How do I calculate battery life from amp-hours?

Battery life in hours equals the amp-hour (Ah) capacity divided by the load current in amperes: runtime = Ah / I. For example, a 100 Ah battery running a 5 A load will last 100 / 5 = 20 hours. This assumes 100% discharge efficiency and a constant discharge rate. In practice, lead-acid batteries typically derate to 50% usable capacity to extend cycle life, so a 100 Ah lead-acid battery effectively delivers 50 Ah.

What is the difference between amp-hours and watt-hours?

Amp-hours (Ah) measure charge, which is the product of current and time. Watt-hours (Wh) measure energy, which is the product of power and time. To convert amp-hours to watt-hours, multiply by the battery voltage: Wh = Ah x V. A 100 Ah, 12 V battery stores 1,200 Wh or 1.2 kWh of energy. Watt-hours are useful when comparing batteries of different voltages or sizing a solar or backup power system.

What is the C-rate of a battery?

The C-rate describes the rate of discharge relative to the battery's total capacity. A C-rate of 1C means the battery is discharged in one hour. A C-rate of 0.5C (or C/2) means it is discharged in two hours. High C-rates reduce the effective capacity of many battery types, especially lead-acid, due to internal resistance effects. Lithium batteries generally tolerate higher C-rates without significant capacity loss.

Amp Hour and Battery Life Calculator

This calculator tells you how long a battery will last given its amp-hour capacity, its nominal voltage, and the current or power your load draws. It returns the runtime in hours, the total energy stored in watt-hours, and the C-rate of the discharge. These figures are useful for sizing off-grid solar systems, selecting a UPS battery, planning a camping or boating power setup, choosing a battery for a remote sensor installation, or checking whether an electric vehicle auxiliary battery will last a long trip. The fundamental relationship is simple: runtime in hours equals the battery's amp-hour capacity divided by the load current in amperes. A 100 Ah battery running a 5 A load will last 100 / 5 = 20 hours. To work from watts instead of amps, divide the wattage by the battery voltage to get the current first. A 50 W load on a 12 V battery draws 50 / 12 = 4.17 A, giving a runtime of 100 / 4.17 = 24 hours. These are theoretical figures at constant discharge. In practice, lead-acid batteries are typically limited to 50 percent depth of discharge to protect cycle life, so a 100 Ah lead-acid battery delivers closer to 50 Ah usably. Lithium iron phosphate (LiFePO4) batteries can typically be discharged to 80 to 90 percent. Internal resistance, temperature and discharge rate also affect real-world runtime. Enter the battery capacity, voltage and load below and the results update instantly.

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Battery capacity

Battery voltage

Load current

Or load in watts (overrides amps if > 0)

20.0 h

battery runtime at 100% discharge

Energy stored1,200 Wh

Effective current5 A

C-rateC/20

Runtime at 100% discharge. For lead-acid batteries allow 50% depth of discharge; for LiFePO4 allow up to 80-90%. Actual runtime varies with temperature and discharge rate.

How it works

If the watts field is greater than zero, the effective current is I = W / V, where W is the load power and V is the battery voltage. Otherwise the current from the amps field is used. Runtime is T = Ah / I hours. Energy stored is E = Ah x V watt-hours. The C-rate is I / Ah; a C-rate of 1 means full discharge in one hour. The C-rate is expressed as C/n where n is the number of hours to full discharge (n = Ah / I).

Worked example

A 100 Ah, 12 V battery powers a 5 A load. Runtime is 100 / 5 = 20.0 hours. The battery stores 100 x 12 = 1,200 Wh (1.2 kWh). The C-rate is 5 / 100 = 0.05C, often written as C/20. To run a 50 W load instead, the current would be 50 / 12 = 4.17 A and the runtime would be 100 / 4.17 = 24.0 hours. These match the default current values above.

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