What size generator do I need for my house?
Short answer: Add the running watts of everything you will run at once. Then add the starting watts of the single largest motor, which Home Depot estimates at 3 times its running watts. Keep the result under 90% of the generator's rating. Our essentials example (fridge, sump pump, furnace fan, lights, router, microwave) needs about 10,300 W.
How do I calculate what size generator I need?
Four steps, following Home Depot’s sizing guide:
- List the essentials. Write down only what must run during an outage.
- Add the running watts. Use the label or manual for each item. Home Depot warns its own numbers are “general examples.”
- Add the largest starting surge. Motors draw extra power for a moment when they start. Home Depot estimates starting watts at about 3 times running watts. Add this only for the single biggest motor, since motors rarely start at the same moment.
- Leave margin. Home Depot advises using no more than 90% of a generator’s capacity. Divide your total by 0.9.
The formula:
Generator size = (sum of running watts + starting watts of largest motor) ÷ 0.9
Lights and coffeemakers do not need extra starting power, per Home Depot.
Example: sizing a generator for household essentials
Typical values only; check your own labels.
| Item | Running watts | Source of typical value |
|---|---|---|
| Refrigerator/freezer | 800 | Home Depot (top of 600–800 W range) |
| Sump pump | 1,500 | Home Depot |
| Furnace fan (gas furnace) | 875 | Honda (largest listed option) |
| Lights, 6 incandescent bulbs | 360 | Honda (60 W each) |
| Wi-Fi router and modem | 20 | Assumed; check the adapter label |
| Microwave | 1,200 | Home Depot |
| Total running | 4,755 |
The sump pump is the largest motor here. Its estimated starting draw is 1,500 × 3 = 4,500 W.
| Step | Math | Watts |
|---|---|---|
| Running total | sum of table | 4,755 |
| Add largest starting surge | 1,500 × 3 | 4,500 |
| Peak load | 4,755 + 4,500 | 9,255 |
| Stay under 90% of capacity (Home Depot) | 9,255 ÷ 0.9 | 10,283 |
So this list needs a generator rated around 10,300 W. To shrink that, skip the microwave while the pump runs, or use LED bulbs. A modern ENERGY STAR fridge may draw far less; Honda lists one at 192 W. See how many watts a refrigerator uses for details.
The 3× rule is a rough estimate; your appliance’s rated starting watts beat it. To run your own numbers, try the power station size calculator, which uses the same add-up method.
Typical running watts for common appliances
From Home Depot’s guide. Ranges vary by model.
| Appliance | Running watts |
|---|---|
| Central air conditioning | 2,000–4,000 |
| Window air conditioner | 600–1,500 |
| Water pump | 1,000–2,000 |
| Water heater (electric) | 3,000–4,500 |
| Electric range, one element | 2,500 |
| Electric oven | 5,000 |
| Space heater | 1,250 |
| Television | 100–350 |
| Coffeemaker | 400–800 |
Central AC, water heaters, and ranges push a home well past the example. Some, like well pumps, run on 240 V circuits; ask an electrician.
Portable vs inverter vs standby generator
| Type | What it is | Best for |
|---|---|---|
| Portable | Movable, relatively affordable | Emergency power for a few essentials |
| Inverter | Quieter and more fuel-efficient | Sensitive electronics, RVs, lighter loads |
| Standby (whole-house) | Hardwired to the home; uses a transfer switch | Automatic backup for larger loads |
Descriptions follow Home Depot’s guide. For a quiet, fume-free option for small loads, compare a battery unit in power station vs generator.
Do I need a transfer switch?
Yes, if you want to power home circuits. Never plug a generator into a wall outlet. Feeding power back through house wiring is called backfeeding. ESFI warns it can electrocute utility workers, and says never to connect a generator to household wiring without a transfer switch. Have a licensed electrician install a transfer switch or interlock.
Without one, plug appliances into the generator with heavy-duty outdoor extension cords. More options are in power station vs generator.
Home Depot also warns that a generator without a circuit breaker can overload and overheat.
Generator carbon monoxide safety
Generator exhaust contains carbon monoxide (CO). CPSC guidance:
- Run it outdoors only, at least 20 feet from the house.
- Never run it inside a home, garage, basement, crawlspace, shed, or on a porch.
- Keep it away from doors, windows, and vents.
- Install battery-operated CO alarms, or alarms with battery backup, outside sleeping areas.
Home Depot adds that open windows or doors do not make indoor use safe. See carbon monoxide detector placement.
Quick answers
What size generator do I need to run a refrigerator? Home Depot’s example: a 350 W fridge needs 350 + 1,050 = 1,400 W. Check your fridge’s label for its real draw.
Is a 5,000-watt generator enough for a house? For a short list, yes. Our example needs about 10,300 W, so trim or stagger loads.
Can a portable generator run central air? Home Depot lists central AC at 2,000–4,000 running watts, plus starting surge. That usually calls for a large or standby unit; confirm with an electrician.
Can I run a generator in my garage? No. CPSC says never run one in a garage, and Home Depot adds that open doors or windows do not make it safe.