Home Backup Generator Size Calculator
Choose the household items you want to run during an outage. The calculator estimates a practical generator class using running load, motor startup surge, and a safety margin.
Start with common choices
Select what applies. If there is a quantity box, adjust the count before fine-tuning the estimate below.
Fine-tune the estimate
How this calculator estimates it
Estimated generator class = selected running load + the largest estimated motor startup surge + manual extras + safety margin. Multiple motors usually do not all start at exactly the same moment, so this uses the largest startup event as the planning surge.
Common mistakes
- Sizing only for running watts and ignoring pump, refrigerator, freezer, AC, or furnace startup surge.
- Trying to run every major appliance at once instead of choosing essentials.
- Forgetting that electric heat, electric water heaters, dryers, and ranges can overwhelm many portable generators.
Before you choose
- Central air can push you into a much larger generator class. If AC is important, verify the outdoor unit label and consider a soft-start kit where appropriate.
- For a house, the safest setup usually involves a transfer switch or interlock installed correctly instead of extension-cord improvising.
- Fuel type, runtime, noise, weather protection, and where the generator sits are just as important as wattage.
- If you only need essentials, leave central air, electric ranges, dryers, electric heat, and large water heaters off the plan.
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