Last verified: October 1, 2026
Most homes need a 10–14 kW standby generator for essentials or an 18–26 kW unit for the whole house including central air. A portable generator for essentials typically needs 5,000–8,000 running watts. The exact answer depends on two numbers: your running load and the start-up surge of your biggest motor. This guide shows how to work them out, with worked examples.
Want the quick version? Use our generator size calculator.
Step 1: List what you want to run
Every appliance has a running wattage (what it draws while working) and, if it has a motor, a starting wattage (a short surge when it switches on). Typical values:
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Chest or upright freezer | 500 | 1,500 |
| Sump pump (1/2 HP) | 1,050 | 2,150 |
| Well pump (1 HP) | 2,000 | 4,000 |
| Gas furnace blower (1/2 HP) | 875 | 2,350 |
| Central AC, 3 ton | 3,500 | 9,500 |
| Central AC, 5 ton | 5,800 | 15,000 |
| Microwave (1,000 W) | 1,000 | 1,000 |
| Electric water heater | 4,500 | 4,500 |
| Electric dryer | 5,400 | 6,750 |
| Lights (whole house, LED) | 300 | 300 |
These are typical planning values; your appliances may differ. The nameplate on each appliance (or the AC condenser’s data plate, which lists running amps and locked-rotor amps) is the real reference. To convert amps to watts for a 240-volt appliance, multiply by 240.
Step 2: Add the running watts
Add the running watts of everything that might run at the same time. Don’t forget that the fridge, furnace and pumps cycle on and off by themselves, so assume they can all be on together.
Step 3: Add the biggest start-up surge
Motors rarely start at exactly the same instant, so you only need to add the largest single surge: take the appliance with the biggest difference between starting and running watts and add that difference to your total. This gives the peak load the generator must handle for a second or two.
Step 4: Add headroom and round up
A generator should not run near 100% of its rating for hours. For a standby unit, pick a size that covers at least 120% of your running load and also your peak start-up load. For a portable, check both numbers on the spec sheet: running (rated) watts must cover the running load plus about 10%, and starting (surge) watts must cover the peak.
Worked examples
Example 1: Essentials only
Fridge, sump pump, furnace blower, LED lights, TV and Wi-Fi.
- Running: 700 + 1,050 + 875 + 300 + 300 = 3,225 W
- Biggest surge: fridge, 2,200 − 700 = 1,500 W → peak 4,725 W
- Standby: 120% of running = 3,870 W, peak 4,725 W → a 10 kW unit is the smallest common size and leaves plenty of room
- Portable: at least 3,600 W running and 4,800 W starting
Example 2: Rural home on a well
Example 1 plus a 1 HP well pump and a microwave.
- Running: 3,225 + 2,000 + 1,000 = 6,225 W
- Biggest surge: well pump, 4,000 − 2,000 = 2,000 W → peak 8,225 W
- Standby: 10 kW still works (needs 8.2 kW)
- Portable: about 6,900 W running and 8,300 W starting, a large portable
Example 3: Whole house with central air
Example 1 plus microwave, washer, dishwasher and a 3-ton central AC without a soft starter.
- Running: 10,375 W
- Biggest surge: AC, 9,500 − 3,500 = 6,000 W → peak 16,375 W
- Standby: 18 kW. With a 5-ton AC instead, the same house needs about 22 kW.
- Portable: not realistic for this load
Common sizing mistakes
- Using the generator’s propane rating for a natural gas install. Air-cooled generators produce less power on natural gas. Generac’s 22 kW, for example, is rated 22 kW on propane but less on natural gas. Size for the fuel you will use.
- Ignoring the AC start-up surge. It is often the single biggest number in the calculation. A soft starter can lower it substantially.
- Counting every appliance at once. Nobody runs the oven, dryer, water heater and AC at the same moment during an outage. Load management or simple habits can save you a size step.
- Buying “as big as possible”. Bigger units cost more to buy, install and fuel. A generator running at a very light load most of the time is not ideal either.
How to find your real appliance wattages
Typical values are a starting point. Your own numbers are better, and they’re usually easy to find:
- Nameplates: most appliances have a label with watts, or volts and amps. Watts ≈ volts × amps. A 120 V appliance rated 6 A draws about 720 W; a 240 V well pump rated 10 A, about 2,400 W while running.
- Motor horsepower: for pumps and blowers without a watt rating, horsepower and the starting surge are the key figures; use the typical values in our table or ask an electrician.
- AC data plate: running amps (RLA) and locked rotor amps (LRA). See What Size Generator to Run Central Air.
- Your installer’s measurements: many installers measure actual loads or use your utility’s demand data during the load calculation.
120 V and 240 V loads
Household outlets are 120 V, while large appliances such as central AC, electric ranges, dryers, water heaters and many well pumps use 240 V. A standby generator feeds both through the transfer switch. A portable generator can only run 240 V loads through a 240 V outlet connected to your panel by an interlock kit or transfer switch, which is one more reason to check the outlets before buying a portable. See Generator Size for a Well Pump.
Heat pumps and electric heat
Homes heated with heat pumps need special care. The compressor is sized like an AC, but backup electric heat strips can draw many kilowatts on their own in cold weather. Most generator installations either lock the strips out on generator power or use load management so they only run when nothing else large is on. If your home has electric resistance heat (baseboards or a furnace with elements), include it explicitly in the load calculation; it can double the generator size you need.
Sizing summary table
| What you want to run | Typical standby size | Typical portable rating |
|---|---|---|
| Essentials (fridge, sump, furnace blower, lights, internet) | 10 kW | About 3,600 W running / 4,800 W starting |
| Essentials + 1 HP well pump + microwave | 10 kW | About 6,900 W running / 8,300 W starting |
| Most of the house + 3-ton AC | 18 kW | Not realistic |
| Most of the house + 5-ton AC | 22 kW (26 kW on natural gas) | Not realistic |
From the worked examples above, using our method; always confirm with a professional load calculation.
Frequently asked questions
Is it better to oversize?
A little headroom is good (that’s why we add 20% to the running load for standby units), but a generator that is far too big costs more to buy, install and fuel, and may run at a very light load most of the time.
Can I size by square footage?
Only as a very rough guide. Two houses of the same size can need very different generators depending on the AC size, electric heat, well pump and appliances. Size by loads, not floor area.
Get it confirmed
An installer should do a load calculation for your home, and many will measure your actual usage. Bring your own numbers from this guide or the calculator so you can tell whether a quote is oversized. For what each size typically costs, see our cost guides in Sizing & Costs.