Backup generator size calculator guide for home outage loads.
The fastest way to overspend on a generator is to size for everything. The fastest way to undersize one is to ignore startup surge. A better estimate starts with essential loads and then checks motors, pumps, refrigerators, freezers, and air conditioning separately.
How to size a backup generator for planning
- Choose the loads that must run during an outage.
- Add running watts for loads that can run at the same time.
- Add startup surge for well pumps, sump pumps, refrigerators, freezers, compressors, and AC equipment.
- Add a safety margin so the generator is not planned at its absolute limit.
- Use actual nameplate data before buying or installing equipment.
- For hardwired standby systems, verify transfer equipment, fuel supply, and installer sizing.
Generator sizing examples and caution points
Essentials-only backup
A refrigerator, freezer, furnace blower, lights, internet, sump pump, and well pump can often require much less generator than a whole-house setup.
Well pump and sump pump
Pumps can have high startup surge. Use a cautious planning estimate until the actual nameplate or measured load is known.
Central air conditioning
Central AC can be the load that changes the entire generator class. Treat it separately from refrigerator and lighting loads.
Standby generator sizing
A standby generator estimate should be checked by the installer against fuel supply, transfer switch type, actual loads, and local requirements.
Queries this guide supports
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Common questions
What size generator do I need for my house?
It depends on which loads must run together and which motors need startup surge. Start with essential loads, then add pumps, refrigerators, freezers, and AC only if needed.
Is a standby generator sized differently than a portable generator?
The load math is related, but standby generator selection also depends on transfer equipment, fuel supply, installation details, and actual load data.
Why does startup surge matter?
Motors and compressors can require far more power for a short startup period than they use while running.