Adds the surge correctly — running watts plus the single largest motor start, not every starting watt added together, which is what makes most estimates twice the size they need to be.
Tick everything that might run at the same time during an outage. That is usually far less than everything you own.
%
Generators should not run at full output continuously. 20% headroom keeps it in its efficient range and leaves room for something you forgot.
Generator size to buy
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Total running watts—
Largest starting surge—
Peak demand—
Loads selected—
Running amps at 120 V—
Running amps at 240 V—
Recommended in kW—
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The calculation
Two numbers matter and they are not the same. Running watts is what everything draws continuously. Starting watts is the brief surge a motor pulls as it spins up, typically two to three times its running draw.
The mistake almost every estimate makes is adding every appliance's starting watts together. Motors do not all start at the same instant, and if they did the generator would stall regardless. The correct figure is total running watts plus the single largest surge:
Fridge (700 W), lights (400 W), TV and router (300 W) and a furnace blower (800 W) run at 2,200 W. The largest surge above its own running draw is the furnace blower at 2,350 − 800 = 1,550 W. Peak demand is therefore 2,200 + 1,550 = 3,750 W, and with 20% headroom you want a 4,500 W generator — not the 9,000 W a naive sum would suggest.
Appliance
Running
Starting
Refrigerator
700 W
2,200 W
Sump pump ½ hp
800 W
2,000 W
Well pump 1 hp
1,000 W
3,000 W
Furnace blower
800 W
2,350 W
Central AC 3 ton
3,500 W
10,500 W
Electric water heater
4,500 W
4,500 W
Resistive loads — heaters, water heaters, ranges, kettles — have no surge at all. Their starting and running watts are identical, which is why they inflate the running total but never the peak.
The exercise is really about deciding what you are willing to live without for a few days. Backing up a refrigerator, some lights, a furnace blower and phone charging is a modest job that a mid-size portable handles easily. Backing up central air conditioning and an electric water heater is a fundamentally different, and far more expensive, proposition.
Stagger the loads
You do not have to run everything at once. Running the well pump and the washing machine at different times, rather than simultaneously, can drop the generator you need by a whole size class. Most people naturally do this during an outage anyway.
Connect it safely
This is the part that genuinely matters. A generator must connect to house circuits through a transfer switch or a listed breaker interlock, installed by a qualified electrician. Plugging a generator into a dryer or range outlet — backfeeding — energises the utility lines outside your house and can kill line workers restoring power. It also bypasses every protective device in the panel.
Run the generator outdoors only, well away from windows, doors and vents. Carbon monoxide from a generator in a garage, even with the door open, kills people every year. Models with a CO shutoff sensor are worth the small premium.
Portable, inverter or standby
Conventional portables give the most watts per dollar and the most noise. Inverter generators are quieter, more fuel-efficient at partial load and produce cleaner power for electronics, at maybe twice the cost per watt. Standby units start automatically, run on natural gas or propane and require professional installation — worth it above roughly 12 kW or where outages are frequent.
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Frequently asked questions
What size generator do I need for my house?
For a fridge, lights, furnace blower and electronics, roughly 4,000 to 5,000 watts. Adding a well pump or window AC pushes it to 7,000 to 9,000. Backing up central air conditioning and electric water heating generally needs 12,000 watts or more, which usually means a standby unit.
Do I add up all the starting watts?
No, and this is the most common sizing mistake. Motors start at different moments, so the correct figure is total running watts plus the single largest starting surge above its own running draw. Adding every surge together typically doubles the generator size you think you need.
What size generator runs a refrigerator?
A refrigerator draws about 700 running watts but surges to roughly 2,200 when the compressor starts, so a generator needs at least 2,200 watts of surge capacity for the fridge alone. A 3,000 to 3,500 watt unit runs a fridge plus lights and small electronics comfortably.
Can I plug a generator into a wall outlet?
No. Backfeeding through a dryer or range outlet energises the utility lines outside your home and can electrocute line workers, as well as bypassing your panel's protection. Use a transfer switch or a listed breaker interlock installed by a qualified electrician.
How much headroom should I leave?
About 20%. Generators are not designed to run at full rated output continuously, and staying below that keeps the engine in its efficient range, reduces noise and wear, and leaves capacity for a load you did not plan for.