Motors in refrigerators, pumps, furnaces and air conditioners need a burst of extra power for a moment as they start. A generator has to cover everything running plus the largest of those bursts, which is why it must be bigger than the running total alone.
How to use this calculator
- Enter the number of each appliance you need to keep running. Leave the rest at zero.
- Under More options, add other running watts, such as lights, and set the margin.
- Shop for a generator whose running (rated) watts cover the result.
Worked examples
The essentials
Refrigerator: 1 · Sump pump: 1 · Furnace blower: 1 · TV: 1 · Laptop computer: 1
- Generator size
- 5,000 watts or more
- Running watts
- 2,395 watts
- Peak while the biggest motor starts
- 4,595 watts
- Running watts plus 25% margin
- 2,994 watts
A well and a freezer too
Refrigerator: 1 · Freezer: 1 · Well pump: 1 · Furnace blower: 1 · TV: 1 · Laptop computer: 1
- Generator size
- 6,500 watts or more
- Running watts
- 4,115 watts
- Peak while the biggest motor starts
- 6,315 watts
- Running watts plus 25% margin
- 5,144 watts
Summer, with a window air conditioner
Refrigerator: 1 · Sump pump: 0 · Furnace blower: 0 · Window air conditioner: 1 · TV: 1 · Laptop computer: 1
- Generator size
- 4,500 watts or more
- Running watts
- 1,895 watts
- Peak while the biggest motor starts
- 4,095 watts
- Running watts plus 25% margin
- 2,369 watts
Central air on a standby generator
Refrigerator: 1 · Sump pump: 0 · Well pump: 1 · Furnace blower: 0 · Central air conditioner: 1 · Microwave oven: 1
- Generator size
- 9,000 watts or more
- Running watts
- 6,620 watts
- Peak while the biggest motor starts
- 8,820 watts
- Running watts plus 25% margin
- 8,275 watts
How the size is worked out
Generator makers all use the same rule: add the running watts of everything on at once, then add the single largest starting surge, because motors that start one at a time never surge together. Generac puts a typical starting draw at about 2 times a motor's running watts, and Mississippi State Extension notes motors draw about 3 times their running current as they start.
Briggs & Stratton advises a margin of 20 to 30 percent on top, and Generac says a generator runs best at 70 to 85 percent of its rating, so a little extra capacity saves fuel and strain.
Running and starting watts
- Refrigerator: about 725 watts running, plus about 2,200 watts for a moment as it starts.
- A small sump pump: 800 watts running, 1,300 watts extra to start.
- A small well pump: 1,000 watts running, 2,000 watts extra to start.
- A furnace blower: 700 watts running, 1,400 watts extra.
- A window air conditioner: 1,000 watts running, 1,100 watts extra.
- Microwave oven 925 watts and space heater 1,125 watts: no starting surge, but large running loads.
Run it safely
- Never run a generator indoors, in a garage or a shed, even with the door open: its exhaust carries deadly carbon monoxide.
- Keep it more than 20 ft from the house, doors and windows, as the CDC advises, and use a carbon monoxide alarm inside.
- Connect it to house wiring only through a transfer switch installed by an electrician; plugging it into an outlet can backfeed the power line and kill a lineworker.
Common questions
What size generator do I need to run my house?
Add the running watts of everything you need at once, add the largest starting surge, and keep 20 to 30 percent on top. The calculator does it for the appliances you pick.
How many watts does a refrigerator need on a generator?
About 725 watts running and about 2,200 watts more for a moment as the compressor starts.
Can a generator run a central air conditioner?
Often only a large one: a typical central unit runs at about 3,800 watts and needs more to start, so it usually takes a standby generator.
How far from the house should a generator be?
More than 20 ft from the house, doors and windows, by the CDC's guidance, and never indoors or in a garage.