How to size whole-home backup (step by step)
Total your continuous loads, find the largest surge, and add safe headroom for outages.
Quick take
List your critical circuits, total the watts, check the biggest surge, and add 20% headroom.
How to use the calculator
- Open the calculator linked below.
- Enter the numbers that match your setup.
- Use our rules of thumb to sanity-check the result.
Rules of thumb
- Size to the higher of continuous vs surge.
- Add 10–30% headroom for comfort and longevity.
- Mind 240V split‑phase needs before buying.