What Size Generator Do I Need?

Almost every generator that disappoints its owner was bought the same way: someone stood in an aisle, looked at a number on a box, guessed it sounded like enough, and found out during the second night of an outage that it was not. The good news is that generator sizing is arithmetic, not judgment. There is one calculation, it takes about ten minutes at the kitchen table, and once you have done it you never have to guess again. Here it is.

Quick Answer

Add up the running watts of everything you want on at the same time, then add the single largest starting surge among those items. That total is the minimum starting wattage your generator needs. For a refrigerator alone, using the industry reference figures of 700 running watts and 2,200 additional starting watts, you need roughly 2,900 starting watts, so a 3,000-watt unit. A realistic outage kit (fridge, sump pump, lights, phones, a fan, a radio) lands near 4,300 starting watts, which a 5,000 running / 6,500 starting class portable covers comfortably. Add a 12,000 BTU window air conditioner and the same list jumps to about 9,300 starting watts. Air conditioning and electric heat are what blow up a sizing calculation, not the fridge.

Key Takeaways

  • Two numbers, not one. Motors draw far more power for the first second than they do while running. Lowe's portable generator wattage worksheet puts a refrigerator at 700 running watts but 2,200 additional starting watts, which is why a 2,000-watt generator can fail to start an appliance it could easily run.
  • You only count the largest surge once. The standard method is total running watts plus the highest single starting figure on your list, because two motors almost never kick in at exactly the same instant.
  • Read the nameplate before you trust any chart. The standard method is simple: the wattage is stamped on the appliance or its nameplate, and if it is not, multiply the amperage by the voltage to get watts.
  • Heating and cooling dominate everything else. A space heater is listed at 1,800 running watts and a 12,000 BTU window air conditioner at 3,250 running plus 3,950 starting. A fridge, some lights, and a laptop together do not come close.
  • A solar generator is sized twice. Its watt rating says what it can run, its watt-hour rating says for how long. A unit that comfortably starts your fridge can still be flat by morning.
  • Size is a safety question too. CPSC's guidance is that generators run outside only, never in a home, garage, basement, crawlspace, or shed, and never plugged into a wall outlet to feed the house wiring.

The Only Sizing Math That Matters

Generator capacity is published as two numbers, usually printed as something like "5,000 running watts / 6,500 starting watts." Running watts (sometimes called rated or continuous watts) is what the machine will deliver indefinitely. Starting watts (surge or peak) is what it will deliver for a couple of seconds while a motor spins up.

Anything with a motor or compressor in it (refrigerator, freezer, sump pump, well pump, air conditioner, furnace blower) draws a large inrush current at the instant it starts. Anything that is essentially a heating element or electronics (space heater, coffee maker, lights, laptop, television) draws roughly the same power the whole time it is on, so its starting and running numbers are the same.

The industry sizing method, printed on the same worksheets the retailers hand out with the generators, is four steps:

  1. Choose which devices you want to power at the same time.
  2. Add up the running watts of all of them.
  3. Note the additional starting watts for each one.
  4. Take the single highest starting figure and add it to the total running watts. That is your requirement.

The reason you add only the highest surge, rather than all of them, is that surges last a second or two and are extremely unlikely to coincide. If two large motors on your list genuinely do start together (a well pump and a sump pump during the same storm, for example), size for both and stop reading charts.

Step One: Decide What Actually Has to Run

This is the step people skip, and it is the one that determines whether you buy a $600 machine or a $6,000 one. Write down what has to be on simultaneously, not what you own.

For most households in a multi-day outage, the honest list is short: the refrigerator, the freezer if it is separate, the sump pump if you have a basement that floods, enough light to move around, a way to charge phones, and something to move air. Everything else is comfort, and comfort is what you buy up to if the budget allows, not what you size around.

Two loads deserve a decision rather than a default. If anyone in the house depends on medical equipment, that is a non-negotiable line item and you size around it first. And if you heat with a gas or oil furnace, the furnace itself needs no generator, but its blower and ignition do, which is often the single highest-value load in a winter outage.

Step Two: Get Real Wattage Numbers

Charts are for planning. Your specific appliance is for deciding. The standard Cooperative Extension guidance for estimating appliance energy use is that the wattage of most appliances is stamped on the bottom or back of the unit or on its nameplate, and that if the wattage is not listed you can find the current draw in amperes and multiply it by the voltage to get watts. Most US household appliances run on 120 volts, while larger ones such as dryers and electric cooktops run on 240.

That last part is worth memorizing, because it turns any appliance into a known quantity: amps × volts = watts. A pump tagged at 9 amps on a 120-volt circuit is drawing about 1,080 watts while it runs. Ten minutes with a flashlight and a notepad behind your appliances produces a better list than any online calculator.

For any major appliance there is an even faster route: the yellow EnergyGuide label carries an "Estimated Yearly Electricity Use" in kilowatt-hours, calculated under Department of Energy test procedures. That figure is about running cost over a year rather than instantaneous draw, so it does not replace the wattage number for sizing, but it is the right input if you are also sizing a battery or a solar array. How Many Solar Panels Do I Need? runs that version of the math.

Reference Wattages for Outage Planning

These figures come from Lowe's portable generator wattage worksheet, which is the sort of chart the sizing method above was written around. The worksheet's own footnote is the right caveat: watts listed are approximate, and you should check your appliance for actual requirements. The "additional starting watts" column is on top of the running figure, not instead of it.

Appliance Running watts Additional starting watts Outage priority
Refrigerator / freezer 700 2,200 First thing most people power
Sump pump, ½ HP 1,050 2,150 Critical if your basement floods
Window air conditioner, 12,000 BTU 3,250 3,950 The load that doubles your generator
Space heater 1,800 None Expensive in watts, cheap in dollars
Microwave, 1,000 watt 1,000 None Fine in short bursts
Coffee maker 1,000 None Morale, and it runs for minutes
Computer with a 17 inch monitor 800 None Lower on a laptop
Television, 27 inch 500 None Information, and it costs little
Humidifier 175 None Comfort tier
Box fan 100 None Cheapest cooling watts you can buy
AM/FM radio 100 None Battery version is better anyway
Cell phone charger 25 None Effectively free
RV air conditioner, 13,500 BTU 700 1,500 The deciding load in any camper

Three Worked Examples

What size generator do I need to run a refrigerator?

Running watts: 700. Highest starting surge: 2,200. Total requirement: 2,900 starting watts. A generator advertised at 3,000 starting watts covers it, and one advertised at 2,000 will run the fridge happily right up until the compressor cycles on and stalls the engine. This is the single most common sizing mistake, and it is why the surge column exists.

What size generator do I need to ride out a normal outage?

Fridge (700), sump pump (1,050), a handful of LED lamps (roughly 100 total, since modern bulbs draw about 10 watts each), phone chargers (25), a box fan (100), a radio (100). Running total: 2,075 watts. The largest single surge on that list is the fridge at 2,200. Requirement: 4,275 starting watts.

A common 3,600 running / 4,500 starting portable technically clears that. A 5,000 running / 6,500 starting unit clears it with room to add the coffee maker and the microwave without doing arithmetic in the dark, which is the real argument for buying one size up.

What size generator do I need to run my house?

Take the same list and add a 12,000 BTU window air conditioner. Running total climbs to 5,325 watts, and the largest surge is now the air conditioner's 3,950 rather than the fridge's 2,200. Requirement: 9,275 starting watts.

One appliance more than doubled the machine. That is the honest shape of this problem: air conditioning, electric heat, electric water heating, and electric ranges are in a different weight class from everything else in the house, and any answer to "what size generator do I need for my house" is really an answer to "how much of the heating and cooling am I trying to keep."

Generator Classes and What Each One Actually Covers

Class Typical rating Realistically covers Watch out for
Small inverter ~2,000 starting Phones, lights, laptop, a small cooler or fan. Quiet and light. Will not reliably start a full-size refrigerator.
Mid inverter / small portable ~3,000 to 4,500 starting Fridge plus lights plus electronics. The realistic entry point for outage use. One big motor at a time. No air conditioning.
Full-size portable ~6,500 to 8,000 starting Fridge, sump pump, furnace blower, lights, small appliances together. Needs a transfer switch to feed circuits safely. Loud, heavy, thirsty.
Large portable / small standby ~9,000 to 12,000 starting All of the above plus one air conditioner or a well pump. Fuel consumption becomes the limiting factor, not watts.
Home standby, permanently installed Commonly 10 kW to 24 kW residential Whole-house operation on natural gas or propane, automatic start. Sized by a licensed electrician off a real load calculation, not a chart.

What Size Whole House Generator Do I Need?

This is the one question in the article that a chart genuinely cannot answer, and anyone who gives you a number based on your square footage alone is guessing. Residential standby units are commonly sold in the 10 kW to 24 kW range, with smaller units covering the essentials tier (refrigeration, sump, furnace, lights, some outlets) and larger ones covering central air and electric-heavy homes. But the actual number comes from a load calculation on your specific panel.

There is also a code dimension worth knowing before you talk to an installer. Permanently installed standby systems fall under the National Electrical Code's optional standby rules in Article 702, and when the unit sits on an automatic transfer switch it generally has to be sized for the full calculated load unless it includes automatic load management (load shedding) that prevents an overload. That is precisely why "just get the biggest one" and "just get a cheap one and be careful" are both wrong answers for a permanent install: the code, the transfer switch, and the load calculation are one system. Ask your electrician to show you the calculation, not just the quote.

The load calculation, and why it decides the price

NEC Article 702 points the calculation at Article 220, which is the same load-calculation machinery used to size a service. The practical consequence for a homeowner is that the calculated load of your house, not its square footage, sets the minimum unit, and that calculation is driven by a handful of large fixed loads rather than by how many rooms you have. A 1,800 square foot house with a heat pump, an electric range, an electric water heater and a well pump calculates far higher than a 3,000 square foot house on gas heat, gas cooking and city water.

Which means the first question is not "how big is my house" but "what are my electric-heavy appliances, and which of them do I actually need running at the same time?" That second clause is where the money is.

Load management is usually cheaper than a bigger generator

This is the part most buyers never hear, and it is the single largest cost lever in a standby install.

Under NEC 702.4(B)(2), a system on automatic transfer must either have capacity for the full calculated load or use automatic load management to keep the generator from being overloaded. Load management modules do exactly what the name suggests: they watch the generator's output and temporarily shed the largest discretionary loads, typically an electric water heater, a second air conditioning compressor, a pool pump or an EV charger, before the unit can be overwhelmed. The water heater pauses for twenty minutes. Nobody notices.

The effect on sizing is substantial. A house that calculates out at the top of the residential range may be legally and practically servable by a mid-range unit once two or three deferrable loads are managed, and the module costs a fraction of the difference between generator tiers. So the right sequence when you get a quote is: ask for the calculated load, ask what it would take to serve it with the next size down plus load management, and compare the two numbers. A good installer will have already done this. An installer who will not show you the calculation is selling you a unit, not a system.

The fuel decision changes the output, not just the running cost

Residential standby units generally run on either natural gas or propane, and the choice is usually made on what is already at the house. What catches people out is that the same unit is rated differently on the two fuels, and the natural gas rating is the lower of the two. Propane carries more energy per unit volume delivered to the engine, so a unit advertised at a given kilowatt figure is often quoting the propane number.

This is not a small distinction when you have sized the install with no margin. Check the rating plate and the manufacturer's spec sheet for your specific model: a standby unit's documentation lists both a natural gas and an LP standby rating, and the one that matters is the one for the fuel you will actually burn. If the load calculation came out close to the unit's capacity, confirm which fuel the quoted rating assumed before you sign.

The other half of the fuel decision is supply rather than output. Natural gas is effectively unlimited as long as the utility's gas system is up, which it usually is, and needs no refilling. Propane is independent of the utility entirely, which is the point of it, but a tank is a finite number of hours and somebody has to deliver more. For a long regional outage the propane tank is the more honest plan and the one that requires planning; the shelf-life side of that is covered in how long a propane tank lasts.

What the tiers actually cover

Rough guidance only, and it is not a substitute for the calculation above. Capacities overlap heavily between manufacturers and the fuel caveat applies to every row.

Unit size Realistically covers Usually will not cover
10 to 14 kW Essentials: refrigeration, furnace blower, sump, well pump, lights, outlets, internet Central air, electric range, electric water heater
16 to 20 kW Essentials plus one central air compressor, or an electric water heater with management Everything simultaneously in an all-electric home
22 to 26 kW Most gas-heated homes in full, or an all-electric home with load management Large all-electric homes with two compressors and EV charging unmanaged

Notice that "with load management" appears in two of those rows. That is the point of the previous section: the module moves you down a tier more often than it moves you up one.

The transfer switch is part of the sizing decision

Two things to settle, because they interact with everything above:

  • Whole-panel versus essential-circuits. A service-rated automatic transfer switch moves the entire panel and then relies on load management to behave. An essential-circuits switch feeds a smaller subpanel you have chosen in advance. The second is cheaper and lets a smaller unit do real work, at the cost of deciding now which circuits matter.
  • Automatic versus manual. Automatic is the reason people buy standby units: it starts on its own at three in the morning whether or not anyone is home. A manual switch is cheaper and perfectly legitimate, and it changes the sizing arithmetic because a human is deciding what runs rather than a module.

Whichever you choose, the transfer switch is what makes the installation safe rather than merely convenient. Backfeeding a panel through a generator without one endangers utility line workers and anyone else on the circuit, and it is covered in the safety section below.

What Size Solar Generator Do I Need?

Solar generators, more accurately called portable power stations, are sized on two axes rather than one, and confusing them is the fastest way to be disappointed.

  • Watts (inverter output) tells you what it can run at once, and it has a surge rating just like an engine generator does. This is the number you compare against the sizing math above.
  • Watt-hours (battery capacity) tells you how long it can run. A refrigerator averaging a few hundred watt-hours per hour of real duty cycle will empty a 1,000 Wh unit overnight even though that unit had no trouble starting it.

So the sizing question for a battery unit is two questions. First, is the continuous and surge output high enough for the largest thing on my list? Second, is the capacity, plus whatever solar input I can realistically produce on a cloudy day after a storm, enough to carry those loads for as long as the outage lasts? For the fridge-plus-lights scenario above, that means an inverter comfortably over 2,900 watts of surge and enough stored watt-hours to cover a fridge's overnight duty cycle, with panels to recover during the day.

The tradeoff against an engine generator is genuine in both directions and covered in more depth in Generator vs. Solar: What Actually Keeps the Lights On. Batteries are silent, produce no exhaust, and can run indoors. Engines produce unlimited energy as long as you keep feeding them fuel, and cannot run indoors under any circumstances.

What Size Generator Do I Need for My RV or Camper?

Camper sizing is almost always decided by the air conditioner and nothing else. The reference figure for a 13,500 BTU RV air conditioner is 700 running watts with 1,500 additional starting watts, so roughly 2,200 watts to get it turning. Everything else in a typical camper (lights, water pump, charger, a small television) is small by comparison.

That is why the 2,200-watt inverter generator class is so standard in RV parks, and why owners of 15,000 BTU units end up either buying a larger generator, running two smaller units in parallel, or fitting a soft-start module to reduce the compressor's inrush. If your rig has no air conditioner, or you never run it, your real requirement drops to a few hundred watts.

Buying the Right Size, Then Running It Safely

A correctly sized generator that is used carelessly is more dangerous than an undersized one. The Consumer Product Safety Commission is blunt about why: most of the incidents it receives involving portable generators are carbon monoxide poisonings from generators used indoors or in partially enclosed spaces.

⚠️ SAFETY: Never run a generator indoors, including in garages, basements, crawlspaces, and sheds, even with doors and windows open. CO builds to deadly levels within minutes and lingers for hours after the generator is off. Get to fresh air immediately if you feel dizzy or weak. Verify against current CPSC and CDC guidance and your generator's manufacturer instructions.
⚠️ SAFETY: Never power your house wiring by plugging a generator into a wall outlet. This practice, called backfeeding, is extremely dangerous: it presents an electrocution risk to utility workers and to neighbors served by the same transformer, and it bypasses your home's built-in circuit protection. Use a transfer switch installed by a qualified electrician. Verify against current CPSC guidance and your local electrical code.
⚠️ SAFETY: Install battery-operated or battery-backup CO alarms in your home per the manufacturer's instructions, certified to the current safety standards (UL 2034, IAS 6-96, or CSA 6.19.01), and test the batteries monthly. Verify against current CPSC guidance.
⚠️ SAFETY: Use heavy-duty extension cords rated for outdoor use, and make sure each cord's wattage rating exceeds the total wattage of everything plugged into it. Check the full length of every cord for cuts or tears and confirm the plug has all three prongs. Verify against current CPSC guidance.
⚠️ SAFETY: Never store generator fuel in the home. Keep gasoline, propane, and kerosene outside living areas in properly labeled, non-glass safety containers, and never near a fuel-burning appliance such as a natural gas water heater in a garage. Turn the generator off and let it cool before refueling, because gasoline spilled on hot engine parts can ignite. Verify against current CPSC guidance and your local fire code.

One sizing point that is also a safety point: do not solve an undersized generator by daisy-chaining extension cords or by running the machine at 100 percent of its rated output continuously. Buy for the load you calculated, keep steady operation somewhere below the rated maximum, and you get a machine that starts what it needs to start and lasts more than one bad winter.

FAQ

What size generator do I need for my house? Is there a chart?

There is a chart, and it is the wattage table above, but the chart is only half the answer. The number you want is your own total: running watts of everything on at once, plus the single largest starting surge. Two identical houses can need a 3,000-watt unit and a 12,000-watt unit depending on whether the owner intends to run air conditioning. Do the arithmetic on your own list rather than matching square footage to a generator size.

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

Mostly it costs you money, fuel, weight, and noise rather than causing harm. A large engine running a light load burns more fuel per useful watt than a right-sized one and can suffer from prolonged light-load operation on some models. Buying one size above your calculation is sensible headroom. Buying four sizes above it means hauling and feeding a machine you will resent.

What happens if my generator is too small?

Best case, the overload breaker trips and you unplug something. Worst case, a motor tries to start on insufficient power and stalls, which can overheat the appliance's windings and damage both the appliance and the generator. Undersizing is not a graceful failure, which is the whole reason the starting-watts column matters.

How do I find the wattage of an appliance that has no label?

Find the amperage on the nameplate, which nearly every appliance has, and multiply by the voltage of the circuit it plugs into (120 volts for standard household outlets in the US, 240 for large appliances such as ranges, dryers, and well pumps). That is the standard Cooperative Extension method. For an appliance you can plug into a socket, an inexpensive plug-in power meter will give you a live reading including the surge.

Should I buy a generator or a solar battery system instead?

They solve different problems, and the sizing math differs accordingly. An engine generator is sized on watts alone and runs as long as you keep feeding it fuel. A battery or solar system is sized on watts and watt-hours, and it is silent and safe to run near the house. One point worth knowing before you assume rooftop solar is your backup: a standard grid-tied array shuts down in an outage by design, and needs a battery plus a backup-capable inverter to do anything at all when the grid is down. Generator vs. Solar compares the two directly, and How Many Solar Panels Do I Need? covers the solar sizing side.

Will a generator run my furnace?

Usually yes, and it is often the most valuable thing you can power in a winter outage. A gas or oil furnace uses grid power only for its blower, controls, and ignition, so a modest generator can restore whole-home heat even though it could never run electric heating. The catch is the connection: a furnace is hardwired, so it needs a transfer switch or a properly installed generator inlet, not an extension cord. Have an electrician set that up before the storm, and see How to Stay Warm Without Power for what to do until then.

What size whole house generator do I need?

It comes from a load calculation on your panel, not from your square footage. NEC Article 702 points the calculation at Article 220, and the result is driven by your large fixed loads rather than by floor area, so a smaller all-electric house often calculates higher than a larger house on gas heat and gas cooking. Residential standby units commonly run 10 kW to 24 kW, with 10 to 14 kW covering essentials, 16 to 20 kW adding a central air compressor, and 22 kW and up covering most gas-heated homes in full. Before accepting a quote, ask for the calculated load and ask what the next size down plus automatic load management would cost, because that comparison usually saves money.

Can I install a smaller whole house generator and still cover everything?

Often yes, with automatic load management. NEC 702.4(B)(2) allows a system on automatic transfer to be smaller than the full calculated load when load-shedding equipment prevents the generator being overloaded. A load management module temporarily drops the biggest deferrable loads, usually an electric water heater, a second air conditioning compressor, a pool pump or an EV charger, and restores them when there is headroom. The water heater waits twenty minutes and nobody notices. The module typically costs far less than stepping up a generator tier, so ask for both quotes.

Does a standby generator produce less power on natural gas than on propane?

Yes, and it is worth checking before you size an install with no margin. The same unit carries two different standby ratings, and the natural gas figure is the lower one, because propane delivers more energy to the engine per unit volume. Advertised capacities frequently quote the propane number. Read the rating plate and your model's spec sheet, confirm which fuel the quoted rating assumed, and size against the fuel you will actually burn.

Do I need a transfer switch?

If you want to power anything hardwired (furnace, well pump, hardwired lighting) or feed household circuits at all, yes. The alternative, plugging the generator into an outlet to energize your wiring, is backfeeding, and CPSC identifies it as extremely dangerous to utility workers and neighbors. Extension cords straight from the generator to individual appliances are the only safe way to skip a transfer switch, and they cannot reach hardwired equipment.

How many watts do I need to run a refrigerator and a freezer together?

Using the reference figures, two units at 700 running watts each is 1,400 running, and the largest single surge is still 2,200, giving about 3,600 starting watts. Two compressors kicking on at the same instant is unlikely but not impossible, so a 4,500-starting-watt machine is the comfortable answer if both are on the same generator.

The Bottom Line

Generator sizing has exactly one formula and one common mistake. The formula is running watts plus the biggest single surge. The mistake is ignoring the surge and buying a machine that can run your refrigerator but cannot start it. Spend ten minutes writing down what actually has to be on at once, read the nameplates instead of trusting a chart, decide honestly whether air conditioning is in or out, and buy one size up from the number you get. Then set up a transfer switch and a place to run the machine that is genuinely outdoors, well away from the house, before you need either. Be prepared. Stay savage.

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