Fondsites Tools
Home Energy Lab
Home Battery Runtime Calculator
Translate battery capacity, usable reserve, inverter efficiency, and loads into active runtime and daily-use estimates.
Backup power planning gets clearer when every appliance is turned into watts and hours. A large battery can disappear quickly if the load list is too ambitious.
This calculator is meant for first-pass planning: routers, lights, refrigeration, CPAP machines, laptops, and other essentials. It is not an electrical design.
Interactive Tool
Home Battery Runtime Calculator
Change the inputs and the result updates in your browser. Results are not saved, shared, or added to the URL.
Result
Adjust the fields to calculate a result.
How to use this tool
- Enter the battery capacity, usable capacity percentage, and inverter efficiency.
- Fill in up to four loads with watts and expected hours per day.
- Compare active runtime with daily runtime so you can prioritize what actually needs backup power.
What the result means
Active runtime assumes all listed devices are running at once.
Days of use are based on your daily watt-hour estimate. Real refrigerators, pumps, and compressors cycle rather than draw nameplate watts all day.
Assumptions and limitations
Usable capacity and inverter efficiency are estimates. Cold weather, battery age, surge loads, and manufacturer reserve settings can change runtime.
Generators require ventilation and carbon monoxide precautions. Electrical transfer equipment and critical-load panels should be handled according to manuals, codes, and qualified advice.