Battery Backup Runtime Calculator
How long a battery can run a load.
Runs in your browser. Nothing is uploaded — your input is processed on this device and discarded when you close the tab.
Battery Backup Runtime Calculator
How to use this tool
- Enter your values in the fields above.
- Press Generate. The result appears below the buttons.
- Use Load sample to see a worked example, or Clear to start again.
Worked example
For a 100 Ah, 12 V battery at 85% efficiency running a 200 W load: usable energy is 100 x 12 x 0.85 = 1,020 Wh, so runtime is 1,020 / 200 = 5.10 hours (5 hours 6 minutes).
What this tool does not do / Limits
This is an ideal estimate. Real runtime is shorter with battery age, cold temperatures, inverter losses beyond the efficiency figure, and loads that vary over time.
Frequently asked questions
Why include efficiency?
Inverters and wiring lose energy as heat, so only part of the battery's stored energy reaches the load.
Can I use this for a UPS?
Yes as a rough guide, but a UPS maker may limit usable capacity to protect the battery, so check its own runtime chart.
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