Battery Runtime Calculator
Estimate how many hours a battery will power a device based on its capacity, discharge depth, and system efficiency. Ideal for sizing backup power, solar storage, and portable electronics.
About this calculator
Battery runtime depends on how much usable energy the battery stores and how fast the load consumes it. The formula is: Runtime (h) = (battery_capacity × (depth_discharge / 100) × (efficiency_factor / 100)) / load_current. Battery capacity in amp-hours (Ah) represents total stored charge. Depth of discharge (DoD) limits how much of that charge you can safely use — for example, lead-acid batteries should not be discharged below 50% to preserve cycle life. System efficiency captures losses in inverters, wiring, and the battery's internal resistance. Dividing usable amp-hours by the load current in amps gives runtime in hours. Accurate runtime estimates prevent unexpected shutdowns and help extend battery lifespan.
How to use
Assume a 100 Ah battery, a load current of 10 A, a depth of discharge of 80%, and a system efficiency of 90%. Step 1 — find usable capacity: 100 × (80/100) = 80 Ah. Step 2 — apply efficiency: 80 × (90/100) = 72 Ah. Step 3 — divide by load current: 72 / 10 = 7.2 hours. The battery will run the load for approximately 7.2 hours before reaching its discharge limit. Reducing the load current to 5 A would nearly double runtime to about 14.4 hours.
Frequently asked questions
How does depth of discharge affect battery runtime and battery life?
Depth of discharge defines the percentage of total capacity you use in each cycle. Using a higher DoD gives longer runtime per charge but accelerates battery aging because deeper discharge stresses the electrodes. Lead-acid batteries typically last longest at 50% DoD, while lithium iron phosphate (LiFePO4) cells tolerate 80–90% DoD with minimal impact on cycle count. Balancing runtime needs against longevity goals is key when choosing a DoD setting.
Why does system efficiency reduce effective battery runtime?
System efficiency captures all energy losses between the battery terminals and the load. Inverters converting DC to AC typically lose 5–15%, wiring has resistive losses, and battery internal resistance wastes energy as heat during discharge. A 90% efficient system means 10% of the stored energy never reaches the load. In calculations, this reduces the effective amp-hours available, shortening runtime compared to an ideal lossless system.
What battery capacity do I need to run a 20 A load for 5 hours at 80% depth of discharge?
Rearranging the runtime formula: required capacity = (runtime × load_current) / (DoD × efficiency). With 5 h, 20 A, 80% DoD, and 95% efficiency: capacity = (5 × 20) / (0.8 × 0.95) ≈ 131.6 Ah. You would need at least a 132 Ah battery. Always round up to the next standard size and consider adding a safety margin of 10–20% for aging and temperature effects.