Battery Life Calculator
Battery runtime is estimated by dividing the battery's capacity in ampere-hours (Ah) by the load current in amps, then multiplying by an efficiency factor to account for real-world losses. The formula is: Runtime (h) = Capacity (Ah) / Load (A) * Efficiency. For watt-based loads, first divide load power (W) by battery voltage (V) to get current in amps. The efficiency factor (typically 0.85 to 0.95) accounts for battery internal resistance, Peukert's effect at higher discharge rates, temperature derating, and minimum state of charge before cutoff.
Battery life formula
Runtime (h) = Capacity (Ah) / Load (A) * Efficiency
Load (A) = Load (W) / Voltage (V)
Example: 100 Ah battery, 5 A load, 90% efficiency: Runtime = 100 / 5 * 0.9 = 18.0 hours. For a 60 W load on a 12 V system: current = 60/12 = 5 A, giving the same 18.0 hour result.
Battery sizing considerations
- Deep-cycle lead-acid batteries should not be discharged below 50% of capacity (DoD 50%) to preserve cycle life; lithium batteries can typically reach 80 to 90% DoD.
- Cold temperatures reduce usable capacity; lead-acid batteries lose about 10% capacity per 10 degrees C below 25 degrees C.
- Always verify the load current under actual operating conditions; motors and compressors draw high startup currents (3 to 10 times running current).
- UPS systems are rated in VA and runtime at specific loads; scale the calculation proportionally for partial loads.
Frequently asked questions
How do you calculate battery life?
Battery life (hours) = Capacity (Ah) / Load (A) * Efficiency. Efficiency accounts for Peukert's effect, temperature derating, and DC-DC converter losses, typically 0.85 to 0.95 for a well-designed system.
What is battery capacity in Ah?
Ampere-hour (Ah) capacity is the total charge a battery can deliver. A 100 Ah battery can in theory supply 1 A for 100 hours, or 10 A for 10 hours, though real batteries deliver less at higher discharge rates (Peukert's effect).
What is the Peukert effect?
Real batteries deliver less capacity at higher discharge rates. At twice the current, you get less than half the runtime. Peukert's exponent (typically 1.05 to 1.3 for lead-acid) quantifies this. Lithium batteries have lower Peukert exponents (closer to 1.0) and are less affected.
What efficiency factor should I use?
For a simple battery directly driving a DC load, use 0.85 to 0.95 to account for internal resistance losses and the minimum usable state of charge. For a system with an inverter (DC to AC conversion), use 0.80 to 0.90 to include inverter losses.
How do I convert milliamp-hours (mAh) to ampere-hours (Ah)?
Divide by 1,000. A 3,000 mAh phone battery = 3.0 Ah. For typical consumer electronics drawing 200 mA (0.2 A): life = 3.0 / 0.2 * 0.9 = 13.5 hours at 90% efficiency.
Official sources
- IEEE Standard 485: IEEE 485, Recommended Practice for Sizing Lead-Acid Batteries.
- NIST: NIST SP 811, Guide for SI Units.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.