What Size Generator Do I Need? Real Wattage Math

Most homes need one of three sizes, and the right one depends on what you refuse to lose when the power goes out. To keep the essentials on with a portable, the fridge, the furnace fan, the lights, and one motor like a well or sump pump, you want 5,000 to 7,500 running watts. A standby that covers the essential circuits runs 10 to 14 kW. A whole-house standby that runs everything, nothing switched off, runs 18 to 26 kW.

The number that trips people up is not the running watts. It is the starting surge, the jolt a motor pulls for a second or two when it kicks on. Size for running watts alone and the generator trips at 2am when the furnace and the well pump start together. This guide does the wattage math the spec sheet skips, gives you the appliance numbers, then hands you the three sizes and who each one fits.

If you are still choosing between a permanent standby and a portable you run yourself, read standby vs portable first. The lane changes the size answer.

Quick Answer: The Three Sizes That Fit a Home

What you want to keep on	Size that fits	Lane
Fridge, furnace fan, lights, one motor (well or sump pump)	5,000-7,500 running watts	Portable
The essential circuits, hands-off	10-14 kW	Standby
The whole house, nothing switched off	18-26 kW	Standby

Pick the row that matches what you actually need running, not the biggest number a salesperson can talk you into. Most homeowners who think they need whole-house discover that a well-chosen essentials list covers everything that matters for a fraction of the price.

Running Watts vs Starting Watts

Every appliance has two numbers, and only one of them is printed on the front.

Running watts, sometimes called rated watts, is what a device draws to keep going once it is on. A fridge holding temperature, a furnace fan pushing air, a string of lights. Add up everything you want on at the same time and that is your running total.

Starting watts, also called surge or peak watts, is the extra jolt some devices need for the first second or two. Anything with a motor pulls far more current to get the shaft spinning from a dead stop than it needs to keep it spinning. That spike is brief, but if the generator cannot deliver it, the motor stalls and the breaker trips.

Your generator has to cover your full running total plus the single largest surge on top of it. Not every surge at once, because your motors do not all start in the same instant, but the biggest one has to fit.

Why motors need a running start

A motor at rest is close to a dead short for a moment, and two different numbers describe what happens next. Getting them mixed up is the most common sizing mistake, so here they are separately.

Starting watts are what a generator's surge rating has to cover, and the planning multiple is about 2 to 3 times running watts. SDG&E's own outage guidance says appliances with motors "can require two to three times their normal wattage when the motor starts up." Generac's Standby Generator Sizing Guide is stricter and uses a flat 3 times: a 1 HP motor is listed at 1 kW running and 3 kW starting.

Locked-rotor current is the instantaneous inrush, and it is much higher. NEC Table 430.251(A) sets maximum locked-rotor current for single-phase motors at exactly 6 times the full-load current in Table 430.248, at every horsepower from half a HP to 10. Real nameplates land close to that: across fourteen Carrier and Goodman residential condensers, locked-rotor amps run 4.3 to 6.3 times rated-load amps, and Franklin Electric's submersible well pump motors run 4.6 to 6.4 times.

So a 1,000 watt well pump needs roughly 2,000 to 3,000 watts of surge headroom, while the current it pulls in that first instant is five to six times its running amps. Resistive and electronic loads, lights, a TV, a laptop charger, a microwave, draw close to their rated watts with little or no surge. The surge math is really a motor problem. Count your motors and you have found your sizing problem.

The Appliance Wattage Table

Real ranges, because the exact number depends on the model, the age, and how hard the motor works. Treat these as planning figures, then check the label on your own units.

Appliance	Running watts	Starting surge watts
Refrigerator or freezer	100-800	1,000-2,200
Furnace fan (blower motor)	600-900	1,300-2,400
Sump pump (1/3 to 1/2 HP)	800-1,050	1,300-2,150
Well pump (1/2 to 1 HP, 230V)	670-1,210	2,000-3,600
Window AC (10,000-12,000 BTU)	1,200-1,500	2,200-3,600
Central AC (2 to 5 ton)	2,600-5,300	See the tonnage table below
LED lights (one room)	60-300	no surge

Central air is the giant, which is why it is usually the first thing left off a portable's list. It also deserves real numbers rather than the word "high," so it gets its own table next. The well pump numbers above come from Franklin Electric's published motor data rather than a range of guesses, and they are lower and tighter than the figures that circulate.

Central air conditioning by tonnage, with real nameplate numbers

Manufacturers publish rated-load amps and locked-rotor amps for every condenser they sell. These are read off two of them, both 208/230 volt single phase, alongside the planning figures Generac uses in its own sizing guide.

Nominal size	Carrier 24ABB3, RLA / LRA	Goodman GSX16, RLA / LRA	Generac planning figure
2 ton	10.9 A / 62.9 A	13.5 A / 58.3 A	2 kW running, 67 LRA
3 ton	13.6 A / 79.0 A	14.1 A / 77.0 A	3 kW running, 100 LRA
4 ton	18.3 A / 93.0 A	17.9 A / 112.0 A	4 kW running, 117 LRA
5 ton	22.1 A / 125.0 A	21.4 A / 135.0 A	5 kW running, 145 LRA

At 240 volts, a 3-ton condenser's rated load of roughly 13.6 to 14.1 amps is about 3.3 kW of running draw, and a 5-ton's 21.4 to 22.1 amps is about 5.1 to 5.3 kW. Notice that Generac's planning LRA numbers sit above the real nameplates. That is deliberate on Generac's part, and it is the right direction to be wrong in when you are sizing.

The number to use is the one on your own condenser's data plate, not any row above. Every one of these units prints RLA and LRA on the label.

What a soft start kit actually buys you

A soft start controller ramps the compressor up instead of slamming it, which cuts the inrush and can let a smaller generator carry the same air conditioner. The manufacturers' own claims:

Micro-Air EasyStart Flex: "Up to 75% reduction in start-up current," for AC units up to 6 tons.
Hyper Engineering SureStart: "Achieve over 60% reduction in startup current for your AC unit and pumps," for single-phase motors from 1.75 to 7 HP on 220/230V.

Both are "up to" and "over" claims, so treat 60 to 75 percent as the vendor-stated range rather than a guarantee. Worth noting that Generac's own PWRcell sizing worksheet assumes a 60 percent locked-rotor suppression when a SureStart is fitted, which is an independent party using the conservative end of that range.

We are not going to publish an installed price for a soft start kit, because no manufacturer publishes one.

Well pumps by horsepower and voltage

Franklin Electric publishes full-load watts, full-load amps and locked-rotor amps for its submersible motors, which is exactly what generator sizing needs. From the 2022 AIM Manual, 60 Hz, 3450 rpm, single phase:

Motor	Volts	Full-load amps	Full-load watts	Locked-rotor amps
1/2 HP, 2-wire	115	10.0	670	64.4
1/2 HP, 2-wire	230	5.0	670	32.2
3/4 HP, 2-wire	230	6.8	940	40.7
1 HP, 2-wire	230	8.2	1,210	48.7
1.5 HP, 2-wire	230	10.6	1,770	66.2
1.5 HP, 3-wire	230	n/a	1,620	51.4
2 HP, 3-wire	230	n/a	2,025	53.1
3 HP, 3-wire	230	n/a	3,000	83.4
5 HP, 3-wire	230	n/a	4,830	129

Two things worth acting on. Wiring the pump for 230 volts instead of 115 halves the locked-rotor current for the same motor, 32.2 amps against 64.4 on a half-horse. And 3-wire motors have meaningfully lower locked-rotor current than 2-wire at the same horsepower, 51.4 amps against 66.2 at 1.5 HP, because the start capacitor and relay sit in the control box rather than inside the motor. If you are specifying a pump and a generator together, both choices make the generator's job easier.

A heat pump needs its own calculation. Compressor draw changes by model and outdoor temperature, while electric backup strips can become the largest load in the house. If heat is a must-run outage circuit, get the proposed system's minimum circuit ampacity, maximum overcurrent protection, heat-strip kW, and control sequence before sizing the generator. The network's cold-climate heat pump comparison explains why published low-temperature operation and delivered capacity are different numbers.

The Three Sizing Answers

Essentials-only portable: 5,000 to 7,500 watts

This is the sweet spot for most homes that lose power a few times a year. It covers the fridge, the furnace fan, the lights, a few outlets, and one motor load like a well or sump pump, with headroom for that motor's surge. A 7,500 watt unit gives you room to run the well pump and the furnace without choreographing which one starts when.

What it will not do is run central air and everything else at once. That is the trade for spending $700 to $2,500 on the machine instead of five figures on a standby. If a portable fits your outage pattern, start with the best portable generators for home backup.

Essential-circuits standby: 10 to 14 kW

A permanent standby wired to a transfer switch that feeds your important circuits: heat, water, fridge, some lights and outlets, maybe one AC zone. It starts itself within seconds of an outage and runs on your gas line, so nobody is pouring fuel at midnight. Ten to 14 kW covers the essentials for most homes without paying to back up the pool pump and every closet light.

Whole-house standby: 18 to 26 kW

This runs the house the way you live in it, central air included, nothing switched off. It is the right size if you have multiple AC tons, a well, and you refuse to think about load at all. It is also the most expensive to buy, install, and fuel. Most people who ask for it actually want the 14 to 20 kW middle, sized to their real simultaneous load rather than the sum of every nameplate in the house. A whole-house standby lands in the $12,000 to $18,000 installed range, because the install costs about as much as the machine.

Why Oversizing Wastes Money, and Undersizing Trips at 2am

Undersizing is the loud failure. The generator runs fine until the furnace and the well pump surge together, the voltage sags, and the breaker drops your power in the middle of the night. You paid for a generator and still got woken up.

Oversizing is the quiet failure. A generator loafing at a small fraction of its capacity costs more up front, costs more to install, and burns more fuel than the job needs, and running an engine far below its rated load for long stretches is not good for it either. Bigger is not safer once your real surge fits with headroom.

Generac's own sizing guide puts a number on the right amount of margin: a generator rated about 20 percent above the maximum calculated load is optimal, so the unit runs at roughly 80 percent of capacity, and for residential work it says to add 10 to 20 percent to the calculated load in kW. Generac is explicit that this is its own recommended practice rather than an NEC requirement. So the goal is your true simultaneous load, plus the largest single surge, plus 10 to 20 percent. Not the sum of every nameplate in the house.

Getting this wrong is one of the most common and most expensive generator buying regrets, in both directions.

Run Your Own Numbers

You do not have to do this on a napkin. Our sizing calculator asks what you want to keep running, then returns a target wattage for a portable or a kW range for a standby, plus a rough fuel burn. Start there, confirm the surge on your biggest motor, and you will walk into any dealer or hardware aisle knowing your number before they tell you theirs.

Frequently Asked Questions

What size generator do I need to run my whole house?

For a true whole-house standby that runs central air and everything else with nothing switched off, plan on 18 to 26 kW, installed in the $12,000 to $18,000 range. Most homes are covered for less. A 10 to 14 kW standby on your essential circuits, or a 5,000 to 7,500 watt portable on the fridge, heat, lights, and one pump, keeps the important things on for a fraction of the cost.

What is the difference between running watts and starting watts?

Running watts keep a device going once it is on. Starting watts are the extra jolt a motor needs for a second or two to spin up from a stop. Your generator has to cover your full running total plus the single largest starting surge on top of it. Lights and electronics use their rated watts with almost no surge. Motors are where the surge lives.

Why does my well pump need so many more watts to start?

A motor at a dead stop draws far more current to break free than it does to keep spinning. Franklin Electric's published data for a 1 HP submersible motor on 230 volts is 8.2 full-load amps against 48.7 locked-rotor amps, so the inrush current is about six times the running current. In watts, the surge headroom a generator needs is smaller than that ratio suggests, roughly 2 to 3 times running watts, which is the multiple SDG&E and Generac's own sizing guide both use. Either way, if your generator cannot deliver that spike, the pump stalls and the breaker trips. Wiring the same motor for 230 volts rather than 115 halves the locked-rotor current.

Will a 7,500 watt generator run my house?

It will run the essentials of most homes: fridge, furnace fan, lights, outlets, and one motor like a well or sump pump, with headroom for the surge. It will not run central air plus everything else at the same time. If that trade fits how often you lose power, a 7,500 watt portable is the best value in backup power for a lot of buyers.

Can a generator run my central air conditioner?

Yes, if it is sized for the surge. Central air is the biggest single load in most homes. Real nameplates show a 3-ton condenser at roughly 13.6 to 14.1 rated-load amps and 77 to 79 locked-rotor amps, and a 5-ton at 21.4 to 22.1 rated-load amps and 125 to 135 locked-rotor amps, all at 208/230 volts. A soft start kit ramps the compressor instead of slamming it, and manufacturers claim 60 to 75 percent reductions in start-up current, which can let a smaller generator carry the same unit. Check your own compressor's LRA on its data plate and size for that, not the running watts.

Is it bad to buy a generator that is too big?

Past the point where your real surge fits with headroom, bigger just costs more to buy, install, and fuel. A generator running far below its capacity wastes gas, and running an engine that light for long stretches is not good for it. Generac's own sizing guide recommends a unit rated about 20 percent above your maximum calculated load, so it runs near 80 percent of capacity, and says to add 10 to 20 percent to a residential load calculation. Size for your true simultaneous load, plus the largest surge, plus that margin. Not the sum of every nameplate in the house.

How do I add up the watts I need?

List everything you want on at once. Add their running watts for your base total. Then find the single appliance with the largest starting surge and add that one surge on top. That sum is your minimum. The sizing calculator does this for you and returns a portable wattage or a standby kW range.

What we could not verify

Soft start kit installed prices. Neither Micro-Air nor Hyper Engineering publishes one, and we will not invent a range. Their reduction claims are quoted as the "up to" and "over" figures they actually print.
The Generac sizing guide's first-party URL. The document we read is Generac's own, part number A0000641269 revision G dated October 3, 2025, but the copy we could retrieve is hosted by a distributor. Generac's own consumer sizing tool is a five-input calculator that publishes no wattage table and does not disclose its formula.
A single authoritative appliance wattage table. There is not one. The running and surge figures in the general appliance table above are planning ranges, and the tonnage and well-pump tables below them are the ones with named manufacturer sources. Your own data plates beat all of them.