Generator Size Calculator: Running Watts, Starting Watts and Fuel
Total running watts and the largest motor on the list, and it returns the starting watts the generator has to make. The surge is added to everything else already running, not to the whole load twice, which is the error that sells people a generator two sizes too big.
When the figures look right, the generator sizing list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- Total running watts3500 W
- Largest motor, running watts800 W
- Starting multiplier3x, standard induction motor
- Reserve headroom20 %
- Supply voltage240 V
- FuelGasoline
- Tank size5 gal
Generator to buy5,100 starting watts
| Running watts | 3,500 Weverything on at once |
|---|---|
| Largest motor, running | 800 W |
| That motor starting | 2,400 W3x its running draw |
| Peak starting watts | 5,100 Weverything else running while it starts |
| Running watts with reserve | 4,200 W20 percent headroom |
| Amps at 240 V | 17.5 Aat the running load with reserve |
| Inlet and transfer switch | 20 Anext standard rating above 17.5 A |
| Fuel burn | 1 gal/hour3.5 kWh from a gallon of gasoline |
| Runtime on the tank | 5 hours5 gal at the running load |
| Fuel for 24 hours | 24 galone full day of outage |
The material list this produces
| Generator, starting watts | 5,100 |
|---|---|
| Transfer switch, inlet and cord rating, amps | 20 |
| Fuel for 24 hours, gal | 24 |
What this calculator assumes
- A generator is sized on the worst moment, not the average one. That moment is everything else already running while the largest motor starts, which is why the peak here is the total running watts with the largest motor swapped for its starting draw rather than the sum of every starting watt on the list.
- Starting multipliers are the common ranges, not a spec. An inverter or ECM motor starts soft at about twice its running draw, a standard induction motor takes about three times, and a well pump or an air conditioning compressor can take four or more. The nameplate on the appliance beats all three.
- Reserve headroom is there because a generator run at its rated ceiling all night is a generator that does not last. Twenty percent is the common allowance and the running number on this page already includes it.
- Fuel is figured from how much electricity a gallon actually produces in a small air cooled set, which is about 3.5 kilowatt hours for gasoline, 2.6 for propane and 4.5 for diesel. Those are real world figures rather than the energy in the fuel, which is several times higher.
- A generator runs outdoors only, well clear of doors, windows and vents, and never in a garage with the door open. Carbon monoxide from portable generators kills people every storm season and no amount of ventilation makes an enclosed space safe.
- Anything permanently wired to the house needs a transfer switch or an interlock. Backfeeding a panel through a dryer outlet energizes the utility line outside and is how linemen get killed, so that part is an electrician's job and not a weekend one.
Starting watts is not running watts plus the surge
Take 3,500 running watts with an 800 watt motor in it. That motor starts at three times its running draw, so 2,400 watts. The peak is everything else running while it starts: 3,500 minus 800 plus 2,400, which is 5,100 watts.
Adding the surge on top of the whole load gives 5,900 instead, and that is the common mistake. The motor's own running watts are already inside the 3,500, so counting them again once at 800 and once inside the 2,400 buys 800 watts of generator nobody needs.
Only one motor is assumed to start at a time, which is the normal case and the assumption every sizing chart makes. If a well pump and a compressor genuinely cut in together, take the two surges and add them by hand, because that is a different and much larger machine.
One motor decides the whole size
Keep the running load at 3,500 watts and swap the 800 watt motor for a 2,000 watt well pump at four times. The pump starts at 8,000, the peak becomes 9,500, and a generator that was comfortable at 5,100 is now nearly twice the machine.
Nothing else about the house changed. The lights, the fridge and the furnace are drawing exactly what they were. That is the whole reason this page asks for the largest motor separately instead of taking one total and multiplying it by a rule of thumb.
The multiplier is what to argue about. An inverter or ECM motor starts at about two times, a standard induction motor at three, and a well pump or an air conditioning compressor at four. A soft start kit on the compressor pulls that four down toward two, and on a borderline job it is cheaper than the next generator up.
Amps, the inlet and the transfer switch
Amps come off the running load with its reserve, not off the surge, because the surge lasts a second or two and the wire never gets warm. At 4,200 watts and 240 volts that is 17.5 amps, so the inlet and the transfer switch land on the next standard rating up, which is 20.
The same 4,200 watts at 120 volts is 35 amps, which is past what a normal cord and receptacle should carry, and the page warns about it. That is the practical reason a whole house generator feeds at 240 and a 120 volt cord runs one tool at a time.
Never backfeed a dryer outlet. The generator has no idea the utility is off, and whatever it pushes back through the meter goes out onto lines a lineman is working on. An interlock or a transfer switch is the cheap part of this job and it is not optional.
Fuel is where the plan usually fails
A gallon of gasoline gives a generator about 3.5 kilowatt hours of real output. At 3,500 running watts that is one gallon an hour, so a 5 gallon tank runs 5 hours and one day of outage takes 24 gallons. That is five jerry cans nobody bought before the storm.
Propane gives about 2.6 kilowatt hours a gallon, so the same load burns 1.35 gallons an hour and a 20 gallon tank runs 14.9 hours. A 20 pound barbecue cylinder holds 4.7 gallons, which is under four hours. Propane keeps for years though, and gasoline in a can does not.
Diesel is the efficient one at about 4.5 kilowatt hours a gallon, 0.78 gallons an hour on that load. Whichever fuel it is, the generator runs outdoors and well away from any window or door, because carbon monoxide from a generator in a garage kills people every single storm.
Questions people ask
What size generator do I need?
Enough starting watts for your running load with the largest motor surging. A 3,500 watt load with an 800 watt induction motor peaks at 5,100 watts. Swap that motor for a 2,000 watt well pump and the same load peaks at 9,500.
How do I calculate starting watts?
Take the running total, subtract the largest motor's running watts, then add that motor's starting watts. Do not add the surge on top of the whole load, because the motor's running watts are already counted inside the total.
How much fuel does a generator use per hour?
About a gallon an hour at 3,500 watts on gasoline, which gives roughly 3.5 kilowatt hours a gallon. Propane is about 2.6 and diesel about 4.5, so the same load burns 1.35 gallons an hour on propane and 0.78 on diesel.
Do I need a transfer switch for a generator?
Yes, a transfer switch or an interlock. Backfeeding through a dryer outlet sends power out onto the utility lines and can kill a lineman. Size the switch off the running amps, so 17.5 amps at 240 volts takes a 20 amp rating.
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