How this calculator works
Charge capacity measures how much current a battery can deliver over time. Multiplying charge by nominal voltage estimates stored energy. A C-rate of 1 means a full discharge in one hour in the ideal model. The voltage varies during discharge, so nominal energy and nameplate capacity are approximations.
Formula
E = VQ; I = Q(C) x C-rate / 3600; C-rate = 3600I/Q; t = 3600/C-rate seconds. One ampere-hour is 3600 coulombs.
Worked example
A 12 V, 10 Ah battery stores about 120 Wh. At 0.5 C it supplies 5 A for an ideal 2 hours.
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Frequently asked questions
Why are ampere-hours different from watt-hours?
Ampere-hours measure charge; watt-hours measure energy. Voltage connects them.
Does the runtime include a reserve?
No. Use the battery life page to account for usable capacity and duty cycling.
Is rated capacity constant?
Temperature, ageing, discharge rate and the cutoff voltage affect delivered capacity.