BuildMath

Generator size calculator

Tick the loads you will run at once. A fridge, freezer, 3-ton central AC, furnace blower, 10 LED bulbs, and a TV with internet on this page is 8,270 W required, or 14 kW after a 25% margin.

Check what will run at the same time. Watts are the high end of cited ranges and are editable. Lights are LED bulbs, not rooms. 5-ton central AC is omitted: we have no cited wattage.

You need 14 kW standby (8,270 W required, 10,338 W with 25% margin).

Sizing for a permanent standby generator must be confirmed by a licensed electrician using an NEC load calculation. Never backfeed a home without a transfer switch.

Total running watts: 5,770 W

Largest surge: Central air conditioner, 2,500 W extra

Required watts: 8,270 W

Recommended size: 14 kW

Is 10 kW enough?

No. 10,338 W with a 25% margin is over 10,000 W. This list needs 14 kW.

Is 20 kW enough?

Yes. 10,338 W with a 25% margin fits in 20,000 W.

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How we calculated this

Running watts: Refrigerator 400 W × 1; Freezer 500 W × 1; Central air conditioner 3,500 W × 1; Furnace blower (gas furnace, 1/2 HP) 800 W × 1; LED lights 15 W × 10; TV and internet 420 W × 1 = 5,770 W.

Largest extra starting watts (one motor): Central air conditioner, 2,500 W. Quantity multiplies running watts only.

Required watts = 5,770 + 2,500 = 8,270 W.

Safety margin 1.25 (size so the unit runs at about 80% of rated output): 8,270 × 1.25 = 10,338 W.

Round up to the next standard standby size: 14 kW.

Sources

Refrigerator 400 / 1,200 W, freezer 500 / 1,000 W, window AC 10,000 BTU 1,200 / 2,400 W, central AC 2 ton 2,500 / 4,500 W, 3 ton 3,500 / 6,000 W, 4 ton 5,000 / 8,000 W, furnace fan 1/2 HP 800 / 1,600 W, electric water heater 4,500 W, well pump 1/2 HP 1,000 / 2,000 W, sump pump 1/2 HP 1,050 / 2,150 W, washer 1,000 / 2,300 W, electric dryer 6,000 W, dishwasher 2,400 W, microwave 1,200 W, electric range one burner 2,400 W, space heater 1,500 W, LED bulb 15 W, TV 400 W, and internet router 20 W are the high end of ranges on Champion Power Equipment, Generator Wattage Chart (manufacturer, accessed October 24, 2026) (opens in a new tab). Resistive loads on that chart have no extra starting watts. 5-ton central AC is omitted: the chart has no 5-ton row. Lights are per LED bulb, not per room.

Heat pump 4,700 W running and 4,500 W additional starting (9,200 W total starting) and garage door opener 1/2 HP 875 W running and 2,350 W additional starting (3,225 W total starting): Generac GP Series portable generators brochure (manufacturer, accessed October 24, 2026) (opens in a new tab). Champion does not list those two loads. Additional starting watts on that sheet equal starting minus running on this page.

Computer 270 W (CPU 120 W plus monitor 150 W, no extra start): Virginia Cooperative Extension BSE-301NP, Estimating Appliance and Home Electronic Energy Use (university extension / DOE energy series, 2020) (opens in a new tab).

25% size margin: install a unit rated about 20% above the calculated load so it runs at about 80% of capacity (1 ÷ 0.80 = 1.25). Residential jobs may add 10–20% instead. Generac Standby Generator Sizing Guide 2025 (manufacturer) (opens in a new tab). That is a sizing practice, not an NEC requirement. Permanent standby size must be confirmed by a licensed electrician.

Generator size calculator

Tick the appliances and systems you will run at the same time. This page adds their running watts, then the largest extra starting (surge) watts from one motor, then a 25% margin, and rounds up to a standard standby size: 10, 14, 18, 20, 22, 24, 26, or 32 kW.

  1. Check each load. Edit quantity, running watts, and starting watts if your nameplate differs.
  2. Central air uses 2, 3, or 4 tons. We do not list 5-ton: no cited wattage. Lights are LED bulbs, not rooms.
  3. Read total running watts, the largest surge item, required watts, and the recommended kW.
  4. Use Estimate the cost of this size to open the generator cost calculator with that kW filled in.

Required watts = sum of running watts + the largest (starting − running) among selected motors. Recommended kW = required × 1.25, rounded up to the next listed standby size.

What size generator do I need?

Size from the loads you will run at once, not from house square footage. Add every selected appliance’s running watts, then add the single largest extra starting watts (starting minus running) from a motor load such as an air conditioner or well pump. Multiply that required wattage by 1.25 so the unit runs at about 80% of rated output, then round up to 10, 14, 18, 20, 22, 24, 26, or 32 kW. A licensed electrician must confirm a permanent standby with an NEC load calculation.

Running watts vs starting watts

Running watts are the power a load draws while it is on. Starting watts, also called surge watts, are the brief extra draw when a motor starts. A refrigerator might run at 400 W and start at 1,200 W. Resistive loads such as an electric dryer, water heater, or space heater do not add extra starting watts on the charts we cite: starting equals running. This calculator adds only the largest extra start, not every motor at once, because motors are not assumed to start together.

Worked example: fridge, freezer, 3-ton AC, furnace, lights, TV

Worked example

Default job on this page: refrigerator 400 W running / 1,200 W starting; freezer 500 / 1,000; 3-ton central AC 3,500 / 6,000; furnace blower 800 / 1,600; 10 LED bulbs at 15 W; TV and internet 420 W.

Running: 400 + 500 + 3,500 + 800 + 150 + 420 = 5,770 W. Extra starts: 800, 500, 2,500, 800, 0, 0. Largest extra is the 3-ton AC, 2,500 W. Required = 5,770 + 2,500 = 8,270 W. With a 1.25 margin: 8,270 × 1.25 = 10,337.5 W, which rounds up to 14 kW. Ten kW (10,000 W) is short of that margin; 20 kW covers it.

Those wattages are the high end of cited ranges and are editable. Use Estimate the cost of this size for a 14 kW installed range. For the transfer switch, pad, and gas line, see the generator cost calculator.

Safety

A permanently installed standby generator must be sized and wired by a licensed electrician using an NEC load calculation. Never backfeed a home by plugging a generator into a dryer outlet or panel without a transfer switch: that can kill line workers and damage equipment. This page is a planning estimate from published appliance wattages, not an installation design.

Questions

What size generator do I need to run my whole house?
Whole-house size is a load list, not a square-footage rule of thumb. Add running watts for everything that must stay on, add the largest motor surge, then apply a 25% margin and round up to a standard standby size. A small essentials list can fit 10 or 14 kW. A licensed electrician confirms the final size with an NEC calculation.
Is a 20 kW generator enough for a house?
It depends on the loads. On this page, 20 kW is 20,000 W: it is enough if required watts times 1.25 stay at or under 20,000 W. With the default fridge, freezer, 3-ton AC, furnace, 10 LED bulbs, and TV, that margin is 10,337.5 W, which rounds up to 14 kW. Adding an electric dryer (6,000 W running, no extra start) is 17,837.5 W with margin, 18 kW. Adding a heat pump instead (4,700 W running, 4,500 W extra start) is 18,712.5 W with margin, 20 kW. Adding both is 26,212.5 W with margin, 26 kW. Tick the loads on this page to size your own list.
How many watts does a central air conditioner need to start?
Champion Power Equipment’s wattage chart (manufacturer) lists 2-ton central AC at 2,000–2,500 W running and 3,500–4,500 W starting, 3-ton at 3,000–3,500 / 5,000–6,000, and 4-ton at 4,000–5,000 / 6,500–8,000. This page defaults to the high end of each band. We do not list 5-ton: that chart has no 5-ton row. Edit the fields to match the nameplate on your condenser.
What is the difference between running and starting watts?
Running watts are continuous draw. Starting watts are the short surge when a compressor or pump motor starts, often two to three times running watts. This calculator uses total starting watts from the cited charts, subtracts running watts to get the extra, and adds only the largest extra to the running total.
Do I need a transfer switch?
Yes. A standby generator needs a transfer switch so it cannot backfeed the utility. Automatic switches sense an outage and change over the panel. See Transfer switch: what it does and what it costs on the generator cost calculator. Never backfeed a dryer outlet or main panel without a listed transfer switch.

Written by The BuildMath team. Formulas tested against worked examples. Updated October 24, 2026.