Generator Size Calculator

Add up running and starting watts for a refrigerator, sump pump, furnace, well pump, air conditioner and lights to find the generator size in kW you need.

By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size a generator, add the running watts of everything you want on at once, add the single largest motor starting surge, then multiply by 1.2 and round up to the next 500 W. A refrigerator, sump pump, furnace blower, microwave and 600 W of lights draw 3,900 running watts and 5,450 starting watts, so a 7 kW (7,000 W) portable generator.

Your measurements

A length built from stock pieces: total length divided by piece length, rounded up, with the last piece cut.

Refrigerators or freezers (count)About 700 W running, 2,200 W starting each.Sump pumps, 1/3 HP (count)About 800 W running, 1,300 W starting.Furnace blowers (count)About 800 W running, 2,350 W starting for a 1/2 HP PSC blower; a variable-speed ECM blower runs 300 to 500 W. A gas furnace still needs power for the blower and controls.Well pumps, 1/2 HP (count)About 1,000 W running, 2,100 W starting for the common 1/2 HP submersible. A 1 HP deep well pump is 2,000 running and 4,000 starting, and both are 240 V, so the inlet must be 240 V.Window air conditioners, 10,000 BTU (count)About 1,200 W running, 3,600 W starting each.Central air conditioner (tons)About 3,500 W running and 8,000 W starting per 3 tons. The condenser nameplate is exact: RLA x 240 for running, LRA x 240 for the surge. A soft-start kit cuts LRA by 60 to 70 percent.Microwaves (count)About 1,000 W, no motor surge.Lights and electronics (watts)Add up LED lights, TV, router, modem, phone chargers and a laptop; 300 to 1,000 W is typical.Electric water heaters (count)4,500 W; the usual first load to leave off a portable generator.

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Generator size7 kWAll results

Results

Generator size: 7 kW; 7,000 W: starting watts x 1.2 margin, rounded up to the next 500 W
Running watts: 3,900 W; Everything on at the same time, at steady state
Starting watts: 5,450 W; Running watts plus the 1,550 W starting surge of the furnace blower, the largest motor
Generator type: Portable; Portable generators run up to about 9 kW; a standby unit is optional at this size
Planning estimate only: Licensed electrician; Connect through a transfer switch or listed interlock (NEC 702.5) installed by a licensed electrician; never backfeed a panel through a dryer or range outlet

Generator Size formula

Running watts = Sum of (count x running watts) for each appliance + lights and electronics
Starting watts = Running watts + the largest single (starting - running) surge
Generator watts = Starting watts x 1.2, rounded up to the next 500 W
Generator kW = Generator watts / 1,000  (portable up to about 9 kW, standby 10 to 26 kW)

Worked example

A rural home wants a refrigerator, a 1/2 HP well pump, a 10,000 BTU window air conditioner, a microwave, 400 W of lights and an electric water heater. Running watts: 700 + 1,000 + 1,200 + 1,000 + 400 + 4,500 = 8,800 W. Starting surges above running: refrigerator 1,500 W, well pump 1,100 W, window air conditioner 2,400 W; the largest is 2,400 W.

Starting watts = 8,800 + 2,400 = 11,200 W. Times 1.2 = 13,440 W, rounded up to 13,500 W, so a 13.5 kW unit, which is standby territory. Leaving the water heater off drops the need to 4,300 running and 6,700 starting watts, or 8,040 W rounded up to 8,500 W, within reach of a large portable.

Quick reference

Generator size by loads powered (600 W of lights and electronics in every row)
Loads on at once	Running W	Starting W	Generator
Refrigerator, lights	1,300	2,800	3.5 kW
Refrigerator, sump pump, furnace blower, lights	2,900	4,450	5.5 kW
Above plus microwave (defaults)	3,900	5,450	7 kW
Defaults plus 1/2 HP well pump	4,900	6,450	8 kW
Defaults plus 10,000 BTU window AC	5,100	7,500	9 kW
Defaults plus 3 ton central AC	7,400	11,900	14.5 kW
Defaults plus 3 ton central AC and electric water heater	11,900	16,400	20 kW

How to add up generator watts

List what must run during an outage, not everything in the house. Refrigerator, sump pump, furnace blower and some lights are the essentials for most homes; a well pump if you are not on city water. Add the running watts of all of them, then add the starting surge of the single largest motor, since motors start one at a time and the generator only has to absorb one surge on top of the steady load. Multiply by 1.2 so the generator is not running at its limit, which shortens its life and lets voltage sag.

Nameplates give the most accurate numbers. Where the label lists amps, multiply by volts (120 or 240) to get watts; on an air conditioner, RLA is running and LRA is the surge. Resistance loads such as a microwave, toaster or electric water heater have no surge but pull a lot of steady power, and the 4,500 W water heater is usually the first thing to leave off when sizing a portable.

Portable or standby, and connecting it safely

Portable generators top out around 9 kW and cover essentials plus a window air conditioner. Anything with central air, an electric water heater or a well pump on top of the basics pushes into standby territory: an air-cooled unit of 10 to 26 kW on a pad outside, fed by natural gas or propane, that starts itself when the power fails. Sizing every load at once is rarely worth it: the step from 14 to 22 kW is typically $1,000 to $1,500 more installed, while a load-shed module at $100 to $200 per circuit drops the AC while the water heater runs. On propane a 22 kW unit burns about 2.2 gal an hour at half load, so a 500 gal tank (400 usable) covers about a week.

NEC 702.5 requires a transfer switch or a listed interlock so the generator can never feed the utility lines. For a portable, the interlock kit, a 30 or 50 A inlet on the outside wall and the 10-3 or 6-3 feed is typically $500 to $1,200 installed. A standby uses an automatic transfer switch and sits per its listing, typically 18 in from the wall and 5 ft from any window, door or vent, with a gas permit on top of the electrical one. Never plug a generator into a wall outlet or dryer receptacle: it backfeeds the grid, can kill a line worker, and destroys the generator when the power returns.

What this calculator leaves out

The result is a generator rating. Not included: the inlet box and 25 ft 50 A cord (typically $100 to $250), the interlock or transfer switch, the pad, the gas line and meter upgrade or propane tank, permits, and CO alarms on every level (IRC R315). Also the soft-start kit that lets a portable start central air, oil changes every 100 run hours, and the derate of 3 to 4 percent per 1,000 ft of elevation.

Code limits that apply

NEC 240.4(D)checked
10 AWG copper capped at a 30 A breaker, so a 30 A inlet on 10-3 passes 7,200 W at 240 V
If you miss it: A 9 kW portable on a 30 A inlet trips the inlet breaker when the well pump starts; the inlet is redone as 50 A on 6-3 with a 50 A cord
NEC 702.4(B)
Manual transfer: user picks loads; automatic transfer: full house load or a load-shed module
If you miss it: A 14 kW standby on an automatic switch with no load-shed module is refused at final; the 220.82 load calc must fit or a module is added
NEC 702.5
Transfer switch or listed interlock so the generator can never feed the utility lines
If you miss it: A suicide cord into a dryer receptacle backfeeds the grid and can kill a line worker; the connection is refused and the inlet added
NEC 110.3(B)
Interlock kit listed for the exact panel make and model, installed per its instructions
If you miss it: A generic interlock plate on the panel is written up at final; the kit listed for that panel model is bought and fitted
NEC 408.36(D)
The backfed generator breaker is secured with a hold-down clip or fastener
If you miss it: A snap-in generator breaker with no hold-down is flagged; the breaker can pop off the bus live, so the clip is added before sign-off
NEC 250.30
One neutral-ground bond: a bonded-neutral generator needs a switched-neutral transfer switch
If you miss it: A bonded-neutral portable on an interlock trips its own GFCI outlets the moment a load runs; the bond is floated or the switch changed

Provision	Limit	If you miss it
NEC 240.4(D)checked	10 AWG copper capped at a 30 A breaker, so a 30 A inlet on 10-3 passes 7,200 W at 240 V	A 9 kW portable on a 30 A inlet trips the inlet breaker when the well pump starts; the inlet is redone as 50 A on 6-3 with a 50 A cord
NEC 702.4(B)	Manual transfer: user picks loads; automatic transfer: full house load or a load-shed module	A 14 kW standby on an automatic switch with no load-shed module is refused at final; the 220.82 load calc must fit or a module is added
NEC 702.5	Transfer switch or listed interlock so the generator can never feed the utility lines	A suicide cord into a dryer receptacle backfeeds the grid and can kill a line worker; the connection is refused and the inlet added
NEC 110.3(B)	Interlock kit listed for the exact panel make and model, installed per its instructions	A generic interlock plate on the panel is written up at final; the kit listed for that panel model is bought and fitted
NEC 408.36(D)	The backfed generator breaker is secured with a hold-down clip or fastener	A snap-in generator breaker with no hold-down is flagged; the breaker can pop off the bus live, so the clip is added before sign-off
NEC 250.30	One neutral-ground bond: a bonded-neutral generator needs a switched-neutral transfer switch	A bonded-neutral portable on an interlock trips its own GFCI outlets the moment a load runs; the bond is floated or the switch changed

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

A 30 A inlet on 10 AWG passes 7,200 W, so a 9 kW portable needs a 50 A inlet, 6 AWG cord and breaker.
Interlock kits must be listed for the exact panel model (NEC 110.3(B)); a generic plate fails inspection, and the generator breaker needs a hold-down (408.36(D)).
Run a portable 20 ft from any window, door or vent, never in a garage with the door open; put CO alarms on every level.
Standby nameplates are propane ratings; on natural gas the same unit makes about 10 percent less, so 22 kW LP is 19.5 kW NG.
A 22 kW standby burns about 280 cu ft of natural gas an hour at full load; the meter and gas pipe usually need upsizing.
A bonded-neutral portable on an interlock trips its own GFCI receptacles; it needs a floating neutral or a switched-neutral transfer switch (NEC 250.30).

Key facts

Motors draw two to three times their running watts for a second or two at startup, which is why a 700 W refrigerator needs about 2,200 W to start and only the largest motor is counted in the surge.
Generators carry two ratings: running (rated) watts they can supply continuously and starting (peak) watts for a few seconds; size loads against both.
NEC Article 702 requires a transfer switch or listed interlock for an optional standby system; backfeeding through a dryer outlet is illegal and can electrocute utility line workers.
The CPSC advises running portable generators outdoors at least 20 ft from the house with the exhaust pointed away, because carbon monoxide from a generator can kill within minutes indoors.
Air-cooled home standby generators come in sizes from 10 to 26 kW; a 3 ton central air conditioner alone needs about 3,500 running and 8,000 starting watts.

Frequently asked questions

What size generator do I need to run a house?

The essentials, a refrigerator, sump pump, furnace blower, microwave and 600 W of lights, need about 3,900 running and 5,450 starting watts, so a 7 kW portable. Add central air conditioning and you are at 14.5 kW or more, which means a standby generator.

Can a portable generator run central air?

Rarely. A 3 ton central air conditioner needs about 3,500 running and 8,000 starting watts on its own, and with the basics the total is around 14.5 kW, above the 9 kW that portables top out at. A soft-start kit on the compressor lowers the surge, but a standby generator is the usual answer.

How many watts does a refrigerator need on a generator?

About 700 W running and 2,200 W for the second or two the compressor starts. A 2,000 W inverter generator can run a refrigerator alone but stalls if the compressor starts while other loads are on.

Do I need a transfer switch, or is an interlock enough?

NEC 702.5 accepts either. An interlock kit is the cheapest: it uses the existing panel and lets you turn on any breaker, so you manage the load by hand and can overload the generator. A manual transfer switch feeds 6 to 10 circuits you pick in advance and cannot be overloaded that way. A standby generator needs an automatic transfer switch. All three need a permit and an inlet on the outside wall, and the interlock must be listed for your panel model.

Sources and references

What Size Generator Do I Need? Sizing guide for running and starting watts (Generac)
Generator Wattage Worksheet (running and starting watts by appliance) (Generac)
NFPA 70, National Electrical Code (Article 702 Optional Standby Systems) (NFPA)
What to Know About Generators and Carbon Monoxide (U.S. Consumer Product Safety Commission)

Next steps for this project

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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.