Battery Runtime Calculator
How long will your battery run a device? Enter the battery bank’s nominal voltage and amp-hours, plus the device’s running watts. Adjust the battery and inverter settings to get a practical estimate in hours.
Enter Your Numbers
Your Results
Estimated runtime
Energy to your device
Estimated battery current
Battery settings used
How to Use the Calculator
Start with the numbers on your battery and the running watts of the device. If you are powering two devices at once, add their watts. Select a usable-capacity assumption, and leave the inverter box checked only when the load uses an AC outlet.
For multiple batteries, enter the bank’s final voltage and amp-hours. Two 12V 100Ah batteries in parallel make a 12V 200Ah bank; the same pair in series makes a 24V 100Ah bank. Either arrangement has about the same nominal energy. Use the manufacturer’s stated nominal voltage if it differs from the round-number system name: a 12V-class LiFePO4 battery may be labeled 12.8V.
How the Estimate Works
Estimated energy to the device (Wh) = nominal energy × usable battery fraction × (1 − buffer fraction) × inverter efficiency (for AC loads)
Runtime (hours) = estimated energy to the device ÷ running watts
The percentages are adjustable assumptions. The result is a planning estimate, not a battery test. It does not model inverter idle power, wiring or DC converter losses, battery voltage changes, or the high-current capacity drop that can be pronounced with lead-acid batteries.
Example: 12V 100Ah Battery and a 60W Load
Using the sample settings above, a 12V 100Ah bank starts with 1,200Wh of nominal energy. At 90% usable capacity, with a 10% planning buffer and a 90% efficient inverter, about 875Wh reaches the device. Divide that by a steady 60W load and you get about 14.6 hours.
That is an example, not a promise. If your battery label says 12.8V, enter 12.8V instead. If the device cycles on and off, measure or estimate its average running watts rather than assuming it draws the same power all day.
Questions That Change the Answer
Does a 100Ah battery always last 100 hours?
No. Amp-hours describe capacity at a stated discharge rate. Runtime also depends on voltage, the load in watts, the share of the battery you plan to use, and any conversion losses.
Should I include the inverter for a USB or direct DC device?
Uncheck the inverter box if the device runs directly from the battery’s DC system. A USB adapter or DC converter can still waste some energy, and this calculator does not model that loss separately.
Why might my real runtime be shorter?
A battery that starts below full charge, cold weather, age, a heavier load, inverter idle draw, and voltage cutoff can all shorten it. A high discharge rate can reduce usable capacity, especially with lead-acid batteries. A device that runs only part of the time may last longer than a constant-watt example.
Plan the Rest of Your Setup
If the estimated hours are too short, try the battery bank size calculator to work backward from a target runtime. For an AC appliance, check the inverter size calculator as well; continuous watts and startup surge are different questions. The solar panel size calculator helps with a separate recharge plan. You can find more tools on the DIY calculators page.
Safety and Assumptions
Use the result to plan, then check the battery, inverter, fuse, wire, and device ratings for your actual setup. A runtime estimate does not establish that the system can safely supply a load or its startup surge. Follow the equipment instructions and applicable local electrical requirements; get qualified help for wiring or installation when needed.
For more on why capacity changes with discharge rate, see Victron Energy’s battery monitor manual. Use your own battery and inverter specifications for the inputs that matter most.