Generator Sizing Calculator
Tick everything you want to run at the same time during an outage. Wattages are typical values for common appliances, labelled as such; your appliance nameplates are the final word.
Your results
Worked example
An outage kit of a refrigerator, furnace fan, sump pump, LED lighting, and a TV (the items ticked when this page loads):
- Running watts: 150 + 500 + 800 + 100 + 150 = 1,700 W
- Biggest starting surge: the refrigerator needs 1,200 W to start versus 150 W running, an extra 1,050 W
- Peak starting watts: 1,700 + 1,050 = 2,750 W
- Recommended size: 2,750 × 1.2 = 3,300 W, so look at generators rated at least 3,300 W starting watts
How this calculator works
Motors draw a brief surge when they start. This calculator assumes the largest motor starts while everything else is already running, which is the standard planning method. Running a generator at 50 to 75 percent of its rating is also easier on fuel and noise than maxing it out, which the 20 percent headroom allows for. All wattages on this page are typical values, labelled as such; check the metal nameplate on your own appliances for exact figures.
Common questions
What is the difference between starting and running watts on a generator box?
Generators list two ratings: running (rated) watts for continuous use, and starting (surge or peak) watts for a few seconds. Match the recommended size here against the generator's starting watts, and make sure its running watts also cover your total running watts.
Can I run a furnace on a portable generator?
A gas or propane furnace mostly needs power for its fan and controls, which is the 500 W figure used here. Many furnaces need a clean power supply and a proper transfer switch installed by an electrician. Never backfeed a panel through a dryer outlet; it is dangerous and illegal.
Estimates only. Wattages are typical values, labelled as such, and results are provided for planning purposes. Generator installation and transfer equipment must comply with the Canadian Electrical Code and local rules; consult a licensed electrician. Mississippi Maple accepts no liability for purchase or installation decisions made using this calculator.