Quick answer: Running watts keep an appliance going. Starting watts are the brief spike a motor demands to get turning, usually two to three times higher. Get it wrong and you either buy equipment that never starts your furnace, or you pay up to $424 more than you needed to for capacity you never use.
The definition is the easy part, and every manufacturer blog will give it to you. What none of them put a number on is the part that costs money: what happens when you size from the wrong one. There are exactly two ways to get this wrong, and they fail in opposite directions.
This article assumes you already have your loads written down. If not, start with the appliance wattage chart for 60 household loads, which lists both numbers and flags which loads surge at all. Come back with a running total and a largest surge.
Mistake one: sizing from running watts alone
This is the expensive one, because the equipment you buy does not work at all.
Say your winter essentials come to about 1,180 W running: fridge, upright freezer, router, lighting and a PSC furnace blower. A 2,000 W inverter generator covers that on paper, and at around $280 for a WEN 56200i it looks like the sensible buy.
It will not run your furnace. That PSC blower demands between 1,200 and 2,700 W for the instant it starts, on top of everything already running. The generator hits its limit, the breaker trips, and you are standing in a cold house holding a receipt.
It catches portable power station buyers harder, because capacity and output are different specs and only one is on the front of the box. An Anker SOLIX C1000 holds 1,056 Wh and delivers 1,800 W continuous: the watt-hours tell you how long it lasts, the watts tell you whether it starts anything at all. Shop the surge figure too. A SOLIX C2000 buys 2,400 W rated and 4,000 W peak, and that peak decides whether a fridge restarts.
Mistake two: adding up every starting watt
The overcorrection is just as common, and most sizing calculators quietly encourage it. You total the running watts, add every appliance’s surge on top, and arrive at a number no household ever actually draws.
| Method | The arithmetic | Total | What you end up buying |
|---|---|---|---|
| Running watts only | 1,180 W | 1,180 W | 2,000 W class, about $280 — and it never starts the furnace |
| Every surge added together | 1,180 + 1,500 + 1,200 + 2,700 | 6,580 W | 7,500 W class at about $569, or 9,500 W at $824 |
| Running plus the largest single surge | 1,180 + 2,700 | 3,880 W | 5,000 W class, about $400 |
The gap between those last two rows is $169 to $424 of hardware you did not need. An arithmetic choice, not a technical one.
The rule, and the one time it fails
Add every running watt you want on at once. Then add only the largest single starting surge. That is your number.
It works because motors do not coordinate. A fridge and a furnace blower starting in the same tenth of a second is possible but rare. Building for it means paying for a machine that spends its life barely loaded.
One scenario genuinely breaks the rule: the moment utility power returns. Everything cut off mid-cycle restarts at once, and those surges really do add. The fix is not a bigger machine but sequencing, which is part of preparing for the outage itself: bring loads back one at a time, largest motor first.
If your house has a well, the calculation is different again, because a submersible pump’s locked-rotor demand dwarfs everything else in the building. That case is worked through in the generator sizing section.
What oversizing costs to run, not just to buy
The purchase price is the larger penalty, but not the only one. A combustion engine is least efficient per watt when barely loaded: mechanical losses stay roughly constant while useful output falls. The sweet spot sits around 70 to 80 percent of rated load.
| Generator | Load factor at 1,180 W | Typical burn | Fuel per hour | Over a 24-hour outage |
|---|---|---|---|---|
| 5,000 W, right-sized | 24% | about 0.4 gal/h | $1.63 | $39 |
| 7,500 W | 16% | about 0.5 gal/h | $2.04 | $49 |
| 9,500 W | 12% | about 0.6 gal/h | $2.44 | $59 |
Twenty dollars a day is real but modest, and worth stating plainly: for a typical outage the money lost to oversizing sits in the purchase, not the fuel. That only inverts over a week-long run, or against something that burns nothing at all, which is the argument made in the home batteries section.
How we calculated this
- Load figures. Running and starting watts come from the appliance wattage chart, where each one carries its own source and tier.
- Equipment prices. Street prices for named models, checked September 2026: WEN 56200i, A-iPower SUA5000C, Powermate P0081600, Westinghouse WGen9500DF, Anker SOLIX C1000 and C2000. Generator pricing moves with the hurricane season, so treat these as a snapshot rather than a quote.
- Fuel burn. Published consumption ranges by generator class at light load. We used the low end of each range, which flatters the oversized machines rather than our own argument.
- Gasoline price. $4.071 per gallon, the US regular average for the week ending August 31, 2026, from the EIA weekly retail price survey.
- The 2 to 3 times multiple is the conventional motor-start figure used in sizing. Where a manufacturer publishes a locked-rotor rating, that published figure is better and we use it instead.
We do not own a test bench. Everything here is analysis based on published specifications and current retail listings, as set out in our testing and sourcing policy.
Safety note. If an appliance will not start on your generator, the answer is a bigger surge capacity or fewer simultaneous loads. It is never to defeat a breaker, bypass a thermal cutout, or repeatedly retry a stalled motor: a motor that cannot turn draws locked-rotor current until something gives, and that something is usually windings or wiring.
Connecting any generator to house wiring is regulated work for a licensed electrician, under your adopted edition of the National Electrical Code, Article 702. Never power a house by plugging a generator into an outlet, and never run one indoors or in a garage — see the CPSC generator safety guidance.
Frequently asked questions
Are starting watts always 2 to 3 times running watts?
No, it is a planning convention, not a measurement. Resistive loads like heaters and kettles have no surge at all. Induction motors can demand far more: a 1/2 HP submersible well pump is rated 670 W running against a locked-rotor figure near 7,400 VA.
Does a generator’s peak watts rating mean it can hold that?
No. Peak watts is a brief rating, typically seconds, meant only to absorb motor inrush. Continuous load must stay under the running figure. Buying on the peak number and running near it is how generators get cooked.
Why do watt-hours not tell me if a power station will start my fridge?
Because they measure different things. Watt-hours are the size of the tank; watts are the size of the pipe (both are listed per appliance in the wattage chart). A 1,000 Wh unit rated 1,500 W will run a fridge for hours. But if the compressor inrush exceeds its surge rating, it never starts at all.
Do inverter appliances change the sizing?
Yes, in your favour. An inverter-driven compressor ramps its own speed up from zero instead of starting across the line, so its surge is much smaller. If your fridge, heat pump or air conditioner says inverter on the badge, size on the running figure with modest headroom rather than a large multiple.
What if everything restarts at once after the power comes back?
That is the one case where surges genuinely add. Unplug the largest motors before power returns, then bring them back one at a time. It costs nothing and removes the only realistic scenario where the largest-single-surge rule under-sizes you.
This article is general information, not an installation guide and not a substitute for a licensed electrician. Electrical work on a service panel, a transfer switch or a utility connection is regulated in most jurisdictions and normally requires a permit and an inspection. Fuel-burning equipment produces carbon monoxide and must never be run indoors, in a garage or near an opening into the home.