WeekendCalc

Generator Size Calculator

Tick everything you want to power during an outage. You get the running watts, the peak starting watts (motors surge when they start), and the generator size to shop for.

Your project

W
W

The surge above running watts for the biggest motor, from its label.

Results

Generator size to shop for
5,000 running watts
With about 20% headroom
Total running watts
3,550 W
Peak watts at startup
5,100 W
Running load at 120 V
29.6 amps
Appliances selected
6 items
  • Start the biggest motor first, and add loads one at a time. Wattages are typical, so check each appliance label (a W or A rating) and use that if it differs. An inverter generator runs electronics more safely than a conventional one.

How this calculator works

Every appliance has a running wattage, which is what it draws once it is going. Motor-driven appliances (refrigerators, pumps, air conditioners) also have a much higher starting wattage for a second or two when they kick in. A generator has to handle the sum of everything running, plus the surge from the single biggest motor starting up, since you will not start them all at once.

peak watts = total running watts + the largest starting surge above its own running watts

We then size the generator so that its continuous rating is at least 20% above your running load (a generator should not be run flat-out for long), and its peak rating covers the surge. Portable generators typically peak about 25% above their running rating.

Worked example: a refrigerator, sump pump, furnace blower, microwave, lights and a laptop/router add up to 3,550 running watts. The furnace blower has the largest surge (2,350 starting watts against 800 running, an extra 1,550 W), so the peak is 5,100 W. With 20% headroom, a 5,000 running-watt generator is the size to shop for.

Tips before you buy

  • Never run a generator indoors, in a garage, or near windows and vents. Carbon monoxide kills quickly. Keep it at least 20 ft from the house, and use a battery-powered CO alarm indoors.
  • To power a furnace, well pump or other hard-wired equipment safely, you need a transfer switch or an interlock installed by an electrician. Never back-feed a house through a dryer outlet.
  • Run only what you need. Skipping the electric water heater, dryer and central air cuts the generator size you need by thousands of watts.
  • Store fuel safely in approved cans and rotate it. Gasoline goes stale in a few months unless it is treated.
  • Test the generator under load every few months, so you know it starts when the lights go out.

Quick answers by size

For a refrigerator2,000 running wattsFor a sump pump2,000 running wattsFor a well pump2,000 running wattsFor a furnace blower2,000 running wattsFor a window air conditioner3,000 running wattsFor a central air conditioner9,000 running wattsFor an electric water heater5,000 running wattsFor an electric dryer6,500 running watts

Tools & materials for this project

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Portable generator (5,000 W)Inverter generator (3,500 W)Generator transfer switch kitCarbon monoxide alarmFuel stabilizer

Frequently asked questions

What size generator do I need to run a house?

To run essentials (refrigerator, a few lights, furnace blower, sump pump) usually 3,500 to 5,000 running watts is enough. To run everything including central air, an electric water heater and dryer you will need a large standby generator of 15,000 W or more.

What is the difference between running watts and starting watts?

Running watts is what an appliance draws continuously. Starting (or surge) watts is the brief extra draw when a motor starts, often two to three times higher, which the generator must be able to supply.

Can a 3,500 watt generator run a refrigerator?

Yes, easily. A refrigerator needs about 700 W running and 2,200 W to start, so a 3,500 W generator can run it along with a few lights, a sump pump and a TV.

Should I buy an inverter generator?

For electronics, laptops and phones, yes: inverter generators produce cleaner power, and are quieter and more fuel-efficient. They cost more per watt, and are usually smaller. Conventional generators are cheaper for high wattage.

Related calculators & guides

AC Size Calculator (BTU)What BTU air conditioner you need for a room.Tankless Water Heater CalculatorGPM, BTU and kW needed for a tankless water heater.Space Heater Size CalculatorWatts of heater for a room, plus running cost.What Size Generator Do I Need? Wattage Chart for Your HomeGuide

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.