How generator sizing actually works
Ask a generator dealer what size you need and you will get a good answer. Ask the internet and you will mostly get a shrug in a nice font: it depends on your needs, every home is different, contact us for a free assessment. You came here for a wattage, so here is exactly how this page produces one.
Two numbers matter, and almost every appliance has both. Running watts are what it draws once it has settled down. Starting watts, also called surge watts, are the much larger gulp that anything with a motor takes for the first second or two as the motor breaks free and gets up to speed. A refrigerator that hums along at 200 watts will ask for 800 for a moment when the compressor kicks in. A sump pump that runs at 950 wants 1,800 at the instant the float rises.
The rule that turns those two numbers into one answer is short, and it is the thing this page exists to get right:
Add up the running watts of everything you want on at once. Then add the single largest starting surge on the list. Not all of them. One.
Motors do not start in unison. A fridge compressor starts when its thermostat closes. A sump pump starts when water reaches a float. A furnace blower starts when the house gets cold. Those events are seconds or minutes apart, and the chance of two landing in the same half second is small enough that the entire industry sizes on one surge. Add every surge together and you will be sold a generator roughly twice the size you need, which costs more, weighs more, drinks more fuel and is louder for the whole outage.
The opposite error is worse. Ignore the surge entirely, size on running watts alone, and the generator will run happily for an hour and then stall the first time the pump starts, usually in the middle of the night, usually while the basement is filling.
The formula
peak demand = running total + the ONE largest starting surge
capacity needed = running total × 1.20, then ÷ any fuel or altitude derate
size = the smallest real generator class whose running rating clears the capacity
AND whose starting rating clears the peak demand
The 1.20 is headroom. An engine run at 100 percent of its running rating all night runs hot, drinks fuel and wears out early, so the working rule is to keep the steady load near 80 percent of the rating. We apply that to the running requirement only, and deliberately not to the surge, because a generator's starting rating is already a short duration figure. Multiplying both is double counting, and it is why some calculators recommend absurd sizes.
The derate is the part nobody mentions until the generator arrives. Propane costs you about 10 percent of the nameplate output, and a naturally aspirated engine loses about 3.5 percent of its power for every 1,000 feet of elevation. In Denver at 5,280 feet that is 18 percent of the machine you paid for, gone before you plug anything in.
The size classes are real products. Each one on this page is anchored to a unit with a published spec sheet, which is where the tank capacity and the run time come from. There is no point rounding up to a size nobody sells.
Worked example
Riding out an outage: a fridge, a freezer, a sump pump, the furnace blower and ten lights.
Running watts first. The refrigerator is 200, the freezer 150, the sump pump 950, the furnace blower 600, and ten LED bulbs are 120. That is 2,020 running watts (1,380 to 2,900 across the published spread for those items).
Now one surge. The sump pump runs at 950 and starts at 1,800, so it needs 850 watts more than it will use a second later, and that is the biggest jump on the list. The furnace blower is next at 700. Peak demand is 2,020 + 850 = 2,870 watts.
Add every surge instead, the way most pages do it, and you get 4,620 watts. That is the difference between a 3,500 watt generator and a 5,000 watt one, on an identical list of appliances.
With the 20 percent headroom, 2,020 running watts wants 2,424 watts of capacity, and 2,870 of peak has to fit a starting rating. The smallest standard class that clears both is a 3,500 running / 4,500 starting watt generator, a midsize inverter. It leaves about 897 watts of comfortable headroom, which is a coffee maker and not much else.
Run time: 2,020 watts is 58 percent of that machine's rating, and at that load it burns about 0.27 gallons an hour, so a 2.3 gallon tank lasts about 8 hours 24 minutes. Around the clock that is 6.6 gallons a day and just under three fill-ups. Two five gallon cans get you through a long night and most of the next day.
Move the same list to Denver and switch to propane and the picture shifts: the derates turn 2,424 watts of requirement into 3,304, which still fits the 3,500 class but only just, and a 20 pound propane cylinder lasts about 12 and a half hours.
The surge, and why one is the right number
This is the part that separates a real answer from a guess, so it is worth seeing the arithmetic laid out.
- Refrigerator: 200 running, 800 starting, so the surge costs an extra 600 watts.
- Freezer: 150 running, 600 starting, an extra 450.
- Sump pump, 1/2 HP: 950 running, 1,800 starting, an extra 850.
- Furnace blower, 1/2 HP: 600 running, 1,300 starting, an extra 700.
- Air compressor, 1 HP: 1,500 running, 4,500 starting, an extra 3,000. Compressors are the worst offenders on any list.
- Central AC, 3 ton: 3,300 running, 5,500 starting, an extra 2,200.
- LED bulbs, kettles, heaters, chargers, televisions: no surge at all. Anything that only makes heat or light draws the same wattage the instant it switches on as it does an hour later.
The wrong method adds all of the extras. The right method adds the largest one. On a jobsite list of a circular saw, a table saw, an air compressor, two work lights and a charger, the wrong method comes to 12,360 watts, which points at a permanently installed home standby generator, and the right method comes to 8,960, which is a 7,500 watt portable you can put in a truck. Same tools, same day, a different order of magnitude in money.
Two honest caveats, because we would rather say them than have you discover them. First, we assume everything on your list runs at the same time, with no credit for the fact that a fridge compressor is only actually running about a third of the hour, because the one minute it decides to run is the minute the pump also wants to start. Second, two motors genuinely can start in the same second. That is what the headroom is for, and a generator meeting a double start does it with an audible stumble and a recovery rather than a shutdown.
If an air conditioner is the item setting your surge, there is a cheaper answer than a bigger generator. A soft start module fitted to the compressor cuts its inrush by roughly half to two thirds. Reported real installs have taken a 2 ton unit from 62 amps of inrush down to 25, which is the difference between needing a 12,000 watt machine and needing a 7,500 watt one.
Safety, which matters more than the wattage
Two things about generators kill people every storm season, and neither of them has anything to do with picking the right size. They are worth more of your attention than everything else on this page.
Carbon monoxide. A portable generator is an engine, and an engine's exhaust contains carbon monoxide in quantities that will kill a family in a closed space in under an hour. The gas has no smell, no taste and no colour, and the early symptoms are a headache and sleepiness, which is exactly what you would expect to feel at 3 a.m. during a power cut. An average of nearly 100 people a year die of it in the United States from portable generators alone.
Run it outdoors only. Never in a garage, a basement, a crawl space, a shed or a carport, and not on a porch. An open garage door is not ventilation, and this is the single most common fatal mistake: CPSC testing found a generator sitting 10 feet from a house drove carbon monoxide inside above 200 parts per million, a level that can cause unconsciousness. The guidance is at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents, and windows near it closed. Fit battery powered CO alarms on every level of the house and outside the bedrooms.
If you are buying, buy a generator with a CO shutoff sensor, which turns the engine off when carbon monoxide builds up around it. Two voluntary standards cover this, ANSI/PGMA G300 and UL 2201, and the CPSC's own modelling put the death prevention rate at roughly 87 percent for the first and close to 100 percent for the second. There are not many safety features you can buy that carry numbers like that.
Backfeeding. The other way people die is at the far end of the street. Running a cord from the generator into a wall outlet or a dryer outlet, using a cord with a plug on both ends, sends power backwards through your house wiring, out through the meter, and up the utility transformer, which steps it back up to thousands of volts on a line a crew may be working on to restore your power. It also bypasses the protection your wiring was designed around, and puts live prongs on the end of a cord in your hand. It is against the electrical code, it is a fire risk, and it can kill somebody you will never meet.
The correct way to feed your house panel is a transfer switch or a panel interlock kit, which physically prevents the generator and the utility being connected at the same time. That is what the code requires and it is an electrician's job, not a weekend one. Everything else runs on heavy gauge extension cords straight from the generator, which is a perfectly respectable way to get through an outage and how most people do it.
Three smaller ones while we are here. Let the engine cool before refuelling, because petrol spilled on a hot muffler is how generators catch fire. Keep it dry and out of standing water, under an open canopy rather than a tarp draped over it. And run it once every couple of months, because the single most common generator failure is a machine that has not been started since the last storm.
What each size class really covers
Every class below is anchored to a real product with a published spec sheet, so the tank sizes and run times are somebody's warranty rather than our arithmetic.
- 1,800 running / 2,200 starting watts. A small inverter you can carry in one hand, about 0.95 gallons of fuel. A fridge, a laptop, the wifi and a few lights, one at a time. Quiet enough to camp beside. It will not start a well pump.
- 3,500 / 4,500 watts. A midsize inverter, about 2.3 gallons. This is the honest smallest size for riding out a home power cut: fridge, freezer, sump pump, furnace blower and lights, with a little left over. Usually 120 volt outlets only.
- 5,000 / 6,250 watts. A large portable, about 4.2 gallons. All of the above plus a well pump or a window air conditioner. Around here you start to see a 240 volt outlet, which is what a well pump, a range or a dryer needs.
- 7,500 / 9,500 watts. The usual home backup size, about 6.6 gallons and around 11 hours at half load. Essentials, a well pump, and one large 240 volt load at a time. This is the class most people who wire in a transfer switch end up buying.
- 9,500 / 12,000 watts. The largest portable class, and it weighs about 220 pounds, which is a wheel kit and two people rather than a thing you carry. Most of a house except central air conditioning and electric heat.
- 14 kW to 26 kW home standby. Permanently installed, wired to the panel through an automatic transfer switch, fed by natural gas or a propane tank, and it starts itself when the power fails. 22 kW is where most homes land: a whole house including central air, assuming you are sensible about the dryer and the range.
One more note on the ladder. Below about 5,000 running watts most generators have 120 volt outlets only, and a well pump, a central air conditioner, an electric range, a dryer and an electric water heater are all 240 volt loads. All the watts in the world will not help if there is no 240 volt outlet on the panel, so check the outlet strip on the actual model, since a few units in the 3,500 to 4,500 class do offer one and a few larger ones do not.
Fuel, run time, and where our numbers come from
Most run times on the internet are invented. Ours are bent out of published spec sheets, and the method is worth a paragraph because it produced a result we did not expect.
Fuel burn is not proportional to load. An engine drinks even when it is barely working, so halving the load buys you about 50 percent more run time rather than double. To pin down how much, take the two run times Honda publishes for one small generator: 3.2 hours at full load and 8.1 hours at a quarter load, on the same 0.95 gallon tank. Fit a straight line through those two points and the idle portion comes out at 0.24 of the full load increment.
Then check that curve against an engine we did not fit it to. Generac publishes 10 hours at half load and 15 hours at a quarter for its 8,000 watt portable. Our curve, built entirely from Honda's numbers, predicts 15.1 hours. Two manufacturers, two completely different engines, the same shape to within one percent, even though the Generac burns about a quarter more fuel per hour than the curve's level would suggest. So the shape of the fuel curve is a property of engines in general and the level is a property of each model, which is why this page takes each size class's own published run time and bends it to your load rather than guessing.
A few practical figures that fall out of it. A generator carrying a typical essentials load burns roughly a quarter to a third of a gallon an hour, so plan on 6 to 8 gallons a day if you run it around the clock, and rather less if you shut it off overnight, which most people do. Two five gallon cans is a realistic overnight supply for a midsize unit and about half a day for a big one.
Propane costs you twice. Output drops about 10 to 15 percent (Champion publishes 3,500 watts on gasoline and 3,150 on propane for the same machine, exactly a tenth, while DuroMax publishes 9,500 and 8,075, which is nearer a seventh), and the volume burns faster. Champion's own spec sheet gives 14 hours on 2.3 gallons of gasoline and 21 hours on a 4.6 gallon propane cylinder at the same load, which works out at 1.33 gallons of propane for every gallon of gasoline. Interestingly the energy content alone only predicts 1.25, so the engine is slightly less efficient on gas as well as the fuel being less dense. What you get in return is real: propane does not go stale, so a cylinder in the garage is still full of fuel in three years, while untreated gasoline is unusable in about 30 days.
Natural gas means a permanently installed standby unit, and its run time is the whole point: as long as the gas main flows. A 22 kW unit at a typical house load burns roughly 280 cubic feet an hour, which is a line on a utility bill rather than a trip to a filling station in the rain. On propane the same unit is about 3.2 gallons an hour, so a 500 gallon tank (usable fill about 400 gallons) is roughly five days.
The gap a generator cannot cover, and what does
Even a standby unit that starts itself takes 10 to 30 seconds to come up and take the load, and a portable takes as long as it takes you to find your shoes. Anything that must not blink in that window (a desktop mid-document, a NAS mid-write, a router that takes four minutes to negotiate a connection) wants a small battery backup in front of it, and that is a different sizing question with a different answer. Our UPS runtime calculator does that side of it.
One thing to know if you are putting the two together. A UPS is advertised in volt-amperes, not watts, and on consumer units the watt rating is only about 0.6 times the VA number, so a 1500 VA box really offers around 900 watts. That is the same trap as a generator's starting rating in the headline: the bigger number is the one on the label, and the smaller one is the one that binds. Also worth knowing: some cheaper UPS units dislike the electrical output of a cheap open frame generator and will keep flicking to battery all evening. An inverter generator produces cleaner power and gets on with them, which is one of the better arguments for paying for one.
Four numbers that are easy to read the wrong way
We read a lot of sizing guides to build this one, including several published by companies that sell generators. Four figures come up constantly where the number in front of you is not quite the number you need, and once you can spot them the whole exercise gets easier.
The surge column is not meant to be added up. It is the single most expensive misreading in generator sizing, and an easy one to make, because a column of numbers really does look like it wants a total. Total it and you land 50 to 100 percent above what you need on a normal household list. The rule two independent industry sources state, one of them as a formula, is the sum of all continuous loads plus the single highest surge, because motors do not all start in the same instant. That is the rule this page uses, and it is the one that saves you a size class.
Refrigerator wattages date faster than the charts. Plenty of charts still print 700 watts running, which was honest when they were written. A refrigerator sold this decade draws roughly 100 to 250 watts while the compressor runs, and an Energy Star model uses 1 to 2 kilowatt hours across a whole day. We publish 200. If your fridge is older than the century, 400 to 700 is the right figure and the old charts have you covered.
Microwave watts are cooking watts. The number on the front of the door is the cooking output, which is genuinely the useful number when you are following a recipe. It is not what the machine pulls from the wall. A 1,000 watt microwave draws about 1,500, and on a small generator that gap is the difference between reheating soup and tripping the breaker.
Generators are advertised on the starting rating. That is the bigger of the two numbers on the box, so a "9,500 watt" generator is usually a 7,500 watt one in continuous terms. The starting rating is real, it is just good for a couple of seconds. Size on the running rating and treat the peak figure as your surge allowance rather than as capacity, and the machine will do everything the box promised.
One more thing, and this one is not anybody's fault. Published fuel tables are not tidy. In the standby range a 26 kW unit burns less at half load than a 22 kW one, because it is a different engine with different efficiency, not because somebody made a mistake. We print the tables as published rather than smoothing them into a graph that looks more convincing than the data.
What this page cannot tell you
It sizes for wattage. That is a genuinely useful and genuinely limited thing, so here is the boundary.
It does not know whether your panel, your cords or your transfer switch can carry what you are about to ask of them, and those are electrician questions with real answers. It does not know your particular appliances: a nameplate is on the back of every one of them and beats any chart, including ours. It does not do a code compliant load calculation of the sort a permanently installed standby generator needs, which considers your service size and your local code rather than a list of appliances.
It also does not know how you intend to live during the outage. A generator only cares what is running in the same instant, so staggering is free and it works: heat the water in the morning, cook at midday, dry the clothes when the power comes back. Most whole house installations quietly assume exactly that, and the honest version of a whole house answer says so out loud.