What size generator do I need?

By Dave, generator and controls engineer · Last reviewed October 31, 2026

Step 1: Decide what has to run at the same time

This choice moves the answer more than anything else. Keeping the refrigerator, freezer, sump pump, furnace blower and some lights going is a very different job from running central air, an electric dryer and a water heater.

Most people pick one of three goals: essentials only, most of the house, or the whole house. The calculator starts from one of these and lets you add or remove appliances.

Step 2: Add up the running watts

Use the appliance nameplate when you can. If it lists volts and amps instead of watts, multiply them. Otherwise use typical values, like the ones in the calculator. Add up everything on your list: that's your running total.

Step 3: Add the biggest motor's starting surge

Motors need a burst of extra power for a moment while they start. For each motor, the extra is its starting watts minus its running watts. Motors usually start one at a time, so add only the largest extra to your running total. That's your starting requirement. Running watts vs starting watts explains why.

Step 4: Leave headroom

Don't plan to run a generator at 100% of its running rating for hours. Our rule keeps the steady load at or below 80% of the rating, so the generator needs a running rating of at least your running total ÷ 0.8. That's 25% more than your load, not 20%: a 100 W load needs a 125 W rating, because 100 W is 80% of 125 W. It also leaves room for the appliance you forgot.

The starting rating needs to be at least your starting requirement.

Step 5: Check what lowers a generator's output

A generator's headline rating is usually for one fuel at low elevation. Two things cut it:

• Fuel. The same generator usually makes less power on propane or natural gas than on gasoline, and less on natural gas than on propane. Spec sheets list a separate rating for each fuel. Use the one for the fuel you'll actually burn.
• Elevation. Thinner air means less power. Many manufacturers publish an altitude derate in the manual or spec sheet.

Our calculator doesn't apply fuel or altitude derating yet. Until it does, if you'll run on propane or natural gas or live well above sea level, make sure the generator's rating on your fuel, at your elevation covers the result.

Step 6: Check the voltage

Well pumps, central air, electric dryers, electric water heaters and ranges run on 240 V. A portable generator can only power them if it has a 240 V outlet, and it has to feed your house through a transfer switch or interlock.

Never plug a generator into a wall outlet to power the house. This is called backfeeding. It can send power back out onto the utility lines, where it can kill a line worker, and it bypasses the protection your wiring relies on. Have a licensed electrician install a transfer switch or interlock.

A worked example: essentials only

The calculator's "essentials only" list, with typical values from our appliance list:

The well pump has the biggest starting extra, 3,000 W. So this home needs a portable generator with:

• at least 6,050 running watts (4,840 ÷ 0.8),
• at least 7,840 starting watts (4,840 + 3,000),
• a 240 V outlet for the well pump, and those ratings on the fuel you'll use.

Without the well pump, the numbers drop a lot: 4,180 running watts and 4,890 starting watts. One big motor often sets the size of the whole generator.

Home standby and battery power stations

Home standby generators are sized in kW, and the size also depends on how the generator switches on. Under the National Electrical Code (Article 702, optional standby systems), a manual transfer switch lets you choose which loads to run. An automatic transfer switch that connects the whole house means the generator has to carry the whole house's calculated load, unless a load management system holds the load down. The calculator asks for your transfer type.

Battery power stations need three numbers: a continuous output that covers your running total, a surge rating that covers your starting requirement, and enough watt-hours for the outage. The calculator works out watt-hours from how long the outage lasts and how often each appliance actually runs.

Run it safely

Portable generators make carbon monoxide, which kills without warning. The U.S. Consumer Product Safety Commission says to run them outside only, at least 20 feet from the house, with the exhaust pointed away from the house and any building people could be in. Never run one in a garage, basement, crawlspace or shed, even with the doors or windows open. Put battery-operated or battery-backup carbon monoxide alarms on each level of the house.

Size your own list in the calculator.

Running total = Σ running W
  = 700 + 500 + 1,050 + 1,500
    + 800 + 100 + 30 + 100 + 60
  = 4,840 W

Largest extra to start
  = well pump: 4,500 − 1,500
  = 3,000 W

Starting requirement
  = running total + largest extra
  = 4,840 + 3,000
  = 7,840 W

Portable generator
  Rated running W
    ≥ 4,840 ÷ 0.8 = 6,050 W
  Rated starting W
    ≥ 7,840 W

Without the well pump
  Running total
    = 4,840 − 1,500 = 3,340 W
  Largest extra
    = furnace blower: 2,350 − 800
    = 1,550 W
  Rated running W
    ≥ 3,340 ÷ 0.8 = 4,175
    → 4,180 W (rounded up)
  Rated starting W
    ≥ 3,340 + 1,550 = 4,890 W

References

• U.S. Consumer Product Safety Commission, CPSC Urges Caution When Using Portable Generators (2022) and What to Know About Generators and Carbon Monoxide.
• NFPA 70, National Electrical Code, Article 702: Optional Standby Systems. Summarized in our own words.
• ISO 8528-1:2018, Reciprocating internal combustion engine driven alternating current generating sets, Part 1: Application, ratings and performance (how generator power ratings are defined).
• IEEE Std 3002.7-2018, Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems (motor starting current and voltage drop).
• Appliance values: Know Your Watts appliance list, typical values chosen on the high side. Check your own nameplate.

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