Battery Runtime Calculator
Knowing how long a battery will last under a given load is essential for off-grid solar systems, RV and marine setups, backup power installations, and portable power stations. This calculator uses the standard energy-based formula: multiply battery capacity in amp-hours by the voltage to get total watt-hours, then multiply by your system efficiency to find usable energy, and divide by the load in watts to get runtime in hours. The efficiency factor (default 0.85) accounts for inverter losses, wiring resistance, and internal battery self-discharge. Results are shown at full load and at 50% load, giving you a practical operating range. For lead-acid batteries, bear in mind that discharging below 50% depth of discharge reduces battery life significantly. Lithium iron phosphate (LiFePO4) batteries generally tolerate 80% to 100% depth of discharge safely. Always verify against your battery manufacturer's datasheet for accurate derating curves. This calculator works for single batteries and for banks wired in series or parallel. Enter the total bank voltage and total bank amp-hour capacity to get the combined runtime.
Battery runtime formula
The calculation follows three steps:
Total Energy (Wh) = Capacity (Ah) x Voltage (V)
Usable Energy (Wh) = Total Energy x Efficiency
Runtime (hours) = Usable Energy / Load (W)
Worked example
A 100 Ah, 12 V battery powering a 500 W load at 85% efficiency:
- Total energy = 100 x 12 = 1,200 Wh
- Usable energy = 1,200 x 0.85 = 1,020 Wh
- Runtime at full load = 1,020 / 500 = 2.04 hours (2 hours 2 minutes)
- Runtime at 50% load (250 W) = 1,020 / 250 = 4.08 hours (4 hours 5 minutes)
Note: this is the electrical runtime. For lead-acid batteries, apply a 50% depth-of-discharge limit to protect battery life, giving a practical runtime of around 1 hour at 500 W in this example.
Frequently asked questions
What does Ah (amp-hour) mean?
Amp-hours (Ah) measure how much charge a battery can store. A 100 Ah battery can theoretically deliver 100 amps for one hour, or 10 amps for 10 hours, or any other combination that multiplies to 100 amp-hours. Multiplying amp-hours by the battery voltage gives you watt-hours (Wh), the total energy the battery can deliver.
Why does efficiency reduce runtime?
No energy conversion is 100% efficient. Inverters (which convert DC battery power to AC mains power) typically operate at 85% to 95% efficiency, meaning 5% to 15% of battery energy is lost as heat. Wiring resistance and internal battery resistance also cause losses. A realistic efficiency factor of 0.85 accounts for these combined losses in a typical setup.
How does depth of discharge affect runtime?
Lead-acid batteries should not be discharged below 50% capacity (depth of discharge or DoD = 50%) without significantly reducing their lifespan. Lithium-ion and LiFePO4 batteries tolerate deeper discharge, often 80% to 100% DoD. If you are using lead-acid batteries, multiply the calculator result by 0.5 to get safe usable runtime. Enter the usable capacity rather than the nominal capacity for the most accurate result.
What is the difference between 12 V and 24 V battery systems?
Both voltage levels store the same energy if the amp-hour capacity is proportional (a 12 V 200 Ah bank stores the same Wh as a 24 V 100 Ah bank). The advantage of 24 V systems is that for the same power delivery, the current is halved, which reduces wiring losses and allows thinner cable. Larger systems (above 3,000 W) often use 24 V or 48 V to keep current at manageable levels.
How do I calculate runtime for multiple batteries?
Batteries wired in series (positive to negative) add their voltages together but keep the same amp-hour rating. Batteries wired in parallel (positive to positive, negative to negative) add their amp-hour ratings but keep the same voltage. For a bank of two 12 V 100 Ah batteries in parallel, enter 12 V and 200 Ah. For the same batteries in series, enter 24 V and 100 Ah. Both give the same total watt-hours, so both give the same runtime for the same load.
Sources
- U.S. Department of Energy, Office of Scientific and Technical Information: Battery Storage for Solar Energy.
- National Renewable Energy Laboratory (NREL): Battery Test Manual for Plug-In Hybrid Electric Vehicles (PDF).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. General information only. Verify against your battery manufacturer's datasheet.