Generator Power and Sizing Calculator

Running watts and starting watts are two different numbers

Every appliance wears two wattage numbers, and mixing them up is the most common way people buy the wrong generator. Running watts, sometimes called rated watts, are what the appliance pulls steadily while it works. A laptop setup runs around 300 watts, a small chest freezer around 120, and a full size refrigerator around 150 once the compressor is turning.

Starting watts, also called surge or peak watts, are the extra push anything with a motor or compressor needs for one to three seconds at power on. The Department of Energy's appliance energy use guidance explains how to read a nameplate and even measure real draw with a $25 to $50 plug in monitor, which is the only way to know exactly what your own appliances pull. The nameplate number is the running number; the spike is not printed on most labels, which is why sizing tables exist.

The spike is not small. Motors commonly draw two to three times their running wattage at startup, and a locked compressor can pull more. That is why a refrigerator that runs at 150 watts can demand 1,200 for a moment, and why a generator that handles the steady load can still stall the moment the fridge kicks on.

How the sizing math works

The sizing method behind the calculator is the one generator makers publish in their own guides, and it takes about four steps.

Step one, add the running watts of everything you plan to run at the same time. Step two, find the single largest startup spike among your picks, measured as the extra watts above running watts. Step three, add that one spike to the running total, which gives the peak watts the generator has to survive at its worst moment. Step four, round up to the next standard generator class.

You only count the biggest spike, not every spike, because motors almost never start at the exact same instant. Adding every surge together oversizes the purchase by hundreds of dollars. Ignoring the surge entirely undersizes it and trips the breaker the first cold night the furnace fan kicks on.

The running column matters as much as the surge column. A unit with 3,500 surge watts but only 2,800 running watts cannot hold your load if the steady total is above 2,800, no matter how generous its spike rating is.

Worked example: a modest outage load

Take a common winter setup: a refrigerator, a gas furnace blower, five 60 watt light bulbs and a laptop, all running at once.

The running total is 150 + 700 + 300 + 300 = 1,450 watts. Now the startup spikes. The fridge jumps from 150 to 1,200, which is 1,050 extra watts. The furnace blower jumps from 700 to 1,400, which is 700 extra. The lights and laptop have no motors and no spike, so the largest single spike is the fridge at 1,050 watts.

Peak watts are 1,450 + 1,050 = 2,500. On the class table, the 2,200W inverter is too small because 2,500 peak exceeds its 2,200 surge rating, so the pick is the 3,500W class, which holds 2,800 running and 3,500 surge. That tier covers both numbers with room to spare.

Fuel is an estimate built on the load, not the tank size. Portable gasoline generators burn roughly 0.15 gallons per kilowatt hour of load, so 1,450 watts costs 0.15 x 1.45 = 0.22 gallons an hour. Over an 8 hour day that is 1.45 x 0.15 x 8 = 1.74 gallons, or about half a 5 gallon can.

Worked example: a storm that takes out the well

Now a rural household on well water: a refrigerator, a half horsepower sump pump, a third horsepower well pump, a 10,000 BTU window AC, ten 60 watt bulbs, a microwave and an LED TV.

The running total is 150 + 1,050 + 1,250 + 1,500 + 600 + 1,000 + 120 = 5,670 watts. The startup spikes are fridge 1,050, sump pump 1,100, window AC 700 and well pump 2,500, because a 3,750 starting number minus a 1,250 running number is the biggest single jump in the whole table. Peak is 5,670 + 2,500 = 8,170 watts.

That lands past the 7,500W class, so the calculator suggests the 11,000W class, which holds 9,000 running and 11,000 surge. Fuel at this load runs 0.15 x 5.67 = 0.85 gallons an hour, or 0.85 x 8 = 6.80 gallons across an 8 hour day. Two full 5 gallon cans per day is the real budget, which is why load discipline matters more than tank size.

The wattage table behind the calculator

The calculator ships with a default table of typical nameplate values, and you can overwrite any of them. Treat these as starting points, not scripture: a 20 year old fridge can pull double the numbers below.

Anything without a motor, like a space heater, microwave, TV or light bulb, has a starting number of zero, because resistive and electronic loads do not spike. Anything with a compressor, pump or fan does. If your appliance only lists amps, multiply amps by 120 volts for standard outlets, or 240 volts for a dryer or range, to get watts.

Fuel: what an outage costs in gallons

Fuel burn scales with the load you carry, and the estimate in the calculator uses a simple rule: about 0.15 gallons of gasoline per kilowatt hour of load, which matches the run time specs published for midsize portable units. Propane carries less energy per gallon, so the estimate rises to about 0.19 gallons per kilowatt hour, and diesel falls to about 0.14.

Those are estimates, not guarantees. Cold weather, a dirty filter, an older engine or running the unit at a light load all move the number. The practical use of the table is budgeting cans of fuel: an 8 hour day at a modest outage load is one 5 gallon can, while a heavy load can take two.

Store fuel in approved containers, outside living space, and never refuel a hot or running generator. Spilling gasoline on a hot exhaust is its own fire, separate from the CO problem below.

Carbon monoxide: the rule that outranks the math

Nothing in this guide matters more than placement. A portable generator makes carbon monoxide, an odorless gas that kills without warning. The CDC's carbon monoxide guidance is blunt: never use a generator inside your home or garage, even with the doors and windows open, and keep it outside more than 20 feet from any window, door or vent. The CPSC's CO information center adds that one portable generator can produce as much carbon monoxide as hundreds of cars, and that more than 100 US deaths a year are linked to portable generators.

The numbers behind those warnings are grim. CPSC staff counted 900 non fire carbon monoxide deaths tied to engine driven tools from 2011 through 2021, and 770 of them, 86 percent, involved generators. Hundreds of Americans die each year from unintentional non fire CO poisoning in total, and more than 100,000 visit an emergency room, per the CDC.

Three habits carry almost all the protection. Run the unit outside, far from openings, with the exhaust pointed away. Install battery powered CO alarms on every level and near every sleeping area. And never backfeed a generator into a wall outlet, which can electrocute a utility worker down the block. This guide is a sizing and estimating tool, not wiring or installation advice; a transfer switch belongs to a licensed electrician.

Getting the size right without oversizing

Two cheap wins beat buying a bigger machine. The first is staging: start the biggest motor first, alone, then plug items in one at a time. The generator only has to cover one spike at a time, which is exactly what the sizing rule assumes. The second is dropping the loads that do not earn their watts; an electric kettle at 1,500 watts for three minutes is fine, but an electric space heater running all night at 1,800 watts costs more fuel than the fridge, freezer and lights combined.

Buy for the peak, run below the rating. Manufacturers like a generator loaded to about half of its running capacity for long life and clean power for electronics. If your steady total lands at 1,450 watts, a 3,500W class unit loafing at half load is a better buy than a 5,500W unit idling at a quarter load, both for fuel and for the engine.

Check the numbers against the real nameplate on your own appliances before you spend money, and if a load involves wiring into the house panel, stop and call an electrician. The calculator is a planning tool for portable, plug in loads.