HVAC Calculators · Calculator
Generator Sizing Calculator: Running Watts, Motor Surge and AC Starting
A generator has to carry everything that runs at once and also survive the start-up surge of the biggest motor. Add up running watts, then add the largest motor's starting watts minus its running watts. For central AC the surge is roughly LRA × 240 V; a 3-ton unit with a 75 A LRA can push a house from 6 kW running to over 20 kW peak, while a soft-start kit can bring that close to 10 kW.
Generator size calculator
Running watts vs starting watts
Resistive loads such as lights, toasters and electric heaters draw the same power from the first instant. Motors do not. While the rotor is stationary a motor draws several times its running current, for a fraction of a second up to a couple of seconds. Refrigerator compressors, well pumps, sump pumps, furnace blowers and above all air-conditioner compressors all do this. A generator that is big enough for the running load can still stall, trip its breaker or let the voltage sag so far that the motor never starts.
Start watts = LRA × 240 × Soft-start factor (AC compressor)
or Start watts ≈ 3 × Running watts (other motors)
Peak = Running total − Largest motor running + Largest motor start
Size ≥ the larger of Peak and Running ÷ Continuous limit
Only the largest motor's surge is added, because in normal use motors do not all start in the same instant. A standby generator's transfer switch or load manager can enforce that by staggering loads after a power cut.
Worked example
A house with 6,000 W running when everything that matters is on, including a 3-ton central AC drawing 3,500 W. The compressor nameplate lists LRA 75 A.
- AC start watts = 75 × 240 = 18,000 W
- Peak = 6,000 − 3,500 + 18,000 = 20,500 W
- Running check: 6,000 ÷ 0.8 = 7,500 W, smaller than the peak
- Smallest standard size ≥ 20.5 kW = 22 kW
Add a soft-start kit (start ≈ 18,000 × 0.4 = 7,200 W): peak becomes 6,000 − 3,500 + 7,200 = 9,700 W, and a 12 kW generator is enough. On paper that 22 kW unit spends most of its time at about 27% load; the soft starter lets you buy a machine sized much closer to what the house actually uses.
Typical running and starting watts
Approximate figures; always check the nameplate on your own equipment.
| Load | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 150–800 | about 3× running |
| Furnace blower (1/2 hp) | 500–900 | 1,500–2,500 |
| Sump pump (1/2 hp) | 800–1,050 | 2,000–3,000 |
| Well pump (1 hp) | 1,000–1,500 | 3,000–4,500 |
| Window AC, 10,000 BTU/h | 800–1,200 | 2,500–3,500 |
| Central AC, 3 ton | 3,000–4,000 | LRA × 240 (often 15,000–20,000) |
| Central AC, 5 ton | 5,000–6,000 | LRA × 240 (often 25,000–30,000) |
| Electric water heater | 4,500 | 4,500 (resistive) |
| Electric range, one large burner | 2,000–2,500 | same as running |
To estimate running watts of an air conditioner from its rating, divide BTU/h by its EER: a 36,000 BTU/h unit with an EER of 11 draws about 3,300 W. The EER rating guide explains the figure, and average electricity usage lists common household loads.
Reading LRA and the AC nameplate
The outdoor unit's data plate lists RLA (rated load amps, running current) and LRA (locked rotor amps, the start-up current). LRA is a worst-case figure measured with the rotor held still. Real inrush lasts well under a second and is usually a little below LRA, so this calculator is deliberately conservative. Standby generator makers publish how much LRA each model can start with acceptable voltage dip; use that table for the final choice.
A compressor soft starter is a small module wired into the outdoor unit that ramps the motor up gently. Manufacturers commonly claim inrush reductions of around 50–70%. It is the cheapest way to fit central AC onto a mid-size generator, and it also reduces light flicker on weak utility circuits. If the AC keeps tripping its breaker on start-up, a failing start capacitor or hard-starting compressor may be the cause, and that should be diagnosed first.
Standard generator sizes
- Portable / inverter (3.5–12 kW): powers a fridge, sump pump, furnace blower, lights and a window AC or two. Gas furnaces need only a few hundred watts to run their blower and controls, so they work on small generators if wired through a proper inlet and interlock.
- Air-cooled standby (10–26 kW): the common whole-house range. 20–24 kW handles most homes with one central AC; 14–18 kW is typical with load management or a soft starter.
- Liquid-cooled standby (30–48 kW+): large homes, two or more AC systems, electric heat or heat pumps with large strip heaters.
Fuel matters too. Most standby generators produce somewhat less output on natural gas than on propane; check which rating the spec sheet lists.
Adding up running watts
List what must run during an outage and total the running watts of everything that can be on at the same time. Nameplates give either watts or amps; for amps, multiply by the voltage (120 V or 240 V). A practical order for a whole-house list:
- Essentials: refrigerator, freezer, sump pump, furnace blower or boiler pump, a few lighting circuits, internet router.
- Comfort: central or window AC, well pump, microwave, TV.
- Large resistive loads: electric water heater, range, dryer, electric heat strips. These are often left off or put under load management, because each can push the generator up a size or more.
Load-management modules on standby systems shed a large load, such as the AC or water heater, while another starts, which lets a smaller generator serve a larger house. That is often cheaper than stepping up a size.
Safety
Never connect a generator by back-feeding a dryer outlet or panel without a transfer switch or approved interlock; it can electrocute utility line workers and start fires. Installing a standby generator, transfer switch and gas supply is licensed electrical and gas work. Run portable generators outdoors, at least 20 feet from doors, windows and vents, because the exhaust contains deadly carbon monoxide.
Frequently asked questions
What size generator do I need to run a 3-ton air conditioner?
Without a soft starter, usually 20–22 kW for the whole house, because the compressor's start-up surge is around 15,000–20,000 W. With a soft-start kit, many homes can run a 3-ton AC on a 10–14 kW generator, depending on what else is on.
What is the difference between kW and kVA on a generator?
kVA is apparent power; kW is real power. Residential generators are rated at a power factor of 1.0, so the two are the same. Commercial three-phase units are often rated at 0.8 power factor, where kW = 0.8 × kVA.
Can I run my furnace on a portable generator?
Usually yes. A gas furnace blower and controls draw roughly 400–900 W. Connect it through a transfer switch or inlet, since the furnace needs a stable, correctly polarized and grounded supply for its flame sensor to work.
Last reviewed 2026-10-28