How it works
A generator has two ratings. Running watts is what it can supply continuously; starting (peak) watts is the short burst it can deliver when a motor starts. Fridges, pumps, furnace fans and air conditioners briefly draw two to three times their running watts as they start.
We add up the running watts of everything you want on at the same time, add 10% headroom, and then add the single biggest start-up surge, because motors rarely start at exactly the same moment. If you start your largest motor first and switch other things on one at a time, that peak is all the generator has to cover.
The formula
running watts needed = sum of running watts × 1.1 starting watts needed = sum of running watts + biggest (starting − running)
Worked example
Essentials for a winter outage: refrigerator or freezer, sump pump (1/2 hp), furnace fan (1/2 hp, gas furnace), led lights (about 10 bulbs), wi-fi router and modem and phones and laptop charging.
| Load | Running | Starting |
|---|---|---|
| Refrigerator or freezer | 700 W | 2,200 W |
| Sump pump (1/2 HP) | 1,050 W | 2,150 W |
| Furnace fan (1/2 HP, gas furnace) | 800 W | 2,350 W |
| LED lights (about 10 bulbs) | 100 W | 100 W |
| Wi-Fi router and modem | 20 W | 20 W |
| Phones and laptop charging | 100 W | 100 W |
| Running total | 2,770 W | |
| With 10% headroom | 3,047 W | |
| Starting: 2,770 + 1,550 (furnace fan (1/2 hp, gas furnace)) | 4,320 W |
So look for a generator rated at least 3,100 running watts and 4,320 starting watts. A common 3,500 W portable usually meets the running figure; check its peak rating covers the start-up.
Frequently asked questions
What size generator do I need to run a house?
For essentials (fridge, sump pump, furnace fan, lights, internet) most homes need about 3,500 to 5,000 running watts. Adding a well pump, window AC or electric cooking pushes it to 7,500 to 10,000 W. Running central air, an electric water heater and a dryer together usually means a standby generator of 20 kW or more.
What is the difference between running and starting watts?
Running watts is the power an appliance uses once it is going. Starting watts is the brief surge a motor needs to start, often 2 to 3 times higher and lasting a second or two. A generator must cover both.
Can a portable generator run central air conditioning?
Sometimes, but central AC compressors have large start-up surges and many need 7,500 W or more. A soft-start kit can cut the surge substantially. Check the compressor's nameplate and ask an HVAC technician before relying on it.
Do I need a transfer switch?
To power anything hard-wired (furnace, well pump, lights on a circuit) you need a transfer switch or an approved interlock, installed by a licensed electrician. Never plug a generator into a wall outlet: it can kill utility workers and damage your home's wiring.
Where should I run a generator?
Outdoors only, at least 20 feet from the house with the exhaust pointing away from doors, windows and vents, and never in a garage, even with the door open. Fit battery-powered carbon monoxide alarms inside.
Sources
- Appliance running and starting watts: typical figures from generator manufacturers' wattage guides; check your appliance labels.
- US CDC, carbon monoxide poisoning prevention (generator placement).
Related guides
Estimates are for planning only and are not financial advice. Get written quotes before you buy.