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.
How to calculate, step by step
- Pick the capacity unit that matches your load — Ah with amps, or Wh with watts.
- Find the usable capacity — apply the depth-of-discharge window: ~50% of rated Ah for lead-acid, 80–90% for LFP.
- Divide capacity by load — hours = Ah ÷ amps (or Wh ÷ watts).
- Adjust for the load's rate — for heavy loads near 1C, the Peukert effect means the battery delivers less than the C20 nameplate.
- State the answer with its assumptions — which DoD and which discharge rate the runtime figure assumes.
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.
The three common mistakes
- Using the full nameplate Ah — lead-acid's usable window is ~50%, LFP's 80–90%
- Ignoring the load's rate — heavy loads deliver less than the C20 nameplate (Peukert)
- Mixing units — Ah needs amps, Wh needs watts; converting at the wrong voltage breaks the result
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?
The bottom line
Runtime is capacity over load, corrected for the depth you may discharge and the Peukert effect at higher loads. Run the two adjustments or the estimate overshoots reality — and in a backup or off-grid case, that overshoot is the failure.
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.
Prefer the interactive version? Use the calculator. Last reviewed: 2026-09-16