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Quick answer
For a 12 V, 100 Ah battery powering a 100 W load, estimated runtime is 8.64 hours at 80% depth of discharge and 90% efficiency, with full battery health, no idle draw and no Peukert reduction. Your result depends on the battery and load assumptions.
How to use
Enter the known values, check the selected units and assumptions, then calculate. Compare the result with the worked example. Copy, print or export the table when you need a record. Use the total bank voltage and capacity, average load watts and manufacturer assumptions. Efficiency applies to the load; inverter idle draw is then added. Peukert uses the rating period and exponent, capped at the linear energy estimate. Target sizing shows the energy-only capacity.
Formula and method
E = V × Q; P_b = P / η + P₀; I = P_b / V; t = min(E × D × S / P_b, H × (Q × S / (I × H))^k × D)
E is nominal energy (Wh); V is bank voltage (V); Q is capacity (Ah); P is load power (W); P₀ is inverter idle draw (W); P_b is battery-side power (W); I is estimated current (A); t is runtime (hours); H is the capacity rating period (hours); and k is the Peukert exponent. D is usable depth of discharge, S is remaining battery health, and η is efficiency. Use fractions in the formula: 80% = 0.80 and 90% = 0.90. With k = 1, the calculation uses the linear energy estimate.
Worked example
100 Ah × 12 V × 80% × 90% / 100 W = 8.64 h (k=1; S=100%; P₀=0)
Assumptions and limits
Runtime is an estimate, not a discharge curve. Temperature, aging, cutoff voltage, load cycling and surge power can change the result. A fixed-current Peukert approximation is less accurate for constant-power loads. Required Wh excludes Peukert; check the manufacturer’s runtime chart.
Sources and methodology
Method reviewed:
Frequently asked questions
How long will a 12 V, 100 Ah battery run a 100 W load?
About 8.64 hours with 80% usable depth of discharge, 90% efficiency, full battery health, no inverter idle draw and k = 1. The calculation is 12 × 100 × 0.80 × 0.90 ÷ 100. Actual runtime depends on temperature, battery age, cutoff voltage and the load.
What should I check before using the result?
Runtime is an estimate, not a discharge curve. Temperature, aging, cutoff voltage, load cycling and surge power can change the result. A fixed-current Peukert approximation is less accurate for constant-power loads. Required Wh excludes Peukert; check the manufacturer’s runtime chart.
Are my inputs sent to a server?
Calculations run in your browser. This plugin does not send your entered values to a server or save them. Other site analytics are governed by the site privacy policy.