Starting Watts vs Running Watts

Add every running watt, then one surge

Total up your running watts. Then add one starting surge: the largest single start-up gap on your list. That is the entire rule, and it is exactly what the free watts calculator on the front page does with whatever equipment you give it.

The word "gap" is doing real work there. The surge you add is not an appliance's full starting figure. It is the difference between its starting watts and its running watts, because the running watts are already sitting in your total. A 1/3 hp sump pump runs at 800 W and starts at 2,400 W. Its gap is 1,600 W, not 2,400 W.

Here is the home backup list from the calculator, sized both ways.

Running watts, everything switched on at once: refrigerator 150 W, chest freezer 120 W, gas furnace blower 800 W, sump pump 800 W, ten LED bulbs 90 W, router and modem 15 W, a 55-inch TV 100 W, two phone chargers at 20 W each. Total: 2,100 W.

Four of those items have motors. Their start-up gaps are 600 W for the refrigerator, 480 W for the freezer, 1,550 W for the furnace blower and 1,600 W for the sump pump. You add the biggest one and nothing else. Peak: 3,700 W.

Add every item's full starting figure instead and you get 6,350 W. That gap is not academic. A 3,700 W peak sits inside the 5,000 W generator class. A 6,350 W peak pushes you into the 7,500 W class: heavier, louder, more expensive, and burning more fuel every hour to carry the same 2,100 W load.

Why motors surge and elements do not

A motor sitting still is close to a dead short. There is no spinning rotor generating a back voltage to oppose the incoming current, so for the first fraction of a second the supply sees almost nothing but winding resistance. That condition has a name: locked rotor. Current rushes in, the rotor breaks free, speed builds, the opposing voltage builds with it, and the draw collapses back to the running figure. The whole event is usually under a second, often a small fraction of one.

Compressors have it worse than plain motors because they start against pressure. So do air compressors with a charged tank, which is why a 2 hp unit rated 1,500 W running is listed at 4,500 W starting. Anything that has to break a load free from a standstill sits at the bad end of the range.

The equipment that never surges

Resistive equipment has no rotor to get moving. It turns electricity straight into heat through an element, and the element draws the same watts in its first millisecond as in its thousandth. Its starting watts equal its running watts, and its gap is zero.

That covers a large amount of expensive gear: space heaters, toasters, electric kettles, air fryers, griddles, deep fryers, tank water heaters, range elements, hot dog rollers, waffle irons. A single-basket electric fryer draws 3,500 W running and 3,500 W starting. A 24-inch electric griddle is 3,000 W both ways. An espresso machine boiler is an element too, so a commercial one-group machine starts at its 1,800 W running figure.

This produces a result that surprises people. The food truck setup in the calculator draws 9,300 W running, but its peak is only 10,500 W. The two biggest loads on the truck, the fryer and the griddle, contribute nothing to the surge at all. The 1,200 W gap comes from the refrigerated prep table, a 400 W appliance.

The reverse also happens. The coffee cart list draws 4,200 W running and peaks at 7,200 W, and the 3,000 W gap belongs entirely to the commercial blender: an item that runs for a few seconds per drink and uses almost no energy across a whole shift. Peak sizing is not about which appliance is big. It is about which one starts hardest.

Where the surge actually hides

Every appliance below has a start-up gap over 900 W. These are the items that set your peak, computed from the same data the calculator uses.

Read the last column, not the middle one. Whichever row in your own list has the largest number there is the one appliance whose surge you add.

Why the box carries two numbers

Manufacturers print both figures because sizing genuinely needs both, and they answer different questions.

Running watts is a heat question. It is what the wiring, the breaker, the extension cord and the engine have to carry for hours without complaint. Starting watts is a stall question. It is what the supply has to deliver for an instant so the motor actually breaks free.

Generators are labeled the same way, and the marketing name is usually the peak. A "2,200 W" inverter generator is 1,800 W continuous and 2,200 W starting. A "7,500 W" portable is 7,000 W continuous and 9,000 W starting. Battery power stations do the same thing: a 2,400 W station commonly publishes a 4,800 W surge. Compare like with like. Running against running, starting against starting.

One more rule from the sizing engine: do not plan to sit at a generator's continuous rating. Keep the steady load at or below about 80 percent of it, which is the same thing as multiplying your running watts by 1.25 before you shop. That home backup list at 2,100 W running wants roughly 2,650 W of continuous capacity, then 3,700 W of starting capacity on top.

A soft starter rewrites the answer

A soft start kit ramps a compressor up over a second or two instead of switching it on hard. Running watts do not change. The surge does, and dramatically.

The cleanest example is an RV roof air conditioner. A 13,500 BTU unit runs at 1,400 W and surges to 3,500 W, so its peak is 3,500 W and a 2,200 W inverter generator will not touch it. Fit a soft start and the same unit surges to 1,800 W. Peak: 1,800 W, comfortably inside that generator's 2,200 W starting rating and its 1,800 W continuous rating. Nothing about the air conditioner got smaller. Only the first second changed.

Home central air behaves the same way. A 2 ton system runs at 2,500 W and surges to 7,500 W, a 5,000 W gap that on its own decides how big a standby generator has to be. Cutting that spike by roughly two thirds, which is the commonly quoted figure for these kits, is the difference between one generator class and the next.

Put your own list through it

Two numbers matter and they are not the same number. Size the engine or the inverter for your running total. Size the peak for that total plus one surge. Then check energy separately, because watts say nothing about how long anything lasts: see how many watts a refrigerator really uses for what duty cycling does to a nameplate, and generator vs portable power station for which type of power source suits the load you just sized.

When you are ready, build your list in the calculator. It adds the running watts, picks the single largest start-up gap, names the appliance that set it, and tells you which generator and battery classes clear both figures.