Battery Runtime Calculator

It calculates the estimated discharge time (in hours) a battery will sustain a constant load, accounting for Peukert’s effect (for lead-acid and flooded batteries), temperature derating, and capacity loss due to aging. The result reflects real-world runtime—not just ideal C-rate estimates—and is especially valuable for sizing backup systems or validating design assumptions.

Use Peukert’s law when designing or analyzing lead-acid, AGM, or gel batteries under moderate-to-high discharge currents (>0.1C), where capacity drops significantly with increased load. The simple Ah ÷ A method only applies to low-current, near-ideal conditions (e.g., trickle loads) and overestimates runtime by 20–40% for typical UPS or solar applications.

Lead-acid (flooded): 1.15–1.35; AGM: 1.05–1.15; Gel: 1.10–1.25; Lithium-ion: ~1.00–1.05 (often treated as Peukert-free). Higher values indicate greater capacity loss at high currents—always verify with manufacturer datasheets, as Peukert varies by cell design and age.

Battery capacity decreases below 25°C and increases slightly above it—but accelerated degradation and safety risks limit practical operation above 35°C. This tool applies standard lead-acid derating curves (~0.5–1% per °C deviation from 25°C), aligning with IEEE 485 and IEC 60896 standards for conservative engineering estimates.

Batteries lose capacity over time due to sulfation, corrosion, and electrolyte dry-out. Age derating (%) approximates remaining usable capacity relative to new—e.g., a 3-year-old lead-acid battery at 77°F may retain only 70–85%. Use 90% for new, 80% for 2 years, and 70% for 3+ years (adjust based on maintenance history and cycle count).

Yes—with caution. Set Peukert constant to ~1.00–1.03 (or 1.0 if unknown), disable aggressive temperature derating (lithium tolerates wider ranges), and rely more on manufacturer voltage-based capacity curves. Note: This tool doesn’t model BMS cutoffs, cell balancing, or low-temperature Li-ion impedance rise—those require advanced battery modeling tools.

Verify load current accuracy (measure with clamp meter, not nameplate rating), confirm battery is fully charged and stabilized at 25°C, and ensure Peukert constant matches your battery type and datasheet. Also check whether “Capacity (Ah)” refers to the 20-hour rate (C20)—if using C5 or C1 ratings, convert appropriately before input.

This calculator focuses solely on electrochemical battery discharge—not system-level losses. Inverter calculators add AC/DC conversion inefficiency (typically 85–95%), power factor, and surge loads. For full system estimation, use this tool’s output as the *battery-only* runtime, then apply inverter efficiency separately to derive AC-side runtime.

Enter the *continuous average DC current* drawn from the battery terminals. For variable loads (e.g., motors, servers), use the weighted average over a representative duty cycle—not peak or RMS unless DC-coupled. AC loads must be converted to equivalent DC current using inverter efficiency and system voltage before input.

It supports preliminary sizing per NEC Article 480 and IEEE 1184, but formal compliance requires additional factors: ambient temperature correction per NEC 480.4(A), minimum 125% continuous load oversizing, and verification against AHJ-specific requirements (e.g., 90-minute emergency egress lighting). Always document assumptions and validate with certified test reports.