How generator sizing works
Every generator lists two ratings. Running watts is what it can supply continuously. Starting watts is a short burst for motors that need extra power for a few seconds to start. This calculator adds the running watts of everything you selected, then adds the single biggest starting surge, assuming only one motor starts at a time.
Once you've picked a size, use the Generator Runtime Calculator to see how long it will run on your fuel, or browse the verified generator database.
Generator sizing FAQ
›How do I calculate what size generator I need?
Add up the running watts of everything you want on at the same time, then add the largest single starting surge (the extra watts a motor such as a fridge, pump, or air conditioner needs for a few seconds to start). The generator's running watts must cover the first number and its starting watts must cover the second.
›Why does the calculator recommend extra headroom?
Running a generator continuously at its full rating is hard on it and leaves no margin for loads you forgot. Planning for roughly 10–25% more running capacity than your total is a common rule of thumb.
›Are the appliance wattages exact?
No. They are typical planning values. Check each appliance's nameplate or manual and edit the numbers for a better result.
›Can I connect a generator to my house wiring?
Only through a transfer switch or interlock installed to code by a licensed electrician. Never backfeed through an outlet.