Battery Sizing Basics: Ah, C-Rates and Runtime Explained

Amp-hours, C-rates and depth of discharge in plain language, with a simple method to size a battery for your load.

ATB Power · Battery Solutions · 6 min read

Sizing a battery comes down to a few numbers. Get these right and you avoid both under-sizing (short runtime, early failure) and over-paying for capacity you never use.

The key terms

Capacity (Ah): amp-hours the battery can deliver at a stated rate, usually the 20-hour rate (C20).
Energy (kWh): capacity times voltage. A 24 V, 100 Ah battery holds about 2.4 kWh.
C-rate: charge or discharge current relative to capacity. 0.5C of a 100 Ah battery is 50 A.
Depth of discharge (DoD): how much you use before recharging.

A simple sizing method

1. Add up the load in amps (or watts divided by voltage).
2. Multiply by the hours of runtime you need. That is the amp-hours used.
3. Divide by the usable fraction: about 0.5 for lead-acid, about 0.9 for lithium.
4. The result is the rated capacity to specify.

Worked example

A 10 A load for 5 hours is 50 Ah used. On lead-acid at 50% DoD, specify about 100 Ah. On lithium, about 55 to 60 Ah covers the same job.

Do not forget

High discharge rates reduce usable capacity on lead-acid (Peukert effect); size up for heavy loads.
Cold reduces capacity; size for the coldest expected operating temperature.
Check the C-rate the equipment needs is within the battery rating.

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