Understanding your result
The headline is the generator size, with the running load and the peak at compressor start in the sub-line. The bars compare the two. The table shows the air conditioner’s running and starting watts, the blower, heat strips and other loads, the total running, the peak when the compressor starts with everything else already running, the requirement with your margin, and the size.
The peak at start is what the generator must deliver for a moment, and it is usually far larger than the running total. That gap is why a soft start matters: the notes show what it would do to the result if you have not ticked it.
The running watts of the air conditioner are the same figure the AC running cost calculator uses to price an hour of cooling, and the HVAC running cost calculator does it for a heat pump. Sizing the air conditioner itself is the job of the central AC size calculator; a heat pump’s need for strips, which drives generator size in winter, comes from the heat pump size calculator.
How we calculate this
The running watts per ton correspond to about 3,500 W for a typical 3-ton unit of 13 to 15 SEER2; the starting multiplier of four is the middle of the usual three to five times running power when the locked rotor amps are not known. Soft-start manufacturers claim a 60 to 70% reduction in starting current; the calculator uses 65%. Standby generator sizes from 5 to 30 kW are the residential range. The electrical background of efficiency ratings is on the efficiency ratings table, and watts and BTU are converted on the unit conversions page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| AC running watts | 1,170 W per ton | Our estimate for 13 to 15 SEER2 units (3 tons ≈ 3,500 W) |
| Starting surge without nameplate | 4 × running | Middle of the usual 3 to 5 × range |
| Nameplate surge | LRA × 240 V | Locked rotor amps from the outdoor unit nameplate |
| Soft-start reduction | 65% of the surge | Manufacturer claims of 60 to 70% |
| Blower | 600 W | Typical PSC blower; variable speed 100 to 500 W |
| Other loads | 2,000 W | Refrigerator, sump pump, lights and electronics running together |
| Margin | 10% | Common sizing practice |
Assumptions last reviewed October 8, 2026.
The calculator does not derate the generator for fuel (natural gas output is often 10 to 20% below the propane rating), altitude or temperature, does not model voltage dip tolerance, which varies by compressor, and assumes 240 V single-phase. A licensed electrician’s load calculation is the standard for the final size, and the generator, transfer switch and connections must be installed by one. The guide to heat pumps in cold climates covers the strip heat that makes winter backup hard, and the guide to what HVAC installations cost covers the equipment side.
Two worked examples
A 3-ton air conditioner without a soft start
3 tons, no soft start, 600 W blower, no strips, 2,000 W of other loads, 10% margin.
- AC running: 3 × 1,170 = 3,510 W; starting ≈ 14,040 W
- Running total: 3,510 + 600 + 2,000 = 6,110 W
- Peak at start: 2,600 + 14,040 = 16,640 W; with margin 18,304 W
- Generator: 20 kW
The house needs only about 6 kW to run, but 16.6 kW for the instant the compressor starts. The note says a soft-start kit would bring the surge to about 7,200 W and the generator down a size or more.
The same size from the nameplate, with a soft start
RLA 14 A, LRA 75 A, 200 W outdoor fan, soft start fitted, 600 W blower, 1,500 W of other loads, 10% margin.
- AC running: 14 × 240 + 200 = 3,560 W
- Starting: 75 × 240 = 18,000 W; with soft start 3,560 + 14,440 × 0.35 = 8,614 W
- Running total 5,660 W; peak at start 2,100 + 8,614 = 10,714 W; with margin 11,785 W
- Generator: 12 kW
The soft start saves the difference between a 20 kW and a 12 kW standby generator, several thousand dollars, for a few hundred dollars of kit. Using the nameplate’s real LRA rather than the rule of thumb also makes the answer firmer.
Where to find your inputs
RLA and LRA. On the nameplate of the outdoor unit, usually on a side panel; “RLA” or “RL” and “LRA” or “LR” next to the compressor line.
Blower watts. From the furnace or air handler’s specifications, or about 400 to 800 W for a standard blower.
Other loads. Add the running watts of what must run together: refrigerator about 700 W, sump pump 800, well pump 1,500, lights and electronics a few hundred.
Common mistakes
- Sizing on running watts. The compressor’s start decides the size.
- Starting everything at once. The calculator assumes the compressor starts with other loads already running, which is the realistic worst case.
- Back-feeding through an outlet. It is illegal and can kill line workers; use a transfer switch or interlock.
- Planning to run heat strips. They need a very large generator; back up a gas furnace or a cold-climate heat pump instead.
- Ignoring the fuel derating. A generator on natural gas often delivers less than its propane rating.
- Running a generator in a garage. Carbon monoxide is deadly; keep it outside, away from doors and windows.
Questions people ask
- What size generator do I need to run a 3-ton air conditioner?
- A 3-ton air conditioner draws about 3,500 watts running, but its compressor needs three to five times that for a moment to start. With a 600 W blower and 2,000 W of household basics running, the peak when the compressor starts is about 16.6 kW, so a 20 kW standby generator with a 10% margin. With a soft-start kit and the nameplate figures of a typical 3-ton unit (14 A running, 75 A locked rotor), about 12 kW is enough.
- What are RLA and LRA?
- Rated load amps (RLA) is the current the compressor draws running at its rated conditions; locked rotor amps (LRA) is the current it draws for the fraction of a second when it starts, before the motor turns. Both are on the outdoor unit's nameplate. Multiplied by 240 volts, RLA gives the running watts and LRA the starting surge; a 75 A LRA is an 18,000 W surge, which is why a generator that can run the unit may not start it.
- Does a soft start really reduce the generator size?
- Yes, often by a size or two. A soft-start kit ramps the compressor's starting current over a fraction of a second, cutting the surge by about 60 to 70% according to manufacturers. On a typical 3-ton unit that brings the starting draw from about 18,000 W to under 9,000 W, the difference between a 20 kW and a 12 kW generator. Kits cost a few hundred dollars installed and are a common addition when a generator is planned.
- Can a portable generator run central air?
- Some can run a small central air conditioner, especially with a soft-start kit, if their starting watts exceed the compressor's surge plus the other loads. A typical 7,500 to 9,500 W portable generator can start a 2-ton unit with a soft start and little else running. It must connect through a transfer switch or a code-approved interlock installed by a licensed electrician, never by back-feeding an outlet.
- Can a generator run electric heat strips?
- Only a large one. Heat strips are 5 to 20 kW of resistance heat, often the largest load in the house, so running a heat pump with 10 kW of strips adds 10,000 W to the running total and usually pushes the generator to 22 kW or more. Houses that want generator-backed heat are better served by a gas furnace, which needs only its blower, or a cold-climate heat pump that needs little strip heat.
- Should the generator be sized for everything at once?
- Not necessarily. Load-management modules and smart transfer switches can shed loads such as the air conditioner, the water heater or the dryer when the generator nears capacity, so a smaller generator covers the essentials and runs the air conditioner when other loads are off. The calculator sizes for the loads you enter running together with the compressor's start, which is the safe case.