How this calculator works
Power accumulated over time becomes energy. At constant battery voltage, transferred charge multiplied by voltage gives the same energy quantity. Ampere-hours alone do not specify energy because different battery voltages deliver different watt-hours. Actual battery runtime also depends on converter losses, discharge rate, cutoff voltage and ageing; use usable energy and average load power for a realistic estimate.
Formula
E(J) = P(W)t(s) = V(V)Q(C); E(Wh) = E(J)/3600; 1 Ah = 3600 C.
Worked example
A 50 W load running for 2 hours uses 100 Wh, or 360000 J. A nominal 12 V battery delivering 2 Ah supplies approximately 24 Wh.
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Frequently asked questions
Is a watt-hour the same as a watt?
No. Watts describe a rate and watt-hours describe accumulated energy.
Why is voltage required to convert ampere-hours?
Charge does not determine energy without knowing the electrical potential through which it moves.
Does this account for battery efficiency?
No. Enter usable output energy, or adjust the load for losses before estimating time.