Generator Sizing Calculator
Choosing the wrong size generator can mean nuisance tripping or wasted money. Enter your key appliances' running watts and the largest single starting-watt surge to size a generator with adequate headroom.
About this calculator
Calculate the generator wattage you need based on the appliances and systems you want to power during an outage.
Frequently Asked Questions
Why add the largest starting watts instead of all starting watts?
Most appliances only draw their extra starting surge for a fraction of a second when first switched on, and it's rare for multiple large motors to start at the exact same instant. Sizing for your running total plus the single largest surge covers realistic worst-case demand without wildly oversizing the generator.
How much safety margin should I use?
A 20% margin is a common rule of thumb to avoid running a generator at its absolute max, which shortens its lifespan and can cause voltage issues. If you plan to add appliances later, consider a larger margin.
Do I need to include HVAC starting watts?
Yes, air conditioners and well pumps typically have the highest starting watt surges of any home appliance, often 2-3x their running watts, so they're usually the 'largest surge' figure to use.