What size generator do I need?
Sized by the electrical load you want to run, not by the size of your house. Enter your details and the answer appears with every step of the arithmetic shown, so you can check it against any quote you have been given.
This is an estimate from the standard industry rule of thumb, not an engineering calculation. For anything you are about to spend money on, have the figure confirmed by a load calculation.
How generator capacity is worked out
Add up the running watts of everything you want on at once, then add the largest single starting surge on top of that. Motors — fridges, well pumps, air conditioners, furnace blowers — draw two to three times their running watts for a second or two at startup, and a generator that cannot supply that surge stalls even though its continuous rating looks adequate.
The calculation is shown step by step under the result rather than hidden behind the answer. That is deliberate: most sizing guides online are lead capture forms that give you a number and a phone call, and a number you cannot check is a number you cannot argue with when a contractor disagrees. Every adjustment is listed, so you can see what changed the answer and by how much.
Nothing you type is sent anywhere. The arithmetic runs in your browser, there is no form to fill in, and no one calls you afterwards. That is the whole reason this page exists — you should be able to what size generator do i need without trading your phone number for it.
What going too big or too small actually costs
A generator far larger than the load runs inefficiently at light load and, on diesel units, can wet-stack — unburnt fuel building up in the exhaust. It also burns fuel roughly in proportion to its size, which matters during a multi-day outage when fuel is the constraint.
An undersized generator trips its breaker, or stalls when the fridge compressor kicks in, or brownouts the circuit and damages the electronics on it. This is the failure mode that costs real money.
Both directions cost money, which is why it is worth ten minutes to what size generator do i need before you take a quote at face value. The asymmetry is worth knowing though: for most equipment on this site, being slightly under is a comfort problem you notice on a handful of extreme days a year, while being significantly over is a permanent efficiency and equipment-life problem you pay for every month.
Running watts and starting watts
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator or freezer | 700 | 2,200 |
| Furnace blower (1/2 hp) | 800 | 2,350 |
| Well pump (1/2 hp) | 1,000 | 2,100 |
| Sump pump (1/3 hp) | 800 | 1,300 |
| Window air conditioner (10,000 BTU) | 1,200 | 3,600 |
| Central air conditioner (2 ton) | 3,800 | 7,000 |
| Lights throughout the house (LED) | 200 | 200 |
| Microwave | 1,000 | 1,000 |
| Coffee maker | 1,000 | 1,000 |
| TV, router and phone charging | 300 | 300 |
| Washing machine | 1,200 | 2,300 |
| Electric clothes dryer | 5,400 | 6,750 |
| Electric water heater | 4,000 | 4,000 |
| Portable space heater | 1,500 | 1,500 |
| Garage door opener | 875 | 2,350 |
These are typical mid-range figures and your specific appliance will differ, sometimes substantially — check the plate on the back before you commit. The pattern holds regardless: anything with a motor draws two to three times its running watts for a moment at startup, and anything that makes heat draws a lot continuously and does not surge at all.
Where a rule of thumb stops being enough
Every number on this page comes from the standard industry rule of thumb, and a rule of thumb is not an engineering calculation. The real method is a Manual J load calculation, which models your wall and attic insulation, your window area and orientation, how leaky the building envelope is, and how much your ducts lose. It routinely lands a size away from any square-footage estimate — in either direction, which is the part people forget.
So treat this as a sanity check rather than a specification. Ten minutes to what size generator do i need tells you whether a quote is in the right neighbourhood, which is genuinely useful — it is enough to catch a contractor who has quoted you two sizes up because that is what was on the truck. It is not enough to order equipment from.
Running watts and starting watts are both printed on the generator, and the second number is the one people forget to check. Size against your simultaneous load rather than the sum of everything you own — you are unlikely to run the well pump, the air conditioner and the microwave at the same instant, and planning for that costs you a much larger machine.