When someone asks what generator they need to “run the house,” the honest answer is that it depends on which house they mean. Running every light, the central air, the electric dryer and the oven at the same time can take 20,000 watts or more. Keeping the essentials on usually takes a fraction of that.
Here’s how I’d work it out, in four steps.
Step 1: Decide what “run the house” means to you
Most people land in one of these tiers:
| Tier | What runs | Typical generator size |
|---|---|---|
| Essentials | Fridge, freezer, lights, internet, phones, TV | 3,000–5,000 W |
| Comfort | Essentials + furnace blower, sump or well pump, microwave, window AC | 5,000–7,500 W |
| Most of the house | Comfort + central AC (with soft start), washer, gas dryer | 9,000–12,000 W |
| Everything | All circuits including electric range, water heater, dryer | 15,000–26,000 W standby |
Step 2: Add up the running watts
List what you’ll run at the same time and add up the running watts. The numbers are on each appliance’s label, or in our wattage chart.
A typical “comfort” list looks like this:
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 180 | 1,200 |
| Chest freezer | 100 | 800 |
| Sump pump (1/2 HP) | 1,050 | 2,150 |
| Furnace blower (1/3 HP) | 600 | 1,400 |
| Lights (10 LED) | 100 | 0 |
| Router, TV, laptop | 180 | 0 |
| Total | 2,210 |
Step 3: Add the biggest starting surge
Motors need a short burst of extra power to get going. You don’t add up every starting number, though. You only add the single biggest jump between starting and running watts, because you’ll switch the motors on one at a time.
In this list the sump pump has the biggest jump: 2,150 − 1,050 = 1,100 W.
Starting watts needed = 2,210 + 1,100 = 3,310 W
Step 4: Leave some headroom
A generator shouldn’t sit at 100% for hours. Add about 20% to the running total:
2,210 × 1.2 ≈ 2,650 running watts, and at least 3,310 starting watts
So a 4,000–5,000 W generator covers this house comfortably, with room for the occasional microwave.
The generator size calculator runs these four steps for you if you’d rather not do the math.
Connecting it to the house
Extension cords are fine for a fridge and a few devices. Just use heavy-gauge (12 AWG or thicker), outdoor-rated ones.
For anything hardwired, like the furnace, a well pump or a sump pump on its own circuit, you need a manual transfer switch or an interlock kit on your panel, installed by a licensed electrician.
And please don’t backfeed the house through a dryer outlet with a double-male “suicide cord.” It’s illegal, it can kill utility line workers, and it can start a fire.
Fuel
A 5,000–7,500 W portable generator at half load burns roughly 0.5–0.75 gallons of gas an hour. Run it around the clock and that’s about 12–18 gallons a day. Running it in shifts, say two hours on and two off to keep the fridge cold, cuts that a lot. Dual-fuel models can also run on propane, which keeps for years without going stale.
Or skip the fuel
Large battery systems in the 3,600–6,000 W range can now run 120/240 V circuits through a transfer switch, some well pumps included. They cost more up front, but they’re silent, safe indoors and need almost no maintenance. I compare the two approaches in power station vs. generator.