What Size Generator Do I Need for a Power Outage? A Wattage Guide

Key facts

• The formula: total the running watts of everything you want on at the same time, then add the single largest starting surge. That sum is the minimum starting (maximum) rating your generator needs; the running total is the minimum rated (continuous) output (EWEB, Generac).
• Motors surge. Refrigerators, sump pumps, well pumps, furnace fans and air conditioners draw two to three times their running watts for about two to three seconds when they start (Generac).
• Typical figures: refrigerator/freezer about 700 W running, 2,200 W extra to start; half-horsepower sump pump 1,050 W / 2,200 W; furnace fan 700 W / 1,400 W; 12,000 BTU window air conditioner 3,250 W / 3,950 W (EWEB). Your own appliances may differ, so read their labels.
• Rough tiers: a fridge, lights and small electronics come to about 3,500 W at startup by those figures; add a sump pump and a furnace fan and you need about 5,200 W; a window air conditioner pushes the total past 10,000 W; whole-house standby units run 10 to 22 kW (Generac).
• Never backfeed. Plugging a generator into a wall outlet can electrocute line workers and neighbors (CPSC). Use outdoor-rated cords or an electrician-installed transfer switch (NFPA).
• Bigger isn't better. A larger generator burns more fuel for the same job, so size for what you actually need and leave some headroom.

Step 1: Decide what has to run

Start with a list, not a wattage. Walk through the house during a normal evening and sort what you'd want on during an outage into three groups:

1. Must run: the refrigerator and freezer, a sump pump if your basement floods, a well pump if you're not on city water, the furnace blower or boiler controls in winter, medical equipment, and the modem and phone chargers that keep you in touch.
2. Nice to have: a few lights, the TV, a microwave or coffee maker (one at a time), a window air conditioner or space heater.
3. Everything: central air conditioning, the electric range and oven, the electric water heater and dryer, an EV charger. These are the heavy loads that push you from a portable generator to a standby unit.

Our grid-down preparedness checklist covers what you can replace with supplies instead of electricity, which keeps this list short. If someone in your home depends on powered medical equipment, ask the supplier now for its running and starting watts and whether it needs the cleaner power of an inverter generator.

Step 2: Find each appliance's running and starting watts

Every appliance has a label, usually on the back or bottom, and it's the best source for your own numbers. The U.S. Department of Energy's guidance on reading them is simple: the wattage is "usually stamped or printed on a nameplate on the rear or bottom of the appliance." If the label lists VA (volt-amps) instead, that figure approximates watts. If it lists only amps, multiply the amps by the voltage, so 3 amps at 120 volts is about 360 watts (DOE FEMP).

Two things to know about those labels:

• They show the maximum, not the average. DOE notes that nameplate ratings are generally maximum design limits "which could be two to four times the actual power consumed," and a refrigerator "uses the amount it says only when the compressor is on" (DOE FEMP, DOE EERE). For generator sizing that's the right direction to err: plan for the label figure, and you won't be surprised.
• Motors need a surge to start. Anything with a compressor or pump draws a burst of power to get moving. Generac defines running (rated) wattage as "the continuous power needed to run an appliance," and surge or starting wattage as "additional power required for starting, typically lasting 2-3 seconds" (Generac). That burst is "often two to three times the running figure" (Generac). Labels rarely list it, so manufacturer specs or a worksheet are your fallback.

The table below uses the figures in a public utility's sizing worksheet. Treat them as planning numbers, then check your own labels.

Appliance	Typical running watts	Additional starting watts
LED light bulb (15 W)	15	—
Phone charger	20	—
Laptop	60	—
TV	400	—
Refrigerator/freezer	700	2,200
Furnace fan blower, 1/3 HP	700	1,400
Sump pump, 1/3 HP	800	1,300
Sump pump, 1/2 HP	1,050	2,200
Water well pump, 1/3 HP	1,000	2,000
Microwave (1,000 W)	1,000	—
Coffee maker	1,000	—
Toaster oven	1,200	—
Space heater	1,500	—
Window air conditioner, 12,000 BTU	3,250	3,950
Electric water heater	4,500	—

Source: Eugene Water & Electric Board, Generator Sizing Considerations. A retailer worksheet gives nearly identical figures for the refrigerator, sump pump and window air conditioner. Central air conditioning is in a different league: Generac puts a 3-ton unit at about 3.5 to 5 kW running and up to 15 to 18 kW to start (Generac).

Step 3: Add the running watts, then the biggest single surge

Both utility and manufacturer worksheets use the same three moves:

1. Write down the running watts of everything you want on at the same time, and add them up.
2. Find the one item with the largest additional starting watts.
3. Add that single surge to the running total. The result is the minimum starting (maximum) watts your generator needs; the running total is the minimum rated watts.

The worksheets assume you start motors one at a time, which is how you should run a generator anyway: let the refrigerator start, then switch on the sump pump, rather than flipping everything on at once. If two motors can start on their own at the same moment, for example a sump pump and a fridge that both cycle automatically, size for both surges or accept that the generator may stall now and then.

Here are three worked examples using the table above.

Keep the food cold and the phones charged. Refrigerator (700) + six LED bulbs (90) + laptop (60) + two phone chargers (40) + TV (400) = 1,290 running watts. The largest surge is the refrigerator's 2,200. Total: 3,490 starting watts. By the worksheet figures a generator rated around 1,300 running and 3,500 starting watts covers it. The smallest inverter generators, in the 2,000-watt class, are marginal for a fridge on these numbers, although many newer refrigerators start on far less, so this is a case where your own label matters.

Essentials with a sump pump and furnace fan. Add a half-horsepower sump pump (1,050) and a furnace blower (700) to the list above: 3,040 running watts. The largest surge is still 2,200. Total: 5,240 starting watts. This is the classic mid-size portable generator job, roughly 3,500 to 4,000 running watts and 5,500 or more starting watts, with headroom.

Add a window air conditioner. Add a 12,000 BTU unit (3,250 running, 3,950 starting): 6,290 running watts, and the air conditioner now has the largest surge. Total: 10,240 starting watts. You're into the largest portables, typically with a 240-volt outlet and a transfer switch, and close to the point where a standby generator makes more sense.

Round up, then leave headroom. A generator working at its rated limit for hours is louder, thirstier and harder on the engine than one with spare capacity. Dealer and retailer sizing guides commonly suggest a margin of about 20 percent above your running total.

What size portable generator for the essentials?

For most households the answer lands in the 3,000 to 7,500 running-watt range, and the choice is between two designs:

• Inverter generators produce cleaner, steadier power that suits laptops, TVs and medical devices, and they're quieter and more compact. They cost more per watt and most top out in the low thousands of watts, though larger inverter models exist.
• Conventional open-frame generators give you more watts for the money and go well past 10,000 watts. They're louder and heavier, and their power is less refined, which some electronics tolerate better than others.

Check the outlets as well as the watts. Well pumps, central air conditioners, electric dryers and ranges usually run on 240 volts, so a generator that will feed them through a transfer switch needs a 120/240-volt outlet. A generator with only 120-volt outlets is a cord-and-appliance machine.

On a spec sheet, look for the rated (running) watts and the maximum (starting) watts separately, the fuel type (gasoline, propane or dual fuel), tank size and the run time per tank at a stated load, usually 25 or 50 percent. Many newer models add a carbon monoxide shutoff, a worthwhile feature that is not a substitute for keeping the generator outdoors.

What size generator runs a whole house?

The average U.S. home used 10,791 kWh of electricity in 2022, according to the U.S. Energy Information Administration, which works out to an average draw of about 1.2 kW around the clock. Peak demand is far higher, because the air conditioner, oven, dryer and water heater can all be on at once, and that peak is what a whole-house generator has to meet.

Generac's current sizing guidance puts the tiers roughly like this (Generac):

• 10 to 14 kW covers the essential circuits of a home around 1,550 square feet, with a load manager to prioritize an air conditioner or well pump.
• 18 kW for the same home if it has an electric heat pump or high-draw medical equipment.
• 18 to 22 kW for a home around 2,000 square feet, which Generac calls the sweet spot for mid-size homes with central air.

Central air is the deciding load. A 3-ton unit's 15 to 18 kW starting surge would otherwise force a much larger generator, so installers pair a 14 or 18 kW unit with a load manager that holds other circuits while the compressor starts, or a soft-start kit on the air conditioner itself. Generac says a professional site survey that measures your actual panel loads "is the only way to ensure full accuracy," and that professional installation is a warranty requirement, not an option. Most jurisdictions also require a permit and inspection.

Don't forget the connection

How you connect the generator is a safety question before it's a sizing question.

Cords. The Consumer Product Safety Commission says to connect appliances with "heavy-duty extension cords that are specifically designed for outdoor use," to "make sure the wattage rating for each cord exceeds the total wattage of all appliances connected to it," and to use cords long enough that the generator can sit far from doors, windows and vents. Check each cord for cuts and a missing ground prong, and protect it where it passes through a window or door (CPSC). NFPA adds that appliances should be plugged directly into the generator or into a heavy-duty outdoor-rated cord, never daisy-chained (NFPA).

Transfer switch. If you want the generator to power circuits in the house, NFPA's guidance is to "have a qualified electrician install a properly rated transfer switch in accordance with the National Electrical Code" and local codes. A transfer switch keeps generator power and utility power from ever meeting. Without one, power can backfeed along the lines and electrocute anyone touching them, "including utility lineworkers making repairs," as the Electrical Safety Foundation International puts it (ESFI). The same guidance recommends a ground-fault circuit interrupter.

Never backfeed. CPSC's words: "NEVER try to power the house wiring by plugging the generator into a wall outlet, a practice known as 'backfeeding.' This is extremely dangerous and presents an electrocution risk to utility workers and neighbors served by the same utility transformer" (CPSC).

Placement and fuel. Run the generator outdoors only, at least 20 feet from doors, windows and vents, with the exhaust pointed away from the house, and never in an attached garage even with the door open (NFPA, CDC). Store fuel outside living areas in labeled containers, and turn the generator off and let it cool before refueling. Our generator and carbon monoxide safety guide covers the CO risk in detail; read it before the first storm, not during it.

How long will you need to run it?

Sizing is about watts; planning is about hours. Most outages are short, but the long ones are what generators are for. Our report on average power outage duration by state shows how long a typical interruption lasts where you live, and your state outage page lists the biggest events on record since 2014. A generator that runs the fridge for 8 hours on a tank is a different purchase from one that has to carry a week after a hurricane, so check the spec sheet's run time at half load and multiply by the fuel you can safely store.

Remember too that a refrigerator doesn't need power every minute. A closed fridge keeps food safe for about 4 hours and a full freezer for about 48, so running the generator in shifts, a few hours on and a few off, stretches fuel without wasting food. Our guide on how long food lasts in a power outage has the details.

Do you need a generator at all?

Not always. Battery power stations run phones, lights, a modem and a CPAP machine without fuel or exhaust, and a well-stocked kit covers water, food and light for days. For many households the honest answer to "what size generator do I need" is a mid-size portable for the fridge and a pump, or none at all. Our generator safety guide walks through the alternatives.

The bottom line

List what must run, read each label, add the running watts, then add the single biggest starting surge. By a public utility's worksheet figures, a refrigerator with lights and electronics needs about 3,500 starting watts, the same plus a sump pump and furnace fan about 5,200, and a window air conditioner pushes you past 10,000. Whole-house standby units run 10 to 22 kW and deserve a professional load analysis. Whatever size you choose, use outdoor-rated cords sized for the load or an electrician-installed transfer switch, never a wall outlet, and keep the generator 20 feet from the house.

PowerOutageTracker.com. "What Size Generator Do I Need for a Power Outage? A Wattage Guide." Accessed November 5, 2026. https://poweroutagetracker.com/guides/what-size-generator-do-i-need