What Size Standby Generator Do I Need? Watts by Load
A standby generator is sized by the loads you want to keep on. Essentials (refrigerator, lights, sump pump, furnace) come to about 2,550 running watts. Add central air and the size to shop for is 15,000 W. Work out your own total with the generator size calculator.
Part of the project planner: Planning backup power, batteries and solar.
Standby generator size by load
| Load | Running watts | Peak watts | Generator to shop for |
|---|---|---|---|
| Basic outage: refrigerator, lights, laptop and router | 950 | 2,450 | 2,000 running watts |
| Basic plus sump pump and furnace blower | 2,550 | 4,100 | 3,500 running watts |
| Add well pump, microwave, coffee maker and TV | 5,800 | 7,350 | 7,500 running watts |
| Add a window air conditioner | 7,000 | 9,400 | 9,000 running watts |
| Add central air conditioning | 9,300 | 16,300 | 15,000 running watts |
| Add central air, electric water heater and dryer | 18,700 | 25,700 | Over 22,000 W: whole-home standby generator |
Portable or standby?
- Portable generators suit essentials: the refrigerator, lights, sump pump and furnace, run through a transfer switch or a few extension cords.
- Standby generators are fixed outside, run on natural gas or propane and start by themselves. They suit homes that must keep central air, a well pump or a medical device on.
- A power station is quiet and indoor-safe for a few devices: see generator vs portable power station.
Before you buy
- Check the nameplates: the chart uses typical figures, and yours may differ.
- Ask an electrician for a load calculation of your panel, which also shows which circuits the transfer switch should cover.
- Pick the fuel you can get in an outage: natural gas lines, or a propane tank you keep full.
- Allow clearance and a permit: standby units need distance from windows and walls, set by code and the maker.
The appliance-by-appliance figures are in the generator wattage chart, and the longer guide is what size generator do I need. For a quiet battery option see the power station size calculator. These are estimates: a licensed electrician must install a standby generator.
Frequently asked questions
What size standby generator do I need for a house?
It depends on what you want running. Essentials only come to about 2,550 watts; adding central air takes the shop size to 15,000 W, and adding an electric water heater and dryer gives: Over 22,000 W: whole-home standby generator.
How do I choose a standby generator size?
Add the running watts of every circuit you want on, add the largest starting surge, and pick a rating above the total. An electrician can also do a load calculation from your panel.
Is a bigger standby generator better?
Not automatically. An oversized unit costs more to buy and run. Size it for the loads you will really use, with some room to add more later.
Does a standby generator need a transfer switch?
Yes. An automatic transfer switch isolates the house from the grid, so power cannot back-feed into the lines. It is installed by a licensed electrician.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.