What Size Generator Do You Need? Running vs Starting Watts

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Most households that want to keep the refrigerator, furnace blower, sump pump, lights and internet running through an outage land somewhere around 5,000 to 7,500 running watts on a portable generator, while running central air conditioning or an electric range usually pushes the answer into whole-house standby territory of roughly 14 to 26 kilowatts. The exact size depends on which circuits truly matter, how much extra power motors draw for a second or two when they start, and, on the Colorado Front Range, how much output a combustion engine loses in thin air. This guide walks through the rule-of-thumb sizing method step by step, works a full example, explains the safety rules that matter more than the wattage number, and covers how a generator should connect to a house. It summarizes general guidance current as of 2026; the generator’s own manual, the appliance nameplates and a licensed electrician take priority for any specific installation.

Part of our Home Electrical guide — start there for the full picture →

The Short Answer: Size for Running Watts Plus the Biggest Surge

Every generator carries two power ratings. Running watts (also called rated or continuous watts) is what the machine can supply hour after hour. Starting watts (also called surge or peak watts) is the short burst it can deliver for a few seconds while an electric motor gets up to speed. Refrigerators, freezers, sump pumps, well pumps, furnace blowers and air conditioner compressors all pull far more power at startup than once they are running, commonly two to three times their running draw for a brief moment.

The rule-of-thumb sizing method that most generator manufacturers publish works like this:

1. List everything that must run at the same time.
2. Add up the running watts of all of it.
3. Find the single appliance with the largest extra startup surge (starting watts minus running watts) and add only that one surge, because motors rarely start at the exact same instant.
4. Add a margin of roughly 20 to 25 percent so the generator is not running flat out.
5. On the Front Range, adjust upward for altitude derating if the generator burns fuel.

That final number is the minimum running-watt rating to shop for. Treat it as a planning estimate rather than an engineering calculation. Actual draw varies by model, age and condition, so the label on each appliance is the authoritative figure.

These ranges are rules of thumb based on common appliance draws. A home with an electric furnace, heat pump or large well pump can sit well above them.

Typical Appliance Wattages for Planning

The table below lists typical running and starting watts for common household loads. These are rough planning numbers that circulate widely in manufacturer sizing worksheets; they are not measurements of any particular model. Check the data plate on each appliance, which usually lists volts and amps (multiply them to estimate watts) or watts directly. For motor loads, the plate may also show locked-rotor amps, which hints at the startup surge.

Resistive loads, such as heaters, toasters, coffee makers and water heater elements, draw the same power at startup as while running. Motors and compressors are the loads that create surges. That distinction is why a generator can handle a 1,500-watt space heater comfortably but stall when a refrigerator and a sump pump try to start together.

Best Home Generators: our top 3 picks

Product	Best for	Key features	Price	Buy
Generac 12,500W Tri-Fuel	Best overall home backup	9,500 running watts on gas COsense CO shutoff Gas, propane or natural gas	—	Buy now →Check current price
Westinghouse WGen11500TFc	Automatic switch-ready portable	11,500 running watts on gas Pairs with WATS200 automatic switch CO sensor shutdown	$1,499.00as of Nov 13, 2026 12:02 am GMT+0000	Buy now →Check current price
DuroMax XP13000HX	5-year warranty, four GFCI	13,000 watts, gas or propane Four GFCI outlets plus 50A 5-year manufacturer warranty	$1,399.00as of Nov 13, 2026 12:02 am GMT+0000	Buy now →Check current price

Prices are Amazon’s listed prices as of the time shown and change often; check Amazon for today’s price. As an Amazon Associate we earn from qualifying purchases.

See every pick compared: Best Home Generators →

A Worked Sizing Example for a Front Range Home

Picture a 2,000-square-foot house in Arvada or Castle Rock with a gas furnace, a gas water heater, a refrigerator, a basement chest freezer and a sump pump in the window well area. The owners want to ride out a winter outage without frozen pipes, spoiled food or a flooded basement. They do not need air conditioning, the range or the dryer during an outage.

Step 1 and Step 2: List the loads and add running watts

• Refrigerator: 700 W
• Chest freezer: 500 W
• Furnace blower (1/2 hp): 800 W
• Sump pump (1/2 hp): 1,000 W
• Microwave, used occasionally: 1,000 W
• LED lighting on key circuits: 150 W
• Modem, router, TV and chargers: 250 W

Running total: 4,400 watts.

Step 3: Add the single largest surge

Suppose the sump pump label and the manufacturer’s spec sheet suggest it can briefly pull about 2,200 watts at startup. Its extra surge is 2,200 minus 1,000, or 1,200 watts. The furnace blower might surge to roughly 2,000 watts, an extra 1,200 as well. The refrigerator might surge to about 1,800, an extra 1,100. The largest extra surge is about 1,200 watts, so the peak demand estimate becomes 4,400 plus 1,200, or 5,600 watts.

Step 4: Add headroom

Running a generator continuously at its full rating shortens its life and leaves no room for a forgotten load. A 20 to 25 percent margin is a common rule of thumb. At 25 percent, 5,600 watts becomes about 7,000 watts.

Step 5: Adjust for altitude

Air-breathing engines produce less power as elevation rises because each intake stroke pulls in less oxygen. Many engine and generator manufacturers publish a derating figure in the range of roughly 3 to 4 percent of output per 1,000 feet above sea level, sometimes starting above a threshold such as 1,000 or 3,000 feet, and some sell high-altitude carburetor jets. The owner’s manual is the source to use; the numbers here are only an illustration.

At about 5,500 feet, a 3.5 percent per 1,000 feet rule of thumb implies a loss of roughly 19 percent. To deliver 7,000 usable watts, the sea-level rating needs to be about 7,000 divided by 0.81, or roughly 8,600 running watts. In Evergreen, Conifer or Woodland Park, at 7,000 to 8,500 feet, the same calculation pushes the target even higher.

In practice, this household would shop for a portable generator rated around 8,500 to 9,500 running watts, or it could decide to skip the microwave during outages and stagger the freezer and fridge, which might bring a 7,500-watt unit into range. That kind of load discipline is often cheaper than buying the next size up.

Portable, Inverter or Whole-House Standby

Once the wattage target is clear, the next decision is the type of generator. Each has a natural size range and a different installation burden.

Conventional portable generators

Open-frame portables commonly range from about 3,000 to 12,000 running watts. They deliver the most watts per dollar, but they are loud and their output can be less stable than an inverter’s. They need to be wheeled outside, fueled and connected each time the power fails. Typical purchase prices run from several hundred dollars for small units to a couple of thousand for large dual-fuel models, depending on size, electric start and fuel options.

Inverter generators

Inverter models electronically condition their output, run quieter and throttle down at light loads to save fuel. They are popular for sensitive electronics and for RV and camping use. Most portable inverters fall in the 1,000 to 4,500 running-watt range, with some larger models now reaching 7,000 or more. Matching two smaller inverters in parallel is another way to reach a higher total. They usually cost more per watt than a conventional unit.

Whole-house standby generators

Permanently installed standby units sit on a pad outside, run on natural gas or propane, and start automatically through an automatic transfer switch when utility power drops. Residential models commonly range from about 10 to 26 kilowatts, and larger homes or all-electric homes may need more. Many installations use load-management modules that shed the air conditioner or water heater for a moment when another big motor starts, which lets a smaller standby unit serve a larger house. Typical installed costs vary widely, often ranging from several thousand dollars for a small air-cooled unit to well over ten thousand for a large system, driven by generator size, transfer switch type, gas line or propane tank work, the concrete pad, permitting and electrical upgrades.

Gas supply is a frequent surprise on the Front Range. A large standby generator can demand more natural gas than the existing meter and piping were sized for, and the utility may need to upgrade the meter. Altitude matters here too: standby engines derate in thin air just as portables do, so the nameplate kilowatt figure needs the same adjustment.

For buyers who already know their wattage target, our roundups of the best home generators and the best whole-house generators compare current models across those size bands.

Carbon Monoxide: The Safety Rule That Outranks Sizing

A correctly sized generator in the wrong location can kill a household. The CDC describes carbon monoxide as “an odorless, colorless gas that kills without warning” and notes that portable generators are among the household items that produce it. CDC guidance is direct: never use a generator inside a home or garage, even with doors and windows open, and only run it outside, more than 20 feet from windows, doors and vents.

The U.S. Consumer Product Safety Commission says one portable generator can produce as much carbon monoxide as hundreds of cars, and that more than 100 of the 200-plus annual non-fire CO deaths associated with consumer products are linked to portable generators. Its 2026 storm-season warning repeats the 20-foot rule, asks owners to point the exhaust away from the house, and recommends choosing a model with a carbon monoxide shut-off feature. CPSC notes that models certified to the PGMA G300-2023 and UL 2201-2023 standards include CO safety technologies, and that UL 2201 certified models also have reduced CO emissions.

Practical placement points for Front Range homes:

• Not in the garage. Attached garages share walls, doors and sometimes ductwork with the living space.
• Not under a deck or in a window well. Exhaust pools in enclosed spaces and drifts in through gaps.
• Mind the wind. Front Range chinook and downslope winds can push exhaust back toward the house; aim the exhaust downwind and away from openings.
• Snow and rain. Use a purpose-made generator cover or tent that keeps the unit dry without enclosing it. Never move a generator indoors because of weather.
• Working CO alarms. CDC recommends a battery-powered or battery-backup CO detector in the home whenever a generator is running. Our guide on how many carbon monoxide detectors a home needs covers placement on each level and near sleeping areas.

Know the symptoms the CDC lists: headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, often described as flu-like. If anyone feels this way while a generator is running, get everyone outside to fresh air and call 911.

Connecting a Generator to the House Safely

Extension cords from a portable generator to individual appliances are the simplest setup and work for a refrigerator and a few lamps. They cannot power hardwired loads such as a furnace blower or a well pump. For those, the generator has to feed the home’s electrical panel, and that connection must keep generator power from flowing back onto the utility lines.

Transfer switches and interlock kits

A manual transfer switch is a small subpanel wired next to the main panel. A handful of chosen circuits, such as the fridge, furnace and sump pump, are moved into it, and a switch selects utility or generator power for each one. An interlock kit is a mechanical plate on the main panel that physically prevents the main breaker and a dedicated generator breaker from being on at the same time. Both connect through a weatherproof inlet box outside, where the generator cord plugs in. An automatic transfer switch is the standard pairing for standby units.

Whichever method is used, it should be installed by a licensed electrician under a permit from the local building department. Colorado municipalities generally require electrical permits for this work, and the inspection catches mismatched breaker sizes and inlet ratings.

Never backfeed through an outlet

A “double male” cord plugged into a dryer or wall outlet is dangerous. It can energize the utility line and endanger line workers, it bypasses breakers, and the exposed prongs are live. If a home being purchased shows signs of an improvised generator hookup, flag it for an electrician.

Fuel, Runtime and Sizing Mistakes to Avoid

The wattage target also affects how much fuel a household must store. Larger generators burn more fuel per hour, although most run more efficiently at about half load than at full load. Gasoline goes stale in storage without a stabilizer; propane stores indefinitely in a tank and is common for dual-fuel portables; natural gas standby units draw from the utility line and do not need refueling unless gas service is interrupted.

Common sizing mistakes include:

• Adding up starting watts for everything. That leads to an oversized, overpriced unit. Add running watts, then only the single biggest surge.
• Forgetting the furnace. In a Colorado winter outage, the gas furnace still needs electricity for its blower, inducer motor and controls.
• Ignoring altitude. A generator that is barely adequate at sea level will come up short in Douglas or El Paso County.
• Buying for the rare case. Sizing to run central AC during a summer outage can double the cost compared with sizing for the essentials.
• Overlooking power quality. Furnace control boards and modern appliances can be sensitive to unstable voltage. Many households choose an inverter or a generator with a low total harmonic distortion rating for these circuits.

When to Bring In an Electrician or Inspector

A homeowner can do the load list and the math, but an electrician should confirm panel capacity, choose the transfer equipment and handle the permit. For a standby unit, the installer should also check gas supply, clearances from windows and vents specified by the manufacturer, and any HOA or setback rules. Our guide to electrical work and the pros who handle it explains what to ask before signing a quote.

Buyers looking at a home with an existing generator should have the system included in the inspection scope. A general home inspector will typically note the presence of a standby unit and visible concerns, such as a missing transfer switch, a unit installed too close to a window, or an improvised backfeed setup, but load testing usually falls to a generator technician. For a broader view of how inspection scopes are set, see our overview of the home inspection hiring process.

More Questions About Generator Sizing

What size generator is needed to run central air conditioning?

Running a typical 3- to 4-ton central air conditioner usually takes a generator of roughly 10 to 20 kW, depending on the unit’s running load, its starting surge and what else must run at the same time. The compressor’s locked-rotor or starting current is the deciding number.

A soft-start kit on the condenser can cut the starting surge substantially, which may let a smaller generator handle the AC. Most portable generators struggle with central AC. An electrician should confirm sizing using the nameplate data on the outdoor unit.

What size generator is needed for a well pump?

A typical 1/2 to 1 horsepower well pump needs roughly 1,000 to 2,000 running watts and 2,000 to 4,000 or more starting watts, so a generator of at least about 5,000 to 7,500 watts is common once other loads are included. Many well pumps run on 240 volts, which requires a generator with a 240-volt outlet.

Check the pump’s nameplate or the control box for horsepower and voltage. Deep well submersible pumps often have higher starting loads than expected, so leave a margin when sizing.

What size generator is needed for a sump pump?

A typical 1/3 to 1/2 horsepower sump pump uses roughly 800 to 1,050 running watts and can draw 1,300 to 2,200 watts when starting, so even a small 2,000- to 3,500-watt generator can usually handle it alone. Running other loads at the same time requires more capacity.

For homes that depend on a sump pump during storms, a generator sized for the pump, refrigerator, lights and furnace blower is a common plan. A battery backup sump pump adds protection when the generator is not running.

How much does a whole-house standby generator cost installed?

Industry estimates put a whole-house standby generator typically at $10,000 to $20,000 installed, with larger units, long gas line runs, permits and electrical upgrades pushing costs higher. Smaller standby units that power essential circuits can cost less.

Installation includes the generator, concrete pad, automatic transfer switch, gas line work, electrical work and permits. Annual maintenance and periodic service are additional costs. Get quotes from licensed electricians and installers who perform a load calculation instead of guessing.

Can a generator be too big for a house?

Yes. A generator can be oversized for a house, which raises purchase cost and fuel use without adding benefit. Generators run less efficiently at very light loads, and diesel units can suffer problems when run lightly for long periods.

A generator that is too small is the more common problem, since it cannot start large motors and may shut down under load. The goal is a unit sized for running loads plus the largest starting surge and a reasonable margin, usually 10 to 25 percent.

References

• Carbon Monoxide Poisoning Basics — Centers for Disease Control and Prevention
• Carbon Monoxide Information Center — U.S. Consumer Product Safety Commission
• CPSC Warns of Generator, Carbon Monoxide and Fire Hazards Ahead of Hurricane Season — U.S. Consumer Product Safety Commission
• Home Heating Equipment and Carbon Monoxide — U.S. Consumer Product Safety Commission

Buying a Front Range home with a standby generator or an existing transfer switch? Reach out to our team to connect with a local inspector who can look over the installation before closing.