Calculate how long a battery lasts at a given load, allowing for capacity derating.
Lead-acid capacity falls as discharge current rises, described by the Peukert exponent. Lithium has an exponent close to 1.0, so its capacity is nearly independent of discharge rate. Rating a lead-acid bank at the 20-hour rate then discharging it in two hours can lose a fifth of the expected capacity.
Battery Discharge Time
Runtime = usable Ah ÷ load, corrected by (rated current ÷ load)^(k−1) for the Peukert effect
Runtime = usable Ah ÷ load, corrected by (rated current ÷ load)^(k−1) for the Peukert effect Lead-acid capacity falls as discharge current rises, described by the Peukert exponent. Lithium has an exponent close to 1.0, so its capacity is nearly independent of discharge rate.
Rating a lead-acid bank at the 20-hour rate then discharging it in two hours can lose a fifth of the expected capacity.
This calculator takes 4 inputs: Rated capacity, Load current, Usable depth of discharge, Peukert exponent. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.