How long will a battery last?
The simple version of this question — hours = Ah ÷ A — is almost always wrong, and wrong in a way that overestimates runtime. Real batteries have two effects that reduce usable capacity: depth-of-discharge limits (you can’t safely use 100% of the rated capacity) and Peukert’s law (capacity drops non-linearly with faster discharge). This calculator handles both.
How to use this calculator
Pick your battery type first — that auto-fills the Peukert exponent and typical usable DoD for that chemistry. Then enter:
- Capacity in Ah — from the battery label
- Nominal voltage — 12, 24, 36, or 48 V (or custom)
- Load — in watts (for appliance loads) or amps (for direct DC loads)
- Inverter efficiency — set to 1.0 if you’re running DC directly; use 0.85–0.92 for a typical pure-sine inverter
The calculator returns both the naïve runtime and the Peukert-corrected runtime, so you can see how much the correction matters for your case.
Depth of discharge (DoD)
Battery chemistry sets how deeply you can safely discharge without hurting cycle life:
- LiFePO4 — 80–90% usable (some tolerate 100%, but 80–90% doubles cycle life)
- Lithium-ion (NMC/LCO) — 80% typical; deeper cycling shortens cell life quickly
- Lead-acid (AGM, gel, flooded) — 50% for good cycle life; 100% DoD kills lead-acid in dozens of cycles
Manufacturers rate Ah at full discharge, but you never actually use that. A 100 Ah lead-acid battery has only about 50 Ah usable in practice.
Peukert’s law — the missing factor
Peukert’s law is why a 100 Ah battery doesn’t give 100 Ah at every discharge rate. Faster discharge means more internal losses; slower discharge means less. The formula:
t = C_rated × (C_rated / (I × t_rated))^(k−1) / I
Where k is the Peukert exponent — 1.0 for an ideal battery, higher for real ones:
- Lithium (LiFePO4, Li-ion) — k ≈ 1.05, near-ideal, Peukert effect is usually negligible
- AGM sealed lead-acid — k ≈ 1.15, moderate effect
- Gel lead-acid — k ≈ 1.20, somewhat higher
- Flooded lead-acid — k ≈ 1.30, strongest effect
The practical impact: at a 500 W load on a 12 V 100 Ah AGM battery (drawing about 42 A), the naïve calculation gives 1.2 hours of runtime. The Peukert-corrected value is 0.9 hours — a 25% overestimate the beginner calculation misses. This is why lead-acid battery banks are usually sized far larger than the naïve math suggests.
The C-rate — is your load reasonable?
The C-rate is the discharge current divided by the rated capacity. A 100 Ah battery discharging at 50 A is at 0.5C.
- 0.05C (20-hour rate) — the reference rate at which Ah capacity is normally measured
- 0.1–0.2C — comfortable rate, all chemistries handle this well
- 0.5C — high for lead-acid (Peukert losses noticeable), fine for lithium
- 1C — very high for lead-acid; typical maximum for lithium continuous
- 2C+ — requires purpose-built high-rate batteries; standard cells will overheat
The calculator warns you if your calculated C-rate is unrealistic for the selected battery type.
Inverter efficiency
If your load is AC (running through an inverter from a DC battery), you lose 8–15% of the battery’s energy in the inverter itself. Set inverter efficiency to about 0.90 for a typical pure-sine wave inverter, 0.85 for a modified-sine, or 1.0 if your load is running directly on DC (LED lighting, DC pumps, radios). The calculator adjusts battery current draw accordingly.
What this calculator doesn’t account for
Real-world runtime can differ significantly from theoretical calculations because of factors this tool cannot know:
- Battery age — a 5-year-old lead-acid battery may hold 60–80% of its rated capacity
- Temperature — cold temperatures dramatically reduce available capacity (a lead-acid battery at −20°C may deliver only 50% of its rated capacity)
- State of charge at start — if the battery isn’t fully charged, runtime is proportionally less
- Load variation — the calculator assumes a constant load; real loads (fridges cycling, motors starting) draw variable current
- Peukert exponent varies with age and temperature — the k value increases as batteries age
Use this calculator to size battery banks and estimate runtime under best-case conditions, then plan for 20–30% less in real deployment. Undersized battery banks cause the most disappointment in off-grid and backup power installations — always oversize.