Usable capacity is about 50% of a lead-acid nameplate and 80–90% of lithium — and the Peukert effect takes more again at high draw.
Discharging lead-acid past half repeatedly destroys cycle life, so half the label is the working figure before anything else happens. Then the rating itself assumes a 20-hour discharge: pull 50 A from a 100 Ah battery and it delivers about 56 Ah, because the chemistry cannot keep up. Add an inverter taking 15% and the deductions multiply rather than add — a 1,200 Wh nameplate comes down to about 340 Wh of real work: 600 after depth of discharge, about 400 after a hard draw, 340 after the inverter.
Lithium changes the arithmetic enough that the rule stops being useful: at a Peukert exponent near 1.05 the high-draw penalty nearly vanishes, and 90% depth of discharge is routine. And cold reverses the advantage — lead-acid loses a fifth of its capacity at 0 °C, while lithium holds capacity but must not be charged below freezing at all.
Estimate with the rule, then check it against the calculator that models it properly.
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