Home Backup Generator Size Calculator
Select the home items you want to run during an outage. You can adjust quantities and use the advanced fields if you know your actual equipment ratings.
Last updated: May 2026
Buying and sizing notes
- For essential backup, size for the loads you truly need during an outage, not every appliance in the house.
- For hardwired home backup, use proper transfer equipment and qualified electrical work.
- If you want to run central air, well pump, sump pump, and kitchen loads together, the required generator size can jump quickly.
Generator sizing reality check
The right generator size depends more on what starts at the same time than on a simple list of appliances. Pumps, compressors, refrigerators, and air conditioners can need a large startup surge.
Essentials first
For outages, start with the loads you truly need: heat controls, refrigerator, freezer, sump pump, well pump, lights, internet, and a few outlets.
AC is a big jump
Central air can require a much larger generator than an essentials-only setup. Verify the actual equipment label before buying.
Use transfer equipment
Hardwired home backup requires safe transfer equipment and electrical work that meets local requirements.
How to use this estimate
What this includes
- Selected running loads
- Estimated startup loads
- Safety margin
What it does not include
- Transfer switch design
- Fuel supply sizing
- Electrical code compliance
- Exact equipment nameplate ratings
Common mistakes
- Trying to run every appliance at once
- Ignoring motor startup loads
- Using generic AC assumptions instead of the actual data plate
Quick questions
Can this size a whole-house standby generator?
It can provide a rough planning estimate, but final standby generator sizing should be verified from actual loads, fuel supply, transfer equipment, and installer guidance.
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