Will a generator run central air or a heat pump?

Central air is the biggest load in most homes, and the one people miss most in a summer outage. A portable generator can run it, but it takes more than most people expect, mostly because of the start.

How many watts does central air use?

Central AC is sized in tons. One ton is 12,000 BTU per hour, so a 3-ton unit is 36,000 BTU. The size is on the outdoor unit's label.

Size	Running W	Starting W
2 ton (24,000 BTU)	2,000–2,500	3,500–4,500
3 ton (36,000 BTU)	3,000–3,500	5,000–6,000
4 ton (48,000 BTU)	4,000–5,000	6,500–8,000
5 ton (60,000 BTU)	more again	often too much without a soft start

The 2–4 ton rows are from Champion's chart. Other charts differ a lot:

• Honda lists a 24,000 BTU unit at 3,800 running and 4,950 starting, and a 40,000 BTU unit at 6,000 and 7,800.
• LCEC, a Florida co-op, lists the same 24,000 BTU unit at 3,800 running plus 11,400 extra watts to start, three times the running figure.

That spread is the real lesson: the start-up surge depends on your unit, so size with room to spare.

Don't forget the indoor blower

The outdoor unit is only half the system. The indoor fan (in your furnace or air handler) has to run too. Our calculator uses 700 running and 1,800 starting watts for a 1/3 HP furnace fan, from Honda's and LCEC's charts.

A worked example: a 3-ton system and the basics

The calculator above is set up for this:

• Running: 3,500 (AC) + 700 (blower) + 400 (fridge) + 100 (lights) + 15 (Wi-Fi) + 80 (chargers) = 4,795 W
• Biggest surge: the AC, 6,000 − 3,500 = 2,500 W
• Starting: 4,795 + 2,500 = 7,295 W
• With 20% headroom: about 5,800 running and 8,800 starting watts

So a 3-ton system and the basics need a generator of about 7,500 W or more, with a 240 V outlet. Add a sump pump or a well pump and a 10,000 W class unit is safer.

What size generator for central air?

AC size	With the blower and basics	Generator to look at
2 ton	about 3,800 run / 5,800 start	5,000–7,500 W
3 ton	about 4,800 run / 7,300 start	7,500–10,000 W
4 ton	about 6,300 run / 9,300 start	10,000 W+, or a soft start
5 ton	more	A soft start, and the biggest portable or a standby unit

Each row uses the top of Champion's range plus the blower, fridge, lights, Wi-Fi and chargers. Starting figures add the AC's surge.

Will a 7,500 or 10,000 watt generator run central air?

These come up a lot. Short answers:

• 7,500 W: yes for 2 tons; yes for 3 tons if it's rated about 7,500 running. See what a 7,500 watt generator can run.
• 10,000 W: yes for 3 tons, with room for a well pump; a 4-ton AC is borderline. See what a 10,000 watt generator can run.
• A "30 amp generator" usually means one with an L14-30 outlet. That carries up to 7,200 W, enough for 2–3 tons and the basics.

Soft starts: the cheapest way to shrink the surge

A soft start kit is a small box an HVAC tech or electrician wires into the outdoor unit. It ramps the compressor up gently instead of hitting it with full power.

Micro-Air says its home soft starter works on ACs and heat pumps up to 6 tons and is made for running on generators. Customer reports on its page describe big drops in start-up current. One owner reports a 4-ton AC going from about 115 amps at start-up to about 27 amps. Those are owners' reports, not tests, but they show why a soft start can let a smaller generator do the job.

Will a generator run a heat pump?

Yes, sized the same way. ENERGY STAR describes a ducted heat pump as like a central air conditioner that also works in reverse to heat. So a 3-ton heat pump's compressor needs about what a 3-ton AC needs. LCEC lists a heat pump at 4,700 running watts, with 4,500 more to start.

The catch is the backup heat. Many heat pumps have electric heat strips in the indoor unit, often shown as "aux" or "emergency heat" on the thermostat. They're plain electric heaters rated in kilowatts, and they can draw a lot. If they kick on during an outage, they can overload the generator.

• Check the air handler label for the heat kit's kW rating.
• Don't select "emergency heat" while on generator power.
• Ask an electrician whether the heat strips can be kept off the generator circuit.

Ductless mini splits are much easier on a generator. See will a generator run a mini split?

How to connect it

Central AC is hard-wired and usually 240 V. You can't plug it into a generator. It needs:

1. A generator with a 240 V outlet (L14-30R or 14-50R).
2. A transfer switch or interlock, installed by a licensed electrician. See interlock vs transfer switch.

Never backfeed through a dryer outlet or wall outlet. Generac warns it can send power down the utility lines and kill line workers.

Tips for running AC on a generator

• Start the AC with nothing else starting. Let the fridge and pumps settle first.
• Set the thermostat a few degrees higher so the AC cycles less.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.