How to Calculate Battery Runtime
Every battery runtime question is one division — capacity over load — with two honest adjustments on top: how deep you may discharge, and how fast the load drains reality beyond the nameplate.
Worked examples
| Battery | Load | Calculation | Runtime |
|---|---|---|---|
| 100 Ah (C20) | 10 A | 100 ÷ 10 | ~10 h to empty (~5 h at 50% DoD) |
| 100 Ah (C20) | 5 A | 100 ÷ 5 | ~20 h to empty |
| 1,200 Wh (lithium) | 120 W | 1,200 ÷ 120 | ~10 h (usable at 80–90% DoD) |
The Peukert adjustment
The nameplate Ah assumes the slow C20 rate (see C20/C10/C5). 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 2 hours. For loads near or above 1C, check the rate-capacity data instead of trusting the division.
The depth-of-discharge decision
The second adjustment is the usable window: lead-acid survives longest at 50% depth of discharge (so plan half the rated Ah), while lithium LFP allows 80–90% (see cycle life vs depth). The runtime formula stays the same — the difference is which fraction of the capacity you count as usable.
What This Means
My take: Runtime calculation is one honest division plus two honest adjustments — and most planning errors are the adjustments, not the arithmetic: people divide by the nameplate Ah and forget the load's Peukert bite and the depth-of-discharge limit.
Why: The formula is trivial; the usable-window discipline is everything. A plan built on usable capacity — half the rating for lead-acid, 80–90% for LFP, rate-adjusted for heavy loads — is rarely off by much, 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.
What buyers should ask
- At which rate (C20/C10/C5) is the capacity figure rated?
- What depth of discharge does the application allow?
- What is the actual load in amps or watts, including peaks?
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.
Last reviewed: 2026-09-16