Battery Runtime Calculator
One division answers every runtime question — capacity over load — with two honest adjustments on top: how deep you may discharge, and how fast the load drains reality beyond the nameplate.
The calculator
Runtime: 5.0 hours
High-rate warning: at this load the battery delivers less than its nameplate Ah — the real runtime is shorter (Peukert effect, explained below).
Assumes the nameplate Ah at its rated (C20) rate and room temperature. Loads near or above 1C and cold conditions both reduce the real answer.
How does the formula work?
The calculation is three steps, and each step is a decision:
- Usable capacity = nameplate Ah × depth of discharge. Lead-acid's 50% window halves the 100 Ah nameplate to 50 usable amp-hours; LFP's 80% window keeps 80.
- Runtime = usable capacity ÷ load. 50 Ah at 10 A is 5 hours; at 5 A it is 10 hours.
- Check the load's rate. If the load is heavy relative to the rating (roughly above the C20/5 mark), the battery delivers less than the nameplate, and step 1 over-credits it.
Why does only half the lead-acid capacity count?
The 50% window is the cycle-life trade, not a rule of physics: lead-acid discharged to 80% every day dies early, discharged to 50% it earns its rated life (see the cycle-life numbers). The calculator's depth setting exists to encode that choice — the arithmetic is the same either way, the battery's life is not.
Why do heavy loads deliver less than the nameplate?
The nameplate Ah assumes the slow C20 rate. At higher currents the internal losses grow, so the same battery delivers fewer usable amp-hours — a 100 Ah battery drained at 50 A may deliver far less than the division predicts (see C-rate fundamentals). The calculator shows the warning when the load crosses into that territory; treat the result there as an upper bound, not a schedule.
Worked examples
| Battery | Load | Depth setting | Result |
|---|---|---|---|
| 100 Ah lead-acid | 10 A | 50% | ~5 h |
| 100 Ah LFP | 10 A | 80% | ~8 h |
| 200 Ah AGM | 20 A | 50% | ~5 h |
When does this calculator mislead?
- Heavy loads — above roughly the C20/5 mark the real capacity shrinks (see above).
- Cold — available capacity falls with temperature (see cold weather).
- An aging battery — a battery at 80% of its original capacity delivers 80% of the nameplate, and the calculator cannot see its age.
- Mixed or variable loads — the tool divides by one load; real duty cycles need the load averaged over the period.
Why This Matters
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.
Sources
Cross-verified from battery engineering references and manufacturer discharge data.
- Battery engineering references — Peukert behaviour and rate capacity.
- Manufacturer datasheets — capacity ratings at standard rates.
More tools: bank sizing calculator · charging time calculator · Last reviewed: 2026-09-17