Battery Runtime Calculator
Estimate how long a battery will power your load. Enter capacity and either watts or amps. Efficiency covers inverter/charging losses; “Usable %” applies your chosen depth-of-discharge. Turn on Peukert for lead-acid to account for high-current runtime loss.
Inputs
Peukert law (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, voltage sag, inverter idle draw, BMS limits, and aging can reduce real-world runtime.
Frequently Asked Questions
How big should my battery be for a target runtime?
Rearrange the formula: Ah needed ≈ (Watts × Hours) ÷ (Volts × Efficiency × Usable).
(Example: You want 10 hours at 100 W on a 12 V system, 90% efficiency, 80% usable →
Ah ≈ (100 × 10) ÷ (12 × 0.90 × 0.80) ≈ 116 Ah.)
When should I enable Peukert and what values do I use?
Peukert applies to lead-acid when discharge current is high compared to the 20-hour rating. It reduces runtime as current goes up.
Use k ≈ 1.05–1.15 and Rated hours = 20 unless your battery datasheet says otherwise. LiFePO₄ is far less affected, so most users leave Peukert off for lithium.
Why do real-world results differ from the calculator?
Temperature lowers capacity, especially in the cold. Inverter idle draw and startup surges add load. Voltage drop in cables and connectors wastes power. Batteries lose capacity with age and at very high currents. BMS or inverter low-voltage cutoffs can end discharge earlier than expected.
What should I enter for Efficiency and Usable %?
Efficiency covers inverter and system losses. Usable % is the depth of discharge you plan to use.
Typical starting points: LiFePO₄ Efficiency 90–94%, Usable 80–90%. AGM or flooded lead-acid Efficiency ~88–92%, Usable ~50%.
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