What size generator do I need for a house?

For essential circuits (refrigerator, lights, sump pump, phone chargers), a 3,500–5,000W portable generator is sufficient. To run central air conditioning as well, you need 12,000–20,000W. A whole-home automatic standby generator typically requires 20,000–22,000W.

What is the difference between running watts and starting watts?

Running watts is the continuous power a generator produces. Starting watts (surge watts) is the brief peak power available for 1–2 seconds when a motor starts up — typically 2–3× the running wattage. Your total running load must stay below the generator's running rating; each motor's starting surge must not exceed the starting watt rating.

Can I run a generator inside a garage?

No. Generator exhaust contains carbon monoxide (CO), a colorless, odorless gas that kills within minutes at high concentrations. Run generators at least 20 feet from any doors, windows, or vents — and never inside a garage, even with the door open. Install battery-powered CO detectors on every level of your home.

Do I need a transfer switch to use a generator?

For directly plugging appliances into extension cords: no transfer switch needed. For powering hardwired appliances (furnace, well pump, home circuits): yes, a transfer switch or interlock kit is required by the NEC and essential for safety. Never backfeed a generator into your home's main panel without a proper transfer switch.

How long can a portable generator run continuously?

Most portable generators run 8–12 hours per tank of gasoline. Let the generator cool for 30 minutes before refueling, and never add fuel to a running or hot generator. Natural gas and propane standby generators can run continuously as long as fuel is available — typically the limiting factor is manufacturer-recommended maintenance intervals (usually 24 hours of run time for an oil check).

Generator Sizing Guide: How Many Watts Do You Actually Need?

Calculate generator wattage for home backup or job site use. Covers running vs. starting watts, essential circuit loads, and the difference between portable and whole-home standby generators.

Related Calculators

Electrical Load Calculator
Estimate total electrical demand, apparent power and current from connected load, demand factor, voltage, phase factor and power factor.
Generator Sizing Calculator
Estimate generator running and starting capacity from total running watts, the largest additional surge load and an editable reserve margin.
Voltage Drop Calculator
Estimate conductor voltage drop and percentage voltage drop from current, one-way length, conductor resistance and circuit voltage.
Breaker Size Calculator
Estimate minimum circuit ampacity and breaker requirement from continuous and noncontinuous load currents using editable multipliers.
Wire Size Calculator
Estimate the minimum conductor circular-mil area from current, distance, allowable voltage drop and conductor resistivity.

Running Watts vs. Starting Watts

Generator capacity is described in two numbers:

Running watts (rated watts): The continuous power the generator can supply. This is the primary spec — it determines whether the generator can run your loads simultaneously.

Starting watts (surge watts): The peak power available for 1–2 seconds when a motor starts. Electric motors draw 2–3× their running wattage during startup.

The sizing rule: Your total running load must not exceed the generator's running watt rating. The highest single-motor starting surge must not exceed the generator's starting watt rating.

Example: Running a refrigerator (200W run / 800W start) and a 1/3 HP sump pump (300W run / 900W start) simultaneously: - Running load: 200 + 300 = 500W - Starting surge: 200 (running fridge) + 900 (starting pump) = 1,100W - Required generator: 1,000W+ running, 1,500W+ starting → a 2,000W generator handles this comfortably

Essential Load Estimates

Appliance	Running Watts	Starting Watts
Refrigerator (modern)	150–200W	600–800W
Chest freezer	100–200W	400–800W
Sump pump (1/3 HP)	300W	900W
Sump pump (1/2 HP)	500W	1,500W
Window AC (10,000 BTU)	1,200W	3,600W
Central AC (2.5 ton)	3,500W	10,500W
Well pump (1/2 HP)	750W	2,250W
Electric water heater	4,500W	4,500W
Microwave (900W rated)	1,000W	1,000W
Lighting (LED, whole home)	200–500W	200–500W
Phone/laptop chargers	100–200W	100–200W

Generator Size Recommendations by Scenario

Essential circuit coverage (refrigerator, lights, phone chargers, sump pump): → 3,500–5,000W portable generator

Essential circuits + window AC or window unit heating: → 5,000–7,500W portable generator

Whole-home backup without electric heat: → 12,000–16,000W standby generator

Whole-home including central AC: → 20,000–22,000W standby generator

Portable vs. Whole-Home Standby Generators

Feature	Portable	Whole-Home Standby
Cost	$500–$3,000	$7,000–$15,000 installed
Output	1,000–10,000W typically	7,000–25,000W
Fuel	Gasoline, propane, dual-fuel	Natural gas or propane
Auto-start	No (manual)	Yes (automatic, within seconds)
Runtime	8–12 hours per tank	Continuous (gas line)
Maintenance	Store dry fuel; run monthly	Annual professional service

For homes with medical equipment (oxygen concentrators, CPAP machines), automatic standby generators are the safer choice — they power on within seconds of an outage, while portable generators require manual startup and extension cord or transfer switch setup.

Transfer Switches: The Safe Way to Connect

Running a portable generator through extension cords is safe for direct-connected appliances but can't power hardwired systems (central heat, well pump, sump pump) or whole-home lighting circuits.

Manual transfer switch ($200–$500): A sub-panel with selected circuits that can be switched from utility to generator power. Requires an electrician to install.

Interlock kit ($50–$200): A mechanical lock that ensures your main breaker and generator breaker cannot both be on simultaneously. Cheaper than a transfer switch but less flexible.

Never backfeed power into the utility grid through your main panel without a proper transfer switch — this is a felony in many states and has killed utility workers restoring power.

Fuel Considerations

Gasoline: Most common for portable generators. Degrades in 30 days without stabilizer; 1–2 year shelf life with stabilizer. Readily available.
Propane: Longer shelf life than gas, cleaner burning, but requires a propane tank. Many dual-fuel generators support both gasoline and propane.
Natural gas: Continuous supply for standby generators. Pressure issues during widespread outages can affect supply.
Diesel: Most efficient at high continuous loads; used for commercial standby systems.

Add 20% Safety Margin

After calculating your load, add 20% to the running watts before selecting generator size. This accounts for load variation, generator derating in hot weather (generators produce less power as ambient temperature rises), and future added loads.

Related Guides

Frequently Asked Questions

What size generator do I need for a house?: For essential circuits (refrigerator, lights, sump pump, phone chargers), a 3,500–5,000W portable generator is sufficient. To run central air conditioning as well, you need 12,000–20,000W. A whole-home automatic standby generator typically requires 20,000–22,000W.
What is the difference between running watts and starting watts?: Running watts is the continuous power a generator produces. Starting watts (surge watts) is the brief peak power available for 1–2 seconds when a motor starts up — typically 2–3× the running wattage. Your total running load must stay below the generator's running rating; each motor's starting surge must not exceed the starting watt rating.
Can I run a generator inside a garage?: No. Generator exhaust contains carbon monoxide (CO), a colorless, odorless gas that kills within minutes at high concentrations. Run generators at least 20 feet from any doors, windows, or vents — and never inside a garage, even with the door open. Install battery-powered CO detectors on every level of your home.
Do I need a transfer switch to use a generator?: For directly plugging appliances into extension cords: no transfer switch needed. For powering hardwired appliances (furnace, well pump, home circuits): yes, a transfer switch or interlock kit is required by the NEC and essential for safety. Never backfeed a generator into your home's main panel without a proper transfer switch.
How long can a portable generator run continuously?: Most portable generators run 8–12 hours per tank of gasoline. Let the generator cool for 30 minutes before refueling, and never add fuel to a running or hot generator. Natural gas and propane standby generators can run continuously as long as fuel is available — typically the limiting factor is manufacturer-recommended maintenance intervals (usually 24 hours of run time for an oil check).

Last updated 7/28/2026