Battery Capacity: Ah, Wh, CCA and RC Explained
Capacity is the tank — and the tank has two units, two extra ratings, and one honest adjustment most labels leave to the fine print.
What does amp-hours mean?
A 100 Ah battery delivers 10A for about 10 hours — or 5A for about 20 — at its rated (C20) rate and before usable-depth adjustments. The fine print matters: the nameplate Ah assumes the slow 20-hour discharge, and heavy loads deliver less (the Peukert effect). Ah compares batteries only at the same voltage — which is why Wh exists.
What is the difference between Ah and Wh?
Wh removes the voltage ambiguity: Wh = Ah × V. 100 Ah at 12V is 1,200 Wh; 100 Ah at 48V is 4,800 Wh — same amp-hours, four times the energy. Use Ah within one voltage class; use Wh to compare across them (see the full comparison).
What are CCA and RC?
The two automotive extras: CCA (cold cranking amps) is the current the battery can deliver for 30 seconds at −18°C — the starting punch, standard-dependent (see the CCA ladder). RC (reserve capacity) is the minutes at 25A before falling to 10.5V — the "engine off, lights on" endurance, roughly 1.5–1.7× the Ah rating in minutes (see reserve capacity).
Why is the usable capacity smaller than the label?
Because the chemistry's longevity window is smaller than its tank: lead-acid lives longest cycled to ~50% of its nameplate; LFP to 80–90%. The label states the tank; the depth-of-discharge states how much of the tank you should spend (see cycle-life numbers). The runtime calculator encodes the difference directly.
Why This Matters
My position: Capacity is the battery market's most misread number — quoted without its rate, its voltage or its usable depth — and all three omissions run in the same direction: making the battery look bigger than it is.
Why: The three qualifiers are the difference between a plan and a wish. A capacity number that carries its rate, voltage and depth is a usable engineering input; one that does not is advertising. This lesson exists so readers can tell the two apart at a glance.
My editorial view, not a purchasing guarantee.
Capacity arithmetic, before the fine print
A 60 Ah battery, ideally, delivers 1 A for 60 hours — or 10 A for 6 hours, or 60 A for 1 hour. The fine print is Peukert: at higher currents real capacity shrinks, so the same battery might deliver on the order of 50 Ah at a 10 A draw in practice. The lesson worth carrying: a capacity number without its discharge rate is an unfinished sentence — see runtime calculation for the full arithmetic.
Frequently asked questions
What does amp-hours mean on a battery?
Amp-hours (Ah) measure capacity: a 100 Ah battery delivers 10 amps for about 10 hours at its rated 20-hour rate. The rating assumes that slow rate — heavy loads deliver less, the Peukert effect — and the usable amount is further reduced by the depth-of-discharge window.
What is the difference between Ah and Wh?
Wh = Ah × voltage, which makes Wh the voltage-independent energy unit. 100 Ah is 1,200 Wh at 12V but 4,800 Wh at 48V. Compare Ah within one voltage class and Wh across them.
What buyers should ask
- Is the Ah rating quoted at the C20 rate, or a slower rate that flatters the number?
- Should the comparison be in Wh instead, because the batteries run at different voltages?
- What usable depth does the application require — roughly 50% for lead-acid, 80–90% for LFP?
- Does the job need CCA (starting) or reserve capacity (engine-off endurance), not just stored Ah?
The bottom line
Read any capacity number with its rate, voltage and usable depth attached — the label states the tank, and the depth-of-discharge window states how much of it you may actually spend. A capacity figure missing those three qualifiers is advertising, not an engineering input.
Sources
Cross-verified from the engineering references cited on the linked pages.
- Battery engineering references — rate capacity and Peukert behaviour.
- Manufacturer datasheets — Ah, CCA and RC ratings.
Next: Lesson 4 — chemistries · Series index · Sizing calculator · Last reviewed: 2026-09-17