What Size Generator Do I Need to Run My House?

This is the number-one question in home backup power, and most people get it wrong in the same way: they buy too small and find out during the outage. The fix takes about ten minutes with a pencil. Here’s how an electrician would size it for your house.

Running watts vs. starting watts — the two numbers that matter

Every motor-driven appliance has two wattage ratings:

• Running watts: the steady draw while it operates. Your fridge hums along at 150–200 watts.
• Starting (surge) watts: the brief spike when the motor kicks on — typically 2–3x the running watts, lasting a few seconds.

Your generator must satisfy both conditions at once:

1. Its running-watt rating must cover everything operating simultaneously.
2. Its starting-watt rating must cover the biggest surge in your setup.

Miss condition 1 and the generator overloads and trips. Miss condition 2 and the compressor or pump stalls on startup — the generator bogs down, voltage sags, and you get the dreaded brown-out that can damage the appliance motor.

Resistive loads (lights, phone chargers, toasters, space heaters) have no surge — starting watts = running watts. Motors (compressors, pumps, fans) are where the surges live.

The appliance wattage table

These are typical ranges. Your specific appliance may differ — check the nameplate (the label on the back or bottom) for the most accurate number.

Appliance	Running watts	Starting watts
Refrigerator / freezer	150–200	1,000–1,200
Furnace blower (gas/oil heat)	700–900	2,000–2,500
Sump pump (1/3 HP)	800–900	1,800–2,200
Well pump (1/2 HP)	1,000–1,100	2,500–3,000
Central AC, 3-ton	3,000–3,500	9,000–12,000
Central AC, 4-ton	4,000–4,500	11,000–14,000
Central AC, 5-ton	5,000–5,500	13,000–16,000
Window AC (10,000 BTU)	900–1,000	2,200–2,600
Portable AC (12,000 BTU)	1,100–1,300	2,500–3,000
Microwave	1,000–1,200	1,000–1,200 (no surge)
Electric range (one burner)	1,500–2,000	no surge
Electric water heater	4,000–4,500	no surge
Clothes dryer (electric)	5,000	no surge
LED lights (per bulb)	8–12	no surge
TV (55”)	100–150	no surge
Laptop / phone chargers	50–100	no surge
Wi-Fi router + modem	20–30	no surge
CPAP machine	30–60	no surge
Garage door opener	500–600	1,200–1,400
Chest freezer	150–200	1,000–1,200

The pattern to notice: the big surge loads are the motors — AC compressors, well pumps, sump pumps, furnace blowers. Everything else is straightforward.

The 5-minute sizing method

Here’s the exact process:

1. List what you want to run during an outage. Be honest — not “everything,” just what matters.
2. Add up the running watts.
3. Find the biggest surge. Take the appliance with the largest (starting − running) difference, and add that difference to your running total.
4. Add 20% headroom. Multiply by 1.2. Generators last longer, run quieter, and handle unexpected loads better when they’re not pinned at 100%.

That final number is your target running-watt rating. Check that the generator’s starting-watt rating covers your biggest single surge with room to spare.

Worked example 1: Essentials only (any size home)

Fridge + freezer, some LED lights, Wi-Fi, phone/laptop chargers, and a gas furnace blower for winter heat.

Load	Running	Surge extra
Refrigerator	200	1,000
Chest freezer	180	1,000
Furnace blower	800	1,500
10 LED bulbs	100	—
Wi-Fi + chargers + laptop	100	—
Running total	1,380 W

Biggest surge: furnace blower or fridge (1,000–1,500 W extra). Running total + biggest surge = 1,380 + 1,500 = 2,880 W. × 1.2 headroom = ~3,450 W.

Answer: a 4,000–5,000 watt portable (like the Generac GP6500) handles this with room to spare. This is the most common real-world setup — it costs $700–$1,000 and covers the things that actually cost you money when they stop (food, heat, sump pump).

Worked example 2: Medium home, 2,000 sq ft, no central AC

Add a sump pump, a microwave, and a window AC to the essentials.

Load	Running	Surge extra
Essentials (from above)	1,380	—
Sump pump	900	1,300
Window AC 10k BTU	1,000	1,600
Microwave	1,200	—
Running total	4,480 W

Biggest surge: window AC (1,600 W extra). 4,480 + 1,600 = 6,080. × 1.2 = ~7,300 W.

Answer: a 7,500–9,000 watt portable — the Champion 100520 (8,750 starting) or Westinghouse WGen7500 class, $800–$1,100. This is the sweet spot for most suburban homes that don’t need central AC.

Worked example 3: 3,000 sq ft home WITH central AC (3-ton)

This is where the math jumps. The AC compressor is the elephant.

Load	Running	Surge extra
Refrigerator + freezer	400	1,000
Lights, Wi-Fi, chargers, TV	400	—
Furnace blower	800	1,500
Sump pump	900	1,300
Central AC, 3-ton	3,500	8,000
Running total	6,000 W

Biggest surge: the AC — 8,000 W on top of its running draw. 6,000 + 8,000 = 14,000. × 1.2 = ~16,800 W running, with a starting requirement near 17,000–18,000 W.

Answer: a 20–22 kW standby generator. No portable under about 12,000 watts will reliably start a 3-ton central AC — and even then it’s marginal. This is exactly why homes with central AC usually end up with a Generac 22kW Guardian or Kohler 20RCA ($8,000–$15,000 installed).

The cheaper alternative: install a soft starter on the AC (~$300–$400) to cut the starting surge by 60–70%. That can drop the surge to ~3,500 W and bring a big portable back into play. Read the AC sizing guide for the full breakdown.

The “add 20% headroom” rule — and why it exists

Three reasons:

1. Motors age. A 10-year-old well pump draws more starting current than it did new.
2. You forget things. The garage door opener, the dehumidifier, the second fridge in the garage — something always shows up during the outage that wasn’t on the list.
3. Heat derates generators. A generator rated 7,500 W at sea level makes less at altitude and in extreme heat. Headroom absorbs that.

Why oversizing wastes fuel

Go too far the other way and you pay at the fuel pump. A 22kW standby idling at 4kW of load still burns a meaningful amount of fuel per hour — roughly 1.5–2 gallons of propane — while a right-sized 7,500-watt portable at the same load sips about half a gallon of gasoline. Generators are most efficient at 50–80% load. Size to your math plus 20%, not double.

The exception: if you’re buying a standby, the price jump from 14kW to 22kW is often only $800–$1,200 installed, and it buys you the ability to run central AC. That’s a reasonable oversize because the load (AC) is real and intermittent.

Common sizing mistakes

• Counting square footage instead of watts. A 1,500 sq ft all-electric home needs more generator than a 3,000 sq ft home with gas heat and no central AC. Watts, not square feet.
• Forgetting the well pump. Rural homeowners lose water when the power goes out. A 1/2 HP well pump needs 2,500–3,000 starting watts — it belongs on the list.
• Assuming two big motors start at the same time. They rarely do. You size for the biggest single surge, not all surges added together — unless you know they’ll coincide (like AC and well pump on the same thermostat cycle, which is rare).
• Trusting the “peak” number on the box. Some manufacturers advertise the starting watts in big print. Size by running watts for the load, starting watts for the surge. Read both.

What size generator do most houses need? (quick reference)

• Essentials only (fridge, lights, phones, furnace): 4,000–5,500 W portable
• Essentials + sump pump + window AC + microwave: 7,500–9,500 W portable or inverter
• Whole house, no central AC: 10,000–14,000 W portable or small standby
• Whole house with 3-ton central AC: 20–22 kW standby
• Whole house with 4–5 ton central AC: 22–26 kW standby

Keep reading

• The Complete Home Backup Power Guide 2026 — the full landscape: generator types, costs, fuel, batteries, and how to choose.
• What Size Generator Do I Need to Run Central AC? — the AC problem in detail: tonnage tables, why starting watts kill undersized generators, and soft starters.