Generator Size for HVAC: What You Need to Run AC and Heat

The right generator size depends on what you want to run. A gas or oil furnace needs roughly 3,500 to 5,000 watts. A window air conditioner needs a couple of thousand starting watts. Central air, a heat pump with electric heat strips or an electric furnace pushes you into the 7,500-watt-and-up range, and sometimes far beyond. The number that decides it is the startup surge of the compressor, not the running watts printed on a spec sheet.

Below, we walk through the sizing math, give figures for each type of equipment, work one full example, and cover the transfer switch and safety steps that keep the generator and your crew of utility workers safe.

TL;DR

• Size to the starting watts of your largest motor, not to BTUs or tonnage: add all running watts, then add the largest motor’s surge on top, then add about 20% headroom.
• Gas and oil furnaces are the easy case, at about 3,500 to 5,000 watts. Electric furnaces and heat strips can need 20,000 watts or more.
• Central air and heat pumps usually call for a large portable or standby unit, and a soft-start kit or load-management module can bring the size down.
• Hardwiring a generator into your home needs a transfer switch and a licensed electrician. Run the generator outdoors, never in a garage or basement.

Table of Contents

• Can a generator run your air conditioner, furnace or heat pump?
• Running watts versus starting watts
• The sizing formula
• Generator size by type of equipment
• Adjust for heat, altitude and what you really need
• Load management: a smaller generator on purpose
• Our perspective: size for the nameplate, not the brochure
• Connecting a generator safely
• Commercial and industrial buildings
• How Brighton Air Corp can help
• FAQ
• Sources

Can a generator run your air conditioner, furnace or heat pump?

Yes. A generator can run HVAC equipment, but whether it does so reliably depends on the equipment, the generator’s capacity, the startup demand, and whatever else is running at the same moment. Heat strips inside an air handler or heat pump can add a very large load on their own.

That is why the same question gets such different answers online. A small inverter generator will run a window unit and a gas furnace fan without trouble. The same generator will not start a 4-ton central compressor. Neither answer is wrong. They describe different loads.

Running watts versus starting watts

Any motor-driven appliance has two numbers. Running watts are what it draws once it is up to speed. Starting watts are the brief surge as the motor spins up, and for a compressor that surge is several times the running figure. A common rule of thumb is that three times a motor’s rated wattage is a good estimate of startup demand. The nameplate on the outdoor unit lists RLA (rated load amps) and LRA (locked rotor amps), and LRA is the current under starting conditions. Use those instead of a BTU guess.

BTUs and tonnage tell you how much heat the system moves. They do not tell you how much electricity the compressor needs to start. Two 3-ton units can have very different startup draws depending on the compressor, the efficiency rating and whether it has a soft starter.

The sizing formula

Generator load calculations for homes follow the logic of NEC Article 220: sum the loads, then account for the surge of the biggest motor. The commonly used formula is:

Required watts = total running watts of all loads + (largest starting watts − that motor’s running watts)

Only the largest motor’s surge is counted because it is unlikely that every motor starts in the same instant, and that is also the discipline you apply when you switch things on by hand. If you bring appliances online one at a time, you avoid stacking surges beyond what the generator can take.

Then add headroom. A 20% margin over the calculated load is the usual advice, because a generator run at its limit for hours is hard on the engine and the alternator.

A worked example

These numbers are illustrative. Replace them with the values on your own nameplates.

• Total running watts: 3,500 + 875 + 1,000 = 5,375
• Surge of the largest motor, beyond its running watts: 10,500 − 3,500 = 7,000
• Calculated load: 5,375 + 7,000 = 12,375 watts
• With 20% headroom: about 14,850 watts

So a home with a modest central AC and a few essentials lands near a 15 kW generator, even though the running load is only about 5.4 kW. If a soft-start device cuts the compressor’s surge to, say, double its running watts, the same house needs well under 12,000 watts. The surge is where the money goes.

Generator size by type of equipment

Window and portable air conditioners

This is the easy end. A 10,000-BTU window unit is listed at about 1,500 running and 2,200 starting watts, so a 2,500-watt generator will run it with room to spare. That also answers the common question about a 5,000-BTU unit: it draws less than that 10,000-BTU figure, so a 2,500-watt generator should handle it alone. Do not plug a refrigerator into the same small generator and assume it will all work. Add the loads.

Central air

A generator in the 7,500-watt range is often cited as the starting point. A utility’s safety guidance says a 7,500-watt or larger generator, sized to the home, can power essentials such as a furnace or a small central AC, and that central-air suitability depends on the particular equipment and the other loads. Consumer Reports puts a typical home’s essentials, including window ACs, at around 5,000 watts, and notes whole heating and cooling systems generally require more.

So can an 8,000-watt generator run central air? It can run a small or efficient system if little else is on, especially with a soft starter. It will struggle with a 4- or 5-ton unit. Confirm with the LRA on your condenser.

Furnaces

For a gas or oil furnace, you are powering a blower motor, a control board and an igniter, not the heat itself. Honda’s guide breaks furnace fans out by motor size:

• 1/8 hp: about 300 running and 500 starting watts
• 1/4 hp: about 600 and 1,000
• 1/3 hp: about 700 and 1,400
• 1/2 hp: about 875 and 2,350

Those figures come from Honda’s estimation guide. Add your boiler or furnace circulator, a few lights and the refrigerator, and a generator of 3,500 to 5,000 watts covers a typical gas-heated home. An electric furnace is a different animal and is rarely practical on a portable generator.

If you are not sure which kind of furnace you have, or the blower motor size, a technician can read it off the nameplate in minutes. That is a routine part of a furnace inspection.

Heat pumps

Heat pumps need careful sizing because the compressor works in both heating and cooling modes and many systems include electric resistance strips for cold snaps. Those strips significantly raise the total load. Mitsubishi Electric documents a U.S. home where a mid-sized portable generator powers an all-climate heat pump system through a transfer switch, which shows it can be done when the setup is planned around the equipment. If your heat pump is struggling already, our short checklist for a heat pump not heating is worth running before you blame the power.

Adjust for heat, altitude and what you really need

A generator’s nameplate rating is its output under standard conditions. Engines lose power with elevation and heat. Sizing calculators commonly derate roughly 3.5% per 1,000 feet above 500 feet and about 1% per 10°F above 77°F. Most of New Jersey sits low, so altitude is a minor factor here. A July heat wave, with the generator running in the sun next to a hot wall, is the one to plan for.

The other lever is the cooling load itself. A home that overheats upstairs forces the compressor to run longer and harder, and the fixes for upstairs rooms running too hot can reduce how much cooling you ask a backup generator to deliver. Likewise, a correctly sized system matters more than an oversized one. Our guide to Manual J load calculation explains why the old 500-square-feet-per-ton guess leads to systems that cost more to run, and to generators that must be bigger than they should be.

Load management: a smaller generator on purpose

You do not have to size a generator to run everything at once. Under NEC Article 702, an optional standby system connected through an automatic transfer switch must be sized for the full calculated load or have a load-shedding system so it cannot be overloaded. Load shedding through a manual or automatic transfer switch and a standby panel is one accepted way to leave the excess off the generator.

In practice that means two things:

• Smart load management modules can stop an AC compressor from starting at the same time as other big appliances, protecting the generator from overloads.
• A soft-start or hard-start device on the compressor reduces the surge that sets the generator size, which can drop the requirement by several kilowatts.

GenTek Power describes this as a design choice rather than a compromise: powering essential HVAC without an oversized, expensive generator. We agree, and the next section explains why.

Our perspective: size for the nameplate, not the brochure

We see the same mistake on service calls. Someone buys a generator from a chart that matches it to square footage or BTUs, and then the compressor will not start, or it trips on the first hot afternoon. Charts are fine for a first estimate. They cannot see your condenser’s LRA, your blower motor, whether you have heat strips, or whether the previous owner added a second air handler.

Our advice, from working on residential and commercial systems across New Jersey:

1. Decide what you must run during an outage. For most families that is the furnace or boiler controls and blower, the refrigerator, lights, and one cooling zone.
2. Read the nameplates on the outdoor unit, the air handler and the furnace. Write down RLA, LRA and motor horsepower.
3. Ask whether a soft starter or a load-management module makes sense before you pay for 5 extra kilowatts of generator.
4. Have the connection done properly. The generator should never be the weakest link in a system that is otherwise sound.

An old or failing compressor also pulls more startup current than a healthy one, so a unit that is on its way out can break a generator that was sized correctly on paper. If yours is making noises or tripping breakers, read about what causes AC compressor failure first.

— John

Connecting a generator safely

Transfer switch and licensed electrician

If you connect a generator to household wiring, only a licensed electrician should install it. A separate outdoor receptacle and a double-throw transfer switch isolate the generator from the grid and protect utility workers from backfeed. Never plug a generator into a wall outlet to power the house.

Permits are a local matter. As one example of how strict they can be, one Texas city requires building, electrical and plumbing permits for all generators, with an electrical one-line drawing in the submitted plans. Ask your New Jersey municipality’s construction office what it requires before you buy.

Placement and carbon monoxide

Run the generator outside, far from windows, doors and vents, and never in a home, basement, carport, crawlspace or attached garage. Carbon monoxide builds up quickly, and a furnace running during an outage adds another combustion source. Our guide to furnace carbon monoxide warning signs lists what to watch for and the tests you can request.

Cords and plug-in loads

If you power anything directly from the generator, use cords rated for the loads you put on them. An undersized extension cord to a window AC is a heat and fire risk.

Commercial and industrial buildings

The same principles apply, with higher stakes. Rooftop units, refrigeration and several air handlers mean multiple large motors and often three-phase power. Facility managers should list which zones and equipment are truly critical, get the starting current of each large motor and decide whether staged starts are possible. If a rooftop unit stops heating during an outage or after a restart, measuring temperature rise is the first diagnostic step. For warehouses, our notes on managing an HVAC system for a warehouse cover load and zoning.

How Brighton Air Corp can help

We are an HVAC contractor serving New Jersey homes and businesses, with emergency service available 24/7. Before you buy a generator, we can inspect your furnace, boiler or AC, read the nameplate data and tell you what your equipment really draws. Our AC service inspection is $125, and AC repairs are published at $150 to $600 depending on the problem, on our AC repair page. If the system is not running right even on grid power, call us through our main page and we will schedule a visit. We handle HVAC, not the electrical connection, so we will tell you when it is time to bring in your electrician for the transfer switch.

FAQ

What size generator do I need to run central air?

Plan on at least 7,500 watts for a small system, and 14 to 20 kW for a 5-ton unit unless you add a soft starter. The deciding figure is the LRA on the condenser nameplate, so check it before you buy.

Can a generator run a heat pump?

Yes, if it covers the compressor’s startup surge and any electric heat strips. The strips can make the load many times larger than the compressor alone, so ask whether your system can be wired or controlled to keep them off during an outage.

What size generator do I need for a furnace?

A gas or oil furnace usually needs 3,500 to 5,000 watts of generator capacity, including a few other essentials. Electric furnaces need 20,000 watts or more and are poorly suited to portable generators.

Will a 2,500-watt generator run a 5,000-BTU air conditioner?

Usually yes, if nothing else is on the generator. A 10,000-BTU window unit is listed at about 1,500 running and 2,200 starting watts, and a 5,000-BTU unit draws less. Confirm with the label on your unit.

Can I plug my generator into the house?

Not with a standard cord into an outlet. A licensed electrician should install a transfer switch and outdoor inlet, which prevents backfeeding the grid and protects utility workers.

Sources

• Can a Generator Run an HVAC System at Home?
• Standby Generator Load Calculation for Residential Services
• Generator Buying Guide, Consumer Reports
• 2025 Updated Standby Generator Guide
• What Size Generator Do I Need?, HVAC Base
• What Size Generator to Run a 5-Ton AC Unit?, HVAC Base
• Standby Generator Sizing Calculator
• What Wattage Generator Do I Need?, Honda Power Equipment
• Generator safety, Kissimmee Utility Authority
• What Size Generator to Run Central Heating, Jackery
• Heat Pump & Generator Pairing, Mitsubishi Electric HVAC US
• Generators and Heating Safety, New York State Department of Health
• Generator Guidelines, City of Hialeah
• Standard Operating Procedures for Home Generators, Seabrook
• Safe Use of Small Generators, USDA Forest Service
• Over 8°F Upstairs? Homeowners’ Fixes That Work Fast

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• Manual J Calculation: Check Code, Skip the 500 Square Feet per Ton Guess
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