Calculate How Long Your Battery Will Last
Enter your battery capacity and load (watts or amps). Set Efficiency and Usable % for real-world results. Turn on Peukert for lead-acid to account for high-current runtime loss.
Inputs
Peukert (simplified): t ≈ Hr × (Ir / I)k, where Ir = (Capacity × Usable × Efficiency) ÷ Hr.
Results
Effective discharge current
Deliverable energy
Runtime (no Peukert)
Pretty format
Runtime (Peukert-adjusted)
Note: Cold temperatures, inverter idle draw, voltage drop, and aging can reduce real-world runtime.
Frequently Asked Questions
Does changing system voltage (12V vs 24V vs 48V) change runtime?
Runtime for a watt-based load depends on available energy (Wh), not just Ah. If you reconfigure the same batteries in series (higher V, same Ah), total Wh increases, so runtime at the same watts increases proportionally (losses permitting). Higher voltage also lowers current for the same power, reducing cable losses and voltage drop.
How much safety margin should I add for real-world conditions?
Plan +15–30% extra runtime (or capacity) to cover cold temps, inverter idle, voltage drop, aging, and usage spikes. In cold climates or for critical loads, target +40–50%.
When should I enable Peukert’s law (lead-acid)?
Enable it when discharge current is high relative to the 20-hour rating; runtime drops as current rises. Typical settings: k = 1.05–1.15, Rated hours = 20. Lithium (LiFePO₄) is minimally affected, most users leave Peukert off for lithium.
Should I include inverter/standby idle draw? How?
Yes. Add idle watts to your load: Total Watts = Load W + Idle W.
Example: same 864 Wh battery, 60 W device + 10 W inverter idle → 70 W total → Runtime ≈ 864 ÷ 70 ≈ 12.3 hours (~12h 21m).
What if my load is measured in amps instead of watts?
Use: Hours ≈ (Ah × Usable × Efficiency) ÷ Amps
Example: 100 Ah, 5 A load, 80% usable, 90% efficiency → (100×0.8×0.9) ÷ 5 = 14.4 hours.
How do I calculate how long my battery will last (in hours)?
If your load is in watts:
Hours ≈ (Ah × Volts × Usable × Efficiency) ÷ Watts
Example: 100 Ah at 12 V, 80% usable, 90% efficiency, 60 W load → Deliverable Wh = 100×12×0.8×0.9 = 864 Wh → Runtime ≈ 864 ÷ 60 = 14.4 hours.