Battery Runtime Calculator
How long will a battery or power station run your devices?
How long will a battery or power station run your devices? Free, private, and it shows its working.
How to use this tool
Runtime depends on usable capacity, not the label. Lead-acid batteries should not be discharged much below half, while lithium iron phosphate batteries can use most of their rated capacity.
Inverter losses matter for AC devices; for 12-volt devices set efficiency to 100.
This tool is for planning and education only. It does not store your entries. Read our disclaimer.
Worked example
Here is what the Battery Runtime Calculator returns with its starting values. Replace them with your own numbers for a real plan.
| Input | Example value |
|---|---|
| Battery capacity | 200 Ah |
| Battery voltage | 12 V |
| Usable depth of discharge | 80 % |
| Average load | 150 W |
| Inverter efficiency (100 for DC loads) | 88 % |
| Result | Value |
|---|---|
| Estimated runtime | 11.3 hours |
| Usable energy | 1,920 Wh |
| Power drawn from battery | 170 W |
How the calculation works
- Usable Wh = 200 Ah x 12 V x 80% = 1,920 Wh.
- Battery draw = load / inverter efficiency = 170 W.
- Runtime = usable Wh / battery draw.
Tips for more accurate results
- Use usable capacity: most of a LiFePO4 battery’s rating, but only about half of a lead-acid battery’s.
- Set inverter efficiency for AC loads, and use 100 percent for devices running directly from 12 volts.
Related guides
When this tool helps most
Use it to check whether a battery or power station will last a night, a workday or a weekend of boondocking with the devices you actually run.
Common mistakes to avoid
- Using the label capacity instead of usable capacity.
- Leaving out inverter losses for AC devices.
- Forgetting standby draws such as detectors and control boards.
Step-by-step guides that use this tool
These guides explain the decisions behind the numbers, so you can act on your result with confidence.
Understanding your results
- Usable capacity: lithium batteries let you use most of the rating; lead-acid only about half.
- Runtime: how long the battery lasts at the load you entered, before recharging.
- Daily balance: compare daily use with daily charging to see if you can stay put.
Two quick scenarios
Fridge and laptops
A 12-volt fridge plus two laptops is a common remote-work load. The result shows whether one battery bank lasts overnight or needs daily solar.
Inverter losses
Running loads through an inverter wastes some energy as heat, so DC loads stretch a battery further.
Before you rely on the numbers
- Use measured draw where you can.
- Include inverter standby draw.
- Keep lithium batteries above freezing when charging.
- Recheck after adding any new appliance.
Keep planning with RV lithium battery guide and RV Solar Sizing Calculator.
Official references
- U.S. Department of Energy: homeowner’s guide to solar
- ENERGY STAR: certified refrigerators and coolers
Frequently asked questions
Why is runtime shorter than expected?
Cold weather, battery age, high loads, inverter losses, and devices that cycle on and off all reduce real runtime.