Generator sizing calculator for HVAC

Why running watts is the wrong way to size a generator

Most generator sizing guides tell you to add up the running watts of every appliance you want to power and pick a generator that exceeds that total. The math is wrong in a specific way for HVAC systems. Air conditioners and heat pumps draw five to seven times their running current during the brief locked-rotor inrush at startup. A 3-ton AC running at 3,500 watts pulls 15,000 to 20,000 watts during the half-second the compressor starts. That inrush is what the generator has to deliver, not the running number on the nameplate.

Generators have two power ratings. Running watts is the continuous output the unit can sustain. Surge watts (also called peak watts or starting watts) is the brief spike it can deliver for a few seconds. Surge is typically 1.2 to 1.4 times running on most portable generators. A 5,000 watt running generator delivers about 6,500 watts surge. That sounds like enough until you realize a 3-ton AC needs 15,000 to 20,000 watts of surge. The math fails immediately.

Locked Rotor Amperage and how to find yours

Locked Rotor Amperage (LRA) is the current the compressor draws at the instant of startup, before the rotor begins to spin. NEC Article 440 governs the wiring of motor-compressor circuits and references LRA as the basis for short-circuit protection sizing. It is also the right number for generator sizing.

LRA is printed on the data plate of every AC condenser and heat pump. Look at the silver sticker on the side of the outdoor unit. The label typically lists RLA (Rated Load Amps, the running current), LRA (Locked Rotor Amps, the startup current), and FLA (Full Load Amps, sometimes used instead of RLA). Multiply LRA by the system voltage (240 V for residential split-phase) to get starting watts. Typical values:

• 1.5 ton: LRA 35-42 amps, surge ~9,000 W
• 2 ton: LRA 45-55, surge ~12,000 W
• 2.5 ton: LRA 55-65, surge ~14,500 W
• 3 ton: LRA 65-80, surge ~17,500 W
• 4 ton: LRA 90-105, surge ~23,000 W
• 5 ton: LRA 110-125, surge ~28,000 W

These are typical ranges from major-brand AHRI directory listings. Use your nameplate value when you can find it. The calculator above takes an LRA override field for exactly this reason.

Soft-start kits: the single biggest lever in generator sizing

A soft-start kit (Hyper Engineering MicroAir EasyStart is the most common aftermarket brand) ramps the compressor up over 5 to 8 seconds instead of full-voltage start. The result is a 60 to 75 percent reduction in starting surge. A 3-ton AC that needs 17,500 watts surge bare drops to 5,000 to 6,000 watts surge with a soft-start kit. That difference is what lets a 7,500 watt portable generator run a 3-ton AC instead of needing a 12,000 watt unit.

Soft-start kits run $250 to $400 for the part plus $100 to $200 for installation if you do not DIY. Compared against the price difference between a 7,500 W and a 12,000 W generator (typically $1,500 to $3,000), the soft-start kit pays for itself on its first install. For households that already own an undersized portable generator, adding a soft-start kit is the cheapest way to make the existing generator work with the existing AC.

Hard-start capacitors are a related but different product. A hard-start capacitor adds extra startup torque by paralleling a capacitor with the compressor's run capacitor for the first half-second. It reduces inrush about 10 to 20 percent. Not enough to change generator sizing meaningfully, but worth installing on fixed-orifice systems that struggle to start in hot weather. For generator purposes, choose soft-start over hard-start every time.

Furnace electrical loads: smaller than you think

Gas furnaces use far less electricity than people assume. The blower motor (PSC or ECM), the inducer fan, and the ignition system together typically draw 400 to 800 watts running. Surge is about 1.3 times running because the blower has minor startup inrush. A modest 3,500 watt generator has no trouble running a gas furnace plus a refrigerator plus essential lighting. The problem comes when AC also has to run.

Electric resistance furnaces and heat pump strip-heat backup are a completely different category. A whole-house electric furnace pulls 15 to 25 kilowatts at full load. Heat pump strip backup typically runs 10 to 20 kW. No portable generator on the consumer market can handle that load. Households with electric backup heat have to choose between two paths: a whole-home standby generator sized to carry the full strip kW, or a heat-pump-aware thermostat that locks out emergency heat during generator operation (Ecobee, Nest, Honeywell T10 all support this) so the heat pump runs without backup during outages.

Portable vs whole-home standby: the decision tree

Three factors push the decision from portable toward whole-home standby:

• Electric backup heat above 5 kW requires standby. No portable carries the load.
• Total surge above 10,000 watts usually means a 12,000+ watt portable, which is at the upper limit of practical portable territory (heavy, expensive, gas-thirsty). Standby becomes more cost-effective.
• Frequent or long outages shift the math toward standby. Portables run 8 to 12 hours per tank of gasoline. A whole-home standby on natural gas runs indefinitely.

Whole-home standby generators size by NEC 702 (optional standby systems) at 1.25 times the total continuous load. A 4,500 sqft house with central AC, a gas furnace, electric water heater, and typical receptacle loads usually lands at a 20 to 22 kW unit. Houses with electric strip backup or whole-house electric heating land at 24 to 26 kW.

A common standby gotcha: a 22 kW natural gas standby needs about 250 to 300 cubic feet of gas per hour at full load. Most existing 1/2 inch residential gas lines max out around 150 cfh. The gas service line and meter often need upgrading along with the generator install. The calculator flags this when the recommended standby crosses 22 kW.

Transfer switches and what NEC requires

Any generator that powers any part of a home's wiring must connect through a transfer switch that prevents backfeed into the utility grid. NEC 702 sets the requirements. Two common configurations:

• Manual transfer switch: A small subpanel with 6 to 10 circuits that switches between utility and generator power via a manual lever. Typical for portable generator setups. Installed cost $400 to $800.
• Automatic transfer switch (ATS): A larger panel that detects utility loss and starts the generator automatically. Standard with whole-home standby installations. Installed cost $800 to $2,000 above the manual switch.

Wire sizing between the generator and transfer switch follows NEC Article 310 by amperage. Breaker sizing per NEC Article 440 for the AC compressor circuit. For portable setups with an L14-30 (240 V 30 A) connector, 10 AWG cable is the minimum. For larger portables on L14-50 (240 V 50 A), 6 AWG. Whole-home standby installations use service-entry cable sized to the generator's continuous rating.

Other essential loads to factor in

Beyond HVAC and the refrigerator, the loads people forget when sizing a generator:

• Sump pump: 800 to 1,200 W running, 2,500 to 4,000 W surge. Critical in flood-prone areas during storm-related outages.
• Well pump: 800 to 1,500 W running, 2,500 to 4,500 W surge. No water without it.
• Garage door opener: 700 W surge, but easy to forget.
• Internet equipment: 30 to 100 W. Important for staying informed during long outages.
• Medical equipment: CPAP, oxygen concentrators, refrigerated medications. Size for full surge of any motorized unit.

The calculator includes input fields for the most common ones. Sum the surge watts for everything you want running at the same moment the AC compressor starts, since that is the worst-case load the generator has to deliver.