Generator size calculator

How the calculation works

The tool follows the method of Generac’s wattage worksheet, in two steps: add up the running watts of everything that will run at the same time, then add the single highest starting load.

Motor-driven appliances such as the refrigerator, the freezer and the sump pump need more power for a few seconds when they start. Generac’s worksheet puts it at “typically” twice the running watts; the manual for Generac’s GP5700E portable generator says about three times for some induction motors. The tool uses twice, as the worksheet does, and assumes you start one appliance at a time.

If your appliance only lists amps, multiply them by 120 V for an appliance plugged into a regular outlet. In Québec, homes are supplied at 120/240 V: the range, dryer, water heater and baseboard heaters run on 240 V.

What the result doesn’t tell you

The total tells you how many watts your appliances need. It doesn’t tell you whether your panel, circuits and service entrance allow them to be connected to the generator: that’s what the load calculation is for, and our master electricians do it with the quote, free of charge. See our electrical load calculation.

To power circuits in the house, the generator goes through a switching device. Under Hydro-Québec’s Conditions of Service (section 15.2.5), it must have a mechanical interlock that prevents the generator and the grid from being coupled; without one, you need Hydro-Québec’s written authorization before the hookup. See the transfer switch.

In winter, heating weighs heavily

In very cold weather, heating can account for up to 80% of a home’s electricity use, according to Hydro-Québec. A 6-foot baseboard heater draws about 1,500 W at 240 V, by Sinopé’s estimate: that’s more than the refrigerator and the lights combined on Generac’s worksheet. That’s why the tool counts baseboard heaters separately, by the foot; to estimate them room by room, see the baseboard heater calculator.

A portable generator without a transfer switch only powers what is plugged into its own outlets: Québec.ca says to plug appliances directly into it, with extension cords approved by the Canadian Standards Association (CSA), and Hydro-Québec reminds you never to plug it into an outlet in the house. Baseboard heaters, which run on 240 V and are connected to the panel, therefore need a transfer switch.

A freezer in good condition and filled to capacity keeps food for up to 48 hours, according to Québec.ca: unticking it can free up room for something else during a short outage.