What Size Generator for 100, 125, 150 or 200 Amp Service?

People often ask what size generator they need for a 125, 150 or 200 amp service because the amp rating is printed right on the main breaker. It is a natural starting point, but it answers a different question. The amp rating tells you the most your panel can safely deliver. Your generator size depends on what you actually run at the same time.

Quick answer. Your amp service does not set your generator size. For essential circuits, most 100 to 200 amp homes use a 5,000 to 7,500 W portable through an interlock or 30 amp transfer switch, or up to about 12,500 W on a 50 amp switch. For automatic whole home standby, 100 to 150 amp homes often land at 10 to 18 kW and 200 amp homes at 18 to 24 kW, usually with load management on air conditioning and other big loads. Add up your real loads before you buy.

This guide explains the amp math, gives typical ranges by service size for both portable and standby setups, and shows why load management and the transfer switch matter as much as the generator. We have not tested these products ourselves. Figures come from the linked manufacturer pages, spec sheets and code guidance, and anything else is labeled general guidance.

Amps, volts and watts in one minute

A typical North American home has a 120/240 volt split phase service. Multiply amps by 240 volts to get the most power the service could carry in theory.

Service size	Theoretical capacity at 240 V	What it really means
100 amp	24,000 W (24 kW)	Ceiling for the panel, common in older or smaller homes
125 amp	30,000 W (30 kW)	Ceiling for the panel, common in smaller homes and condos
150 amp	36,000 W (36 kW)	Ceiling for the panel, often with gas heat or modest electric loads
200 amp	48,000 W (48 kW)	Ceiling for the panel, standard in many newer homes

Those numbers are arithmetic, not a recommendation. Breakers and service equipment are sized with headroom, and household loads switch on and off. Generac’s own standby generator spec sheet notes that its main line circuit breakers are rated for 80% continuous load, which is a reminder that nobody plans to run equipment at its full rating all the time.

So a 200 amp home does not need a 48 kW generator. It needs a generator large enough for the loads that will be on together during an outage, plus the starting surge of the biggest motor.

The method that works on any service size

Generac’s sizing guide says to size a home standby generator to your peak load, not your square footage. Its method is simple.

1. List the appliances and circuits you want to run during an outage.
2. Add up their running watts.
3. Add the single largest starting surge, because motors draw extra power for a moment when they start.
4. Pick a generator whose rating clears that total with some margin.

Generac notes that starting watts are often two to three times running watts. Virginia Tech’s farm and home generator guide makes the same point and adds that a typical electric motor draws about three times its running current while it starts.

Our generator size calculator does this sum for you, and the appliance wattage chart lists typical running and starting watts.

Typical generator size by amp service

The table below is general guidance to help you set a budget and a shortlist. It assumes a gas or oil heated home unless noted. Your own list of loads always wins.

Service	Essentials only (portable plus interlock or transfer switch)	Whole home standby (automatic)	What pushes it higher
100 amp	5,000 to 7,500 W	About 10 to 14 kW, often with load management	Central air, well pump, electric water heater
125 amp	5,000 to 7,500 W	About 10 to 18 kW	Heat pump, electric range, medical equipment
150 amp	5,000 to 9,500 W	About 14 to 22 kW	Central air over 3 tons, electric dryer, two HVAC systems
200 amp	7,500 to 12,500 W	About 18 to 24 kW, usually with load management	Large or multiple AC units, electric heat, pool equipment

The standby ranges follow Generac’s own examples. It suggests 10 to 14 kW with load managers, or 18 kW when you have a heat pump or medical loads, for a home of about 1,550 square feet. It puts many 2,000 square foot homes at 18 to 22 kW and describes 22 kW as the industry standard for whole home coverage in 2,500 to 3,000 square foot homes. Generac’s air cooled standby range runs from 10 to 28 kW.

The essentials column reflects what a portable can push through a home connection. That limit often comes from the transfer equipment, which we cover next.

The transfer switch can cap your generator first

On a portable setup, the transfer switch or inlet can limit you before your panel does. Reliance Controls’ Pro/Tran 2 spec sheet gives clear limits.

Connection	Inlet	Maximum running watts per Reliance	Best fit
30 amp manual transfer switch (Pro/Tran 2, 306 and 310 models)	L14-30	7,500 W	Portables up to about 7,500 W
50 amp manual transfer switch (Pro/Tran 2, 506 and 510 models)	CS6375 50 amp	12,500 W	Large portables up to about 12,500 W
Interlock kit with a 30 amp inlet	L14-30	Limited by the 30 amp breaker, about 7,200 W at 240 V	Most portables, choose circuits at the panel
Interlock kit with a 50 amp inlet	50 amp	Limited by the 50 amp breaker, about 12,000 W at 240 V	Large portables, more circuits at once

The interlock rows are simple amps times volts math and are general guidance. Your electrician sizes the backfeed breaker, inlet and cable. For the pros and cons of each method, see interlock kit vs transfer switch.

With a portable you choose which circuits are on, so you can keep the load inside the generator’s rating no matter whether your service is 100 or 200 amps. That is why a 7,500 W portable works on a 200 amp home for essentials.

Why automatic standby is different

A home standby generator with an automatic transfer switch switches the house over without anyone choosing loads. That changes the code rules.

Under National Electrical Code section 702.4(B), as explained by EC&M’s code Q&A, a manual transfer system only needs enough capacity for the loads intended to run at one time, and the user selects them. With automatic transfer, the generator must carry the full load that is transferred, calculated under Article 220, or the system must use load management to keep the load within the generator’s capacity.

Local guidance applies that in practice. Leon County, Florida’s generator requirements handout says a load management system is required when the generator output is less than the rating of the main breaker it backs up with an automatic transfer switch. Your local inspector may read it differently, so ask before you design the system.

What this looks like on a 200 amp home

Generac’s 20, 22 and 24 kW Guardian spec sheet shows the numbers.

Generac Guardian model	Rated output on LP	Rated output on natural gas	Amps at 240 V on LP and natural gas (calculated)
20 kW	20,000 W	18,000 W	83.3 and 75.0
22 kW	22,000 W	19,500 W	91.7 and 81.3
24 kW	24,000 W	21,000 W	100.0 and 87.5

The spec sheet lists a 200 amp service rated automatic transfer switch with these units, able to manage up to four air conditioning loads and up to eight more with smart management modules. Generac’s RXG42SHA1 transfer switch page says managing those loads allows you to reduce the size of the generator needed.

So the common 22 kW and 200 amp pairing works because the switch sheds big loads, not because 22 kW equals 200 amps. Also note the natural gas rating is lower than the propane rating, so size on the fuel you will actually use.

Central air is the swing factor

Generac’s guide gives a useful example. A 3 ton central air unit runs at about 3.5 to 5 kW but can need 15 to 18 kW for a moment to start. It suggests pairing a 14 or 18 kW generator with a load manager or a soft start kit rather than jumping to a much larger unit. If you have central air on a 150 or 200 amp service, ask your installer about a soft starter and load management before you pay for extra kilowatts. Ductless systems are gentler, because their inverter compressors ramp up slowly. See what size generator runs a mini split for real watts by BTU.

Worked examples

These are illustrative sums using typical nameplate wattages from the Virginia Tech guide and Generac’s air conditioning figures. Your appliances will differ.

Home	Loads running together	Running watts	Largest start added	Total and suggested size
125 amp, gas heat, essentials	Fridge 725 W, furnace fan 750 W, lights and Wi-Fi 300 W, TV 120 W, laptop 50 W	About 1,950 W	Furnace fan start, about 1,500 W extra	About 3,450 W, a 5,000 W portable leaves margin for a microwave
150 amp, gas heat, well water	Above plus deep well pump up to 1,100 W	About 3,050 W	Well pump start, about 2,200 W extra	About 5,250 W, a 7,500 W portable on a 30 amp switch
200 amp, central air, whole home	AC about 5,000 W, fridge, furnace fan, lights, cooking and laundry about 7,000 W	About 12,000 W	AC start without soft start can reach 15 to 18 kW	18 to 22 kW with a load manager or soft start

The pattern is clear. Two homes with the same 200 amp panel can need very different generators, while a 125 amp home with a well and a 200 amp home with gas heat might need the same portable.

Portable or standby for your service size?

Question	Portable generator	Home standby generator
Who chooses loads	You do, at the interlock or transfer switch	The automatic switch and load managers
Typical size	5,000 to 12,500 W	10 to 28 kW in Generac’s air cooled range
Code sizing rule	Loads intended to run at one time	Full transferred load or load management
Starts by itself	No, you wheel it out and start it	Yes, within seconds of an outage
Installation	Inlet and switch by an electrician	Professional install with permits and inspection, as Generac notes

For a full comparison, read standby generator vs portable generator. If a battery is on your list too, see whole home generator vs whole home battery.

Safety and permits

Any generator that feeds your panel needs a transfer switch or an approved interlock, installed by a licensed electrician where your local rules require it. Never plug a generator into a wall socket to feed the house. The CPSC warns that backfeeding puts utility workers and neighbors at risk of electrocution.

Run portable generators outdoors only. The CPSC’s portable generator safety alert says to keep them at least 20 feet from the house with exhaust pointed away from windows, doors and vents, and never in a garage, basement, crawlspace or shed. Fit battery carbon monoxide alarms on every level.

On a private well, the pump’s starting surge often sets the minimum size. See what size generator you need for a well pump for Franklin Electric’s minimums by horsepower.

Bottom line

Use your amp rating to understand what your panel and transfer switch can handle, then size the generator to your real loads. Essentials on almost any service fit a 5,000 to 7,500 W portable on a 30 amp connection. Whole home automatic backup on a 200 amp service usually means 18 to 24 kW with load management. When in doubt, start with our generator size calculator, compare whole home setups in home backup power options, and ask an electrician for an Article 220 load calculation before you buy.

Frequently asked questions

What size generator do I need for 200 amp service?

It depends on what you run at once, not on the 200 amp rating. For essential circuits through a manual transfer switch or interlock, a 7,500 to 12,500 W portable is common. For whole home standby, Generac says 18 to 22 kW suits many 2,000 square foot homes and calls 22 kW the industry standard for 2,500 to 3,000 square feet, usually with load management on the big appliances.

What size generator for 150 amp service?

A 150 amp service at 240 V can supply about 36 kW in theory, but typical simultaneous loads are far lower. Essentials usually fit a 5,000 to 7,500 W portable. Whole home standby on a 150 amp home often lands between 14 and 22 kW, depending on central air, electric heat, well pumps and whether a load manager can shed the largest loads.

What size generator for 125 amp service?

Many 125 amp homes are smaller or gas heated, so a 5,000 to 7,500 W portable on an interlock or transfer switch covers the essentials. For automatic whole home backup, Generac suggests 10 to 14 kW with load managers or 18 kW for a heat pump or medical loads in a home around 1,550 square feet.

Can a 22 kW generator run a 200 amp house?

Often yes, with load management. Generac's 22 kW Guardian delivers up to 22,000 W on propane or 19,500 W on natural gas, about 92 or 81 amps at 240 V, and ships with a 200 amp transfer switch that can manage up to four air conditioning loads. The code requires the generator to cover the transferred load or use load management, so a licensed electrician should run the load calculation.

Does the generator have to match my main breaker?

No. The transfer switch must be rated for your service if it sits at the service entrance, but the generator can be smaller. Under NEC 702.4(B), an automatic system must either carry the full load or use load management, while with a manual system you choose which loads to run. Some local guidance says a load management system is required when generator output is less than the main breaker rating.

How do I find my home's real load?

An electrician can do a load calculation under NEC Article 220, or use 12 months of utility peak demand data where available. At home you can list what must run together, add their running watts and add the single largest starting surge, which is the method Generac describes. Our generator size calculator does that sum for you.