What Size Generator Do I Need? Wattage by Scenario

The generator that fails during a storm is usually not too small on paper. Somebody added up the watts of everything they wanted to run, bought a unit rated comfortably above that number, and felt prepared. Then the outage came, the lights burned steadily for twenty minutes, and the instant the refrigerator's compressor cycled on, the engine bogged, the voltage sagged, and the whole rig stalled. The rating on the box was never the problem. The list was measured in the wrong unit.

Motor-driven appliances need a burst of extra power to get spinning — often two to three times their running draw — and that spike lasts just long enough to knock over a generator sized to steady watts. Generator sizing is really three numbers, not one: the running watts of everything you'll power at once, the starting surge of the single largest motor, and 20% headroom on top of both. Get all three and the arithmetic takes two lines. Miss the surge and you own a generator that quits at the worst possible second. Add too much and you're paying for capacity, fuel, and noise you will never use.

The formula

Two lines, then a rounding step:

• peak demand = running watts + the single largest starting surge
• recommended size = peak demand × 1.20
• buy: the next standard generator size at or above the recommendation — 3,500, 5,000, 7,500 W and up

With the calculator's defaults — 3,000 running watts and a 2,200 W surge — peak demand is 5,200 W and the recommendation is 6,240 W, which puts you in a 7,000 to 7,500 W unit. If nothing on your list has a motor, the surge term is zero and the whole thing collapses to running × 1.2: a 3,000 W kit of pure lights and electronics needs 3,600 W.

Note what the formula refuses to do. It does not sum the surge of every motor you own, and it does not let you buy a machine that will spend its life at 100% of its rating. Both refusals are deliberate, and both get their own section below.

Quick answer: generator sizes by scenario

If you want the number before the theory, here it is. Every running figure below comes from the appliance energy calculator's typical running wattages, and every row budgets the calculator's default 2,200 W surge for one motor of the refrigerator, air conditioner, or well-pump class.

Treat the rows as templates, not verdicts. If your largest motor is a deep-well pump rather than a kitchen fridge, its surge can run well past the 2,200 W placeholder — the calculator accepts surge figures up to 8,000 W, so enter the real spike for your biggest motor and let the recommendation move. The structure of every row stays the same: one running total, one surge, one multiplication.

Two things in this table are worth staring at. In the first row, the fridge runs on a modest 150 W, yet its motor class commands a 2,200 W surge budget — a spike roughly seven times the steady draw of the entire appliance list. Small kits are surge-dominated, which is why "I only need to run a few small things" still lands you at 3,500 W. And compare rows two and four: a single 1,500 W space heater converts a 5,000 W generator into a 7,000–7,500 W generator. Nothing else changed.

Running watts versus starting watts

This is the concept the whole calculation turns on, and it is the number-one generator-sizing mistake.

Running watts is the steady power a device draws once it's operating: the figure a fridge hums along at, the draw of a TV mid-movie. Starting watts — surge watts — is the brief extra spike a motor needs to go from standstill to spinning. For motor-driven appliances that spike often reaches two to three times the running figure. A window AC that runs at 1,200 W can momentarily pull 2,400 to 3,600 W at start-up, and the generator has to source every watt of that spike for the moment it lasts.

The split follows the hardware. Compressors and pumps — refrigerators, air conditioners, well pumps, sump pumps, furnace blowers — are motor loads, and they all surge. Resistance loads and electronics — space heaters, light bulbs, TVs, laptop chargers — switch on at full draw and stay there. No motor, no spike.

Here is why sizing to running watts alone fails so reliably. Your tally says 3,000 W; you buy a 3,500 W unit; everything works at setup. But compressors are thermostat-controlled — they start when the temperature says so, not when you say so. At some point the fridge or the AC kicks in on its own schedule, instantaneous demand blows past the rating, and the generator stalls or trips its overload protection. Usually at 2 a.m., usually in the exact weather you bought the thing for.

Some appliance manuals and data plates list both running and starting watts; use those figures when you have them. When you don't, the two-to-three-times band is the working rule, and the calculator's 2,200 W default is a sensible budget for one fridge/AC/well-pump-class motor.

Why you only add one surge

The formula adds the surge of the single largest motor, not the sum of every motor on the list. That looks like an omission. It's the entire trick.

Starting spikes last moments. For two surges to stack, two compressors would have to start in the same instant, and in practice they almost never do — the fridge cycles on its thermostat, the well pump on its pressure switch, the AC on its own schedule. Sizing for all of them starting simultaneously means sizing for a coincidence that essentially never occurs, and the price of that phantom scenario is a bigger, thirstier, louder machine. The calculator's approach: cover the realistic worst case — everything running, plus the biggest single spike on top — and stop there.

You also control more of this than it first appears. When you fire up a generator, you plug loads in one at a time anyway, so start-up surges at setup never stack. What you cannot control is the thermostat-driven restart mid-outage, and that is precisely the event the one-surge budget exists to survive.

Build an honest running-watts list

The running-watts input deserves more care than people give it. The question is not "what do I own" or even "what will I use during an outage." It is: what will genuinely be on at the same moment?

Typical running wattages, from the appliance presets this site's calculators share:

A few observations that change how you tally. The refrigerator always makes the list — it runs around the clock and food spoilage is usually the reason you bought a generator — but at 150 running watts it is nearly free to carry. Electronics barely register: a TV, a desktop PC, and a ceiling fan together draw 360 W, roughly an eighth of the default storm kit's budget. The expensive rows are all at the bottom of the table, and they are all heat.

A note on where the numbers should come from. The figures above are typical running wattages, and that is deliberately not the same thing as the nameplate on the back of the appliance. A nameplate lists the maximum the device can pull, and for anything that cycles on a thermostat — fridges, ACs — it overstates the steady draw considerably. For generator sizing, use the running figure from the manual or a preset like these; a plug-in watt meter reads the true draw at the outlet if you want the exact number. Nameplates are acceptable only as a deliberate overestimate for loads that don't cycle.

Be honest about simultaneity in both directions. You will not run the microwave, the air conditioner, and the well pump in the same breath, so don't size for that. But don't kid yourself the other way either — if the plan is TV on, fridge running, and someone microwaving dinner, all three belong in the total, because that evening will happen. Loads you control the timing of (microwave, washing machine) are easy to schedule around; loads on thermostats are not, which is another reason the fridge stays on the list permanently.

The 20% headroom is not padding

Peak demand times 1.20 looks like an engineer's fudge factor. It is closer to a warranty on the whole plan.

A generator run at 100% of its rating is a machine being redlined. It runs hot. It stresses the engine. It burns more fuel per watt delivered, so flat-out operation is also the least economical way to make electricity. And it leaves exactly zero margin for the next thing someone plugs in — which, during a multi-day outage with a bored family in the house, is not a hypothetical.

Sizing to 120% of peak flips all of that. Do the division: with a 6,240 W recommendation carrying a 5,200 W peak, the worst instant of your worst case only ever asks the machine for about 83% of what it can give. Day-to-day running load sits far below even that. The generator lives in its efficient, durable band, and the spare fifth is there on the rare occasion something extra shows up.

The same logic in reverse explains why buying with zero headroom feels fine at first and ages badly. The undersized machine works — at full throttle, all the time, drinking fuel and wearing itself out, one plugged-in coffee maker away from an overload trip.

Worked example 1: the storm kit

Run the calculator's default scenario by hand. The kit: refrigerator (150 W), a window AC (1,200 W), a microwave (1,000 W), TV, desktop PC and a ceiling fan (360 W together), plus lights, phone chargers, and a couple of outlets bringing the tally to roughly 3,000 running watts. The largest motor is the air conditioner, budgeted at 2,200 W of starting surge.

• Peak demand: 3,000 + 2,200 = 5,200 W
• Recommended: 5,200 × 1.20 = 6,240 W
• Buy: the next standard size up — a 7,000 to 7,500 W unit

That last step matters. Nobody sells a 6,240 W generator. The recommendation is a target, and the purchase is the first real machine above it.

One dependency worth flagging: this kit's biggest single load and its surge donor is the window AC, so the size of that unit drives the size of the generator. If you're buying the AC too, size it properly first — an oversized air conditioner would inflate both numbers. The AC sizing guide covers that lookup.

Worked example 2: a tally from scratch

Now build one bottom-up, using the preset wattages. The scenario: a work-from-home household that needs food cold, lunch hot, and a desk running.

• Refrigerator: 150 W
• Microwave: 1,000 W
• Desktop PC: 200 W
• LED TV: 100 W
• Ceiling fan: 60 W
• Running total: 150 + 1,000 + 200 + 100 + 60 = 1,510 W

The only real motor on the list is the fridge, so it supplies the surge budget: 2,200 W.

• Peak demand: 1,510 + 2,200 = 3,710 W
• Recommended: 3,710 × 1.20 = 4,452 W
• Buy: a 5,000 W unit

Now watch how fragile that answer is. It's January, the furnace is out with the power, and someone adds one 1,500 W space heater to the kit. Running total: 1,510 + 1,500 = 3,010 W. Peak: 3,010 + 2,200 = 5,210 W. Recommended: 5,210 × 1.20 = 6,252 W — and you are shopping in the 7,000–7,500 W aisle. One appliance, one full size class, roughly the same jump the entire original kit produced. Decide about electric heat before you buy the generator, not the night you need it.

The loads that break the budget

There's a pattern in both examples: electronics are cheap, motors are moderate, and heat is ruinous. Electric resistance heating — space heaters, the drying elements in dishwashers, clothes dryers — pulls 1,500 to 3,000 watts per appliance, and it dwarfs everything else a household plugs in.

Scale it against the table above. A 1,500 W space heater draws as much as fifteen LED TVs. A clothes dryer, at 3,000 W, consumes the entire running budget of the default storm kit by itself. A dishwasher's 1,800 W outdraws the fridge, the microwave, the TV, the PC, and the fan combined.

On a portable generator, the practical answer is scheduling, not capacity. Laundry waits. The dishwasher waits. If you must heat electrically, run one heater in one closed room and count its full 1,500 W in the tally. Buying enough generator to run heat appliances concurrently with everything else means paying for whole size classes of capacity — plus the fuel burn and noise that come with them — to cover loads a schedule handles free. These same loads dominate a normal electric bill for the same wattage reasons, which is worth understanding even when the grid is up — the electricity cost guide runs that math.

Round up to a machine you can buy

The calculator hands you a precise number like 6,240 W; the market hands you shelves labeled 3,500, 5,000, 7,500 and up. Bridging the two has one rule: the recommendation is a floor. Buy the next standard size at or above it, never below.

Rounding down is tempting exactly once a year, when the smaller unit is on sale. Resist it. Round the space-heater kit's 6,252 W recommendation down to a 5,000 W machine and the result isn't a slightly snug fit — that kit's 5,210 W peak already exceeds the 5,000 W rating, so the generator starts its service life beyond its limit, with the failure modes from the headroom section built in at purchase. The gap between sizes is not wasted money — it is the headroom, bought in steel.

While you're reading spec sheets, you'll notice outlets rated in amps rather than watts: household circuits run at 120 V, large appliances at 240 V, and watts, volts, and amps convert directly into each other. The watts to amps calculator translates your loads into the amp figures printed next to each receptacle, which is how you check that a single outlet can actually feed your heaviest cord.

When a portable generator is the wrong tool

This calculator sizes a portable generator for the essentials you choose to keep running. Two situations fall outside that job, one at each end.

At the top: whole-home backup. If the goal is every circuit live — central HVAC, water heater, range, the works — stop tallying appliances. That is a panel-level load calculation performed by an electrician, paired with a transfer switch so the generator can feed the house safely. The calculator's input tops out at 15,000 running watts, and if your honest list is anywhere near that ceiling, you have already left portable territory and entered standby-generator country. The formula's logic still applies; the execution belongs to a professional.

At the bottom: outages where nothing on your list has a motor. Phones, laptops, a lamp, a router — that is a few hundred running watts, no surge, and an engine is overkill for it. A battery power station covers that scale silently and indoors. Battery capacity is sold in amp-hours and watt-hours rather than watts, and the battery watt-hours calculator converts between them so you can compare a battery's stored energy against what your devices actually consume.

Between those poles — fridge, some comfort, a few days of grid failure — the portable generator and the two-line formula are exactly the right tool.

Common mistakes to avoid

• Sizing to running watts alone. The classic. Everything works until a compressor starts on its own thermostat, then the generator stalls — at night, in the storm.
• Adding every motor's surge. Motors rarely start in the same instant. Budget the single largest spike; summing them buys capacity, fuel burn, and noise you'll never use.
• Padding the list with everything you own. Running watts means on at the same time. A dishonest tally oversizes the machine as surely as a careless one undersizes it.
• Forgetting that heat is enormous. One 1,500 W space heater moves you a full size class. A 3,000 W dryer is a storm kit all by itself. Schedule these loads instead of buying capacity for them.
• Rounding down to the sale unit. The recommendation is a floor. The smaller machine starts its life beyond its rating.
• Buying zero headroom. A generator sized exactly to peak runs hot, wears fast, burns more fuel per watt, and trips the first time someone plugs in a kettle.

Total what will honestly run at once. Add the starting surge of your one biggest motor. Multiply by 1.2, then buy the next standard size above the answer. Four steps, and the generator that results will start the fridge without flinching, run in its efficient band instead of at redline, and have a fifth of its capacity in reserve for whatever the outage adds to the plan.