My position: Runtime estimates fail at the two adjustments, not the division — people
divide by the nameplate and forget the depth-of-discharge window and the load's Peukert bite, and both
errors run in the same direction: over-promising runtime.
Why: The arithmetic is trivial; the honesty is everything. A number built on usable
capacity — half the rating for lead-acid, 80–90% for LFP, rate-adjusted for heavy loads — is rarely
far off, and it is the difference between a system that matches its math and one that disappoints its
owner.
My editorial view, not a purchasing guarantee.
Frequently asked questions
How do you calculate how long a battery will last?
Divide the usable capacity by the load: hours = capacity × usable depth ÷ amps. A 100 Ah lead-acid
battery at a 10 A load, with its 50% usable window, runs about 5 hours — a 100 Ah LFP at 80% runs about
8 hours.
Why does the calculator only count half of a lead-acid battery’s capacity?
Because lead-acid lives longest when discharged to about 50%; the unused half is the margin that protects
cycle life. Lithium LFP allows 80–90%, so its margin is smaller — the calculator’s depth setting
encodes that difference.
