The four steps
Step 1 — write the list, and keep it short
Not everything in the house. The things that genuinely must keep running. For most households that is four or five items:
- Refrigerator, or refrigerator and freezer
- Furnace fan, if you heat with gas or oil in a cold climate
- Sump pump, if you have a basement
- Well pump, if you are not on municipal water
- Lighting and a router
Everything else — the television, the dishwasher, the oven — is a preference, not a requirement, and it belongs in a second list you power only when the first list is off.
Step 2 — find each item's running and starting watts
Two sources, in order of authority. First, the nameplate on your own appliance — a sticker inside the door, on the back, or on the motor, giving volts and amps. Watts = volts × amps, and that figure beats any chart.
Second, when there is no accessible nameplate, the published wattage chart, which carries Champion Power Equipment's and Honda's figures for everything in a normal house.
Step 3 — add the running watts
Straight addition of everything on list one. This is the number that decides whether the machine can carry your house continuously.
Step 4 — add the largest single surge
This is the step people get wrong. Take the item with the largest starting-watts figure, subtract its running watts from your total, and add its starting watts instead.
Peak = total running watts − (biggest item's running watts) + (biggest item's starting watts)
You do not add every appliance's surge. Motors almost never start at the same instant, and summing all the surges produces a number two or three times larger than the house needs — which is exactly how people end up with a machine they cannot lift and cannot fuel.
Three worked examples
A: apartment or small house, no basement
- Refrigerator — 400W running, 1,200W starting
- Six LED bulbs — 90W
- Router and modem — 20W
- Laptop and monitor — 100W
Running total 610W. Largest surge is the fridge at 1,200W. Peak = 610 − 400 + 1,200 = 1,410W.
This household should not buy a generator. A 1,152Wh battery with an 1,800W inverter covers it silently, indoors, with no fuel — see the power station picks.
B: family house with a basement and gas heat
- Refrigerator — 400W running, 1,200W starting
- Furnace fan, ½ HP — 800W running, 1,600W starting
- Sump pump, ⅓ HP — 800W running, 1,600W starting
- Lighting and electronics — 150W
Running total 2,150W. Largest surge 1,600W (furnace fan or sump pump). Peak = 2,150 − 800 + 1,600 = 2,950W.
A 3,500W generator covers this. A 5,300W machine gives comfortable headroom for a microwave on top — see generators under $1,000.
C: rural house on a well, with summer cooling
- Refrigerator — 400W running, 1,200W starting
- Well pump, 1 HP — 2,000W running, 4,000W starting
- Window AC, 10,000 BTU — 1,200W running, 2,400W starting
- Furnace fan, ½ HP — 800W running, 1,600W starting
- Lighting and electronics — 150W
Running total 4,550W. Largest surge 4,000W (well pump). Peak = 4,550 − 2,000 + 4,000 = 6,550W.
This needs 7,500W or more — see the larger machines. The well pump is doing most of the damage, which is worth knowing: sequencing the pump away from everything else meaningfully reduces the requirement.
The two mistakes that inflate the answer
Adding every starting surge. This is the big one and it typically doubles the answer. Example C summing all four surges gives 9,200W instead of 6,550W — a machine two sizes larger, considerably heavier, and burning more fuel every hour to do identical work.
Sizing for everything at once. In a real outage you do not run the well pump, the air conditioner, the furnace and the microwave simultaneously. Deciding in advance what shares the machine — load shedding — is free capacity. Example C run as “well pump or air conditioning, never both” drops from 6,550W to about 4,150W, which is a different and much cheaper machine.
Then check the duration
The wattage number tells you how big. It does not tell you what kind — and that is decided by how long your power is off, not by watts.
A 2,000W requirement for four hours is a battery. The same 2,000W requirement for three days is a generator. Same number, completely different purchase. The duration page settles that half, and it is the half almost every other sizing guide skips.
What about the 240V question?
Most household circuits are 120V. A few large loads are 240V: an electric range, an electric water heater, a well pump, central air conditioning, an electric dryer.
If anything on your list is 240V, the generator must supply 240V — look for an L14-30R or 14-50R outlet and a 120/240V selector switch. And the appliance must be fed through a transfer switch or an interlock kit, because 240V loads are hard-wired and no extension cord reaches them.
Note also that a machine's rated watts may be split across its two 120V legs, so a 5,000W generator does not necessarily deliver 5,000W to one 120V circuit. The watt meters on a transfer switch exist precisely so you can balance the load rather than guess.
