voltage, your load and a realistic depth of discharge, and it gives the stored and usable energy plus the runtime, so you can size a bank that actually lasts the night.

How it is calculated

Watt-hours = amp-hours × battery voltage. Usable energy = watt-hours × depth of discharge. Runtime = usable energy ÷ load in watts. Depth of discharge protects battery life (≈50% lead-acid, 80-100% lithium). Inverter and wiring losses (~10-15%) reduce real-world runtime, so add margin.

Steps

Enter the battery capacity in amp-hours and its voltage.
Enter the load in watts you will run.
Set a realistic depth of discharge (50% lead-acid, 80%+ lithium).
Read the watt-hours and the runtime at your load.
Derate ~10-15% more for inverter losses.

Worked example

A 100 Ah 12 V battery holds 1,200 Wh; at 50% usable depth that is 600 Wh — about 6 hours running a 100 W load.

Common questions

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

Multiply amp-hours by the battery voltage. A 100 Ah 12 V battery is 1,200 Wh.

How long will my battery last?

Usable watt-hours ÷ your load in watts. A 100 Ah 12 V battery at 50% usable runs a 100 W load about 6 hours.

What is depth of discharge?

How much of the battery you actually use. Draining lead-acid past ~50% shortens its life; lithium tolerates 80-100%.

Why is my real runtime shorter?

Inverter and wiring losses (~10-15%), cold temperatures and ageing all cut usable capacity. Size with margin.