what size generator do I need

This is the most important question you will answer in this entire process. Get it wrong and you either waste thousands of dollars on capacity you'll never touch, or your generator trips offline at 2 AM during an ice storm because it couldn't handle the load when your well pump and furnace blower kicked on at the same time.

Both mistakes are expensive. One of them leaves you in the dark.

The generator industry does not make this easy. Manufacturers advertise peak watts that your unit will hit for roughly half a second. Salespeople size by square footage, which tells you almost nothing about your actual electrical load. Online calculators ask you to check boxes but never explain what the numbers mean or why they matter.

I'm going to walk you through this the way I wish someone had walked me through it five years ago, when I was standing in my garage trying to figure out why my 5,500-watt portable kept shutting down every time the refrigerator compressor cycled on.

That's the rule of thumb. But rules of thumb are how people end up with generators that are wrong for their house. The five steps below take about 20 minutes and will tell you exactly what you need.

step 1: list your essential loads

Get a piece of paper. Or open a spreadsheet. I don't care which. Write down every appliance and system in your house that you need running when the power goes out.

Not want. Need.

There's a difference, and it matters here, because every appliance you add to this list costs you money in generator capacity. I'll talk more about essential vs. comfort loads later. For now, start with the things that keep your house livable and your family safe:

• Refrigerator and freezer — food spoils in 4 hours without power. A full freezer buys you 48 hours if you don't open it. But you will open it.
• Well pump — if you're on well water, no power means no water. No water means no toilets, no showers, no drinking water. This is non-negotiable.
• Furnace blower — your gas furnace won't run without electricity. The gas valve, igniter, and blower all need power. If you're in a cold climate, this is a survival item.
• Sump pump — storms knock out power. Storms also bring rain. You see where this is going. A flooded basement is a five-figure problem.
• Lights — you don't need every light in the house. A few circuits covering kitchen, bathroom, and hallways. Figure 300-500 watts.
• Medical equipment — CPAP machines, oxygen concentrators, nebulizers, powered wheelchairs. If someone in your house depends on powered medical equipment, it goes at the top of the list.
• Communication — phone chargers, modem, router. Maybe a radio. Under 200 watts total but easy to forget.

Once you have your list, you've already done the hardest part. Most people skip this step entirely and buy based on whatever the sales guy at Home Depot recommended. Don't be most people.

step 2: add up running watts

Every electrical appliance has two wattage numbers that matter: running watts and starting watts. Running watts is the continuous draw — how much power the appliance uses while it's operating normally. Starting watts is the spike that happens when a motor first kicks on.

For step 2, we only care about running watts. We'll deal with starting watts next.

You can find running wattage in three places:

1. The nameplate — look on the back or bottom of the appliance for a sticker or stamped plate. It will show watts, amps, or both. If it shows amps, multiply by 120 (or 240 for large appliances like a well pump or AC) to get watts.
2. The owner's manual — usually in a specifications section near the back.
3. The reference table below — these are typical values. Your actual equipment may differ. Use the nameplate when you can.

Here's a reference table for common household appliances. These are averages. Your specific equipment may draw more or less.

Add up the running watts column for every appliance on your essential list. Write that number down. Let's call it your base load.

For a typical house running the essentials I listed in step 1 — refrigerator, well pump, furnace blower, sump pump, lights, and basic electronics — you're looking at roughly 3,000 to 3,500 watts of running load. That probably feels low. It is low. The running load is not what kills you. The starting watts are.

step 3: account for starting watts

This is where most people get bitten. This is why my first portable generator kept tripping the overload breaker. I had sized it by running watts and completely ignored the fact that every motor in my house draws 2 to 3 times its running wattage the moment it starts up.

Look at your list again. Anything with a compressor or electric motor has a starting surge:

• Refrigerator compressor — 1,200W starting vs. 150W running
• Well pump — 3,000W starting vs. 1,000W running
• Sump pump — 2,400W starting vs. 800W running
• Furnace blower — 1,500W starting vs. 500W running
• Central AC compressor — 5,250W starting vs. 3,500W running

These surges only last a fraction of a second to a few seconds. But your generator has to handle them or it shuts down.

Here's how to calculate what you actually need:

1. Take your total running watts from step 2.
2. Find the single appliance on your list with the highest starting watt surge (the difference between starting and running watts).
3. Add that surge to your running total.

Why only the highest single surge? Because in most real-world scenarios, your appliances don't all start at the exact same second. They cycle on and off independently. You need enough overhead to handle any one of them starting while everything else runs.

Example: Your essentials draw 3,200W running. Your well pump has the highest surge at 2,000W above running (3,000W starting minus 1,000W running). Your minimum generator capacity is 3,200 + 2,000 = 5,200 watts.

We're not done. There's one more step before you match to a generator size.

step 4: add a 20% safety margin

Take your number from step 3 and multiply it by 1.2.

This isn't me being conservative for the sake of it. There are real reasons for the buffer:

• Nameplate ratings aren't always exact. A refrigerator rated at 150W might pull 180W on a hot day when the compressor works harder. Appliances degrade over time. Voltage fluctuations change real power draw.
• You'll add loads you didn't plan for. It's February, the power's been out for 18 hours, and someone wants to run a space heater. Or charge a laptop. Or use the microwave. Without headroom, you can't.
• Generators shouldn't run at 100% continuously. Sustained full-load operation increases fuel consumption, wear, heat, and noise. Running at 70-80% capacity is where generators are happiest. They run more efficiently, last longer, and stay quieter.
• Altitude and temperature matter. Generators lose roughly 3.5% of output per 1,000 feet above sea level. They also lose capacity in extreme heat. If you're at 5,000 feet in July, that 10,000W generator might only deliver 8,200W.

Continuing the example: 5,200W x 1.2 = 6,240 watts. You'd want at least a 6,500W generator, which in practice means a 7,500W portable is a smart choice since that's a standard size that's widely available and reasonably priced.

step 5: match to generator size

Now you have an actual number. Here's how it maps to generator categories:

portable generators (3,000-12,000W)

These are pull-start or electric-start units that run on gasoline, propane, or both (dual-fuel). You connect them to your house through a manual transfer switch or interlock kit, or run extension cords to individual appliances.

• 3,000-5,000W: Bare minimum. Runs a refrigerator, some lights, phone chargers, and maybe a furnace blower. Won't handle a well pump and refrigerator simultaneously during startup surges. Fine for short outages in mild climates.
• 5,000-7,500W: The sweet spot for most essential-loads-only scenarios. Handles refrigerator, well pump, furnace blower, sump pump, and lights without breaking a sweat. This is what I recommend for most homeowners who want a backup that actually works.
• 7,500-10,000W: Everything above plus a window AC unit or small appliance flexibility. Can handle more simultaneous starting surges. If you have a large well pump or multiple motor-driven appliances, this is your range.
• 10,000-12,000W: Approaching the upper limit of what portables can do. At this point you're getting into territory where a small standby unit might make more sense financially, especially once you factor in installation costs for a proper transfer switch.

standby generators (10-48kW)

These are permanently installed units that run on natural gas or propane. They start automatically when the power goes out — no going outside in the rain, no pull cords, no extension cords. They connect to your electrical panel through an automatic transfer switch.

• 10-16kW: Powers all essentials and several comfort loads. Won't run a large central AC system unless you use load management. Good for smaller homes or homes with gas heat and gas cooking.
• 16-22kW: The most popular residential range. Covers a 3-ton AC system plus all essentials and most comfort loads. This is what most installers will recommend for a standard 200-amp home, and for once, the sales pitch is roughly accurate.
• 22-27kW: Full whole-house coverage for most homes up to about 3,500 sq ft. Runs everything including a large AC, electric water heater, and kitchen appliances simultaneously. If you want to forget the power is out, this is the range.
• 30kW+: Large homes, homes with multiple AC systems, or properties with outbuildings (shops, pools, barns). Diminishing returns for most residential applications.

If you want to see how specific standby models compare, I've reviewed the top options in my best whole-home generator guide. And if you're still working through the numbers, the sizing calculator does all of this math for you — just check the boxes and it spits out a recommendation.

common mistakes that cost people money

I've watched people make every one of these. I've made some of them myself.

sizing by peak watts instead of running watts

The big number on the box — "9,500 PEAK WATTS!" — is marketing. That's the number the generator can hit for a momentary surge. What matters for sizing is running (rated) watts, which is always the smaller number. A generator advertised at 9,500 peak watts might only deliver 7,500 running watts. That's still a fine generator. But if you thought you were getting 9,500 watts of continuous power, you're going to be disappointed and confused when it shuts down under what you thought was a light load.

sizing by square footage

A 2,500 square foot house with gas heat, gas stove, city water, and no AC has wildly different power needs than a 2,500 square foot house with electric heat, electric stove, a well pump, and a 5-ton AC. Square footage tells you almost nothing. Electrical load tells you everything. This is why I keep pointing you to the calculator — it asks the right questions.

forgetting about 240V loads

Your central AC, well pump, electric range, and electric dryer run on 240 volts. Most portable generators under 7,500W don't have a 240V outlet, or they have one with limited capacity. If you need to run 240V equipment, make sure your generator can actually deliver it. This is another reason a transfer switch matters — it connects the generator to your panel, which distributes both 120V and 240V circuits properly.

buying too big

Yes, too big is a real problem. An oversized generator that routinely runs at 20-30% load doesn't burn fuel efficiently. It can also "wet stack" — unburned fuel accumulates in the exhaust system, leading to carbon buildup and shortened engine life. Diesel generators are especially susceptible. Most manufacturers recommend loading a generator to at least 50% of capacity during normal use.

ignoring installation costs

The generator is not the whole price. A standby unit needs a concrete pad, an automatic transfer switch, fuel line connection, electrical work, and permits. These costs often equal or exceed the generator itself. I've broken down the real numbers in my whole-house generator cost guide.

do I need to power my whole house?

No. You don't. And understanding this one point can save you thousands of dollars.

There are two categories of loads during a power outage:

essential loads

Things you need for safety, health, and basic livability. Refrigerator. Heating or cooling (depending on climate and season). Water if you're on a well. Sump pump if you have a basement that floods. Medical equipment. Lights. Communication.

Total for most homes: 3,000-5,000 watts running.

comfort loads

Things that make a power outage feel less like a power outage. Central AC, electric stove, dishwasher, washing machine, dryer, entertainment systems, multiple small appliances running simultaneously.

Total including these: 10,000-25,000+ watts running.

See the gap? That gap is where your money lives.

If you size for essentials only, a $1,000 portable generator and a $500 transfer switch gets it done. If you size for the whole house, you're looking at a $5,000-12,000 standby unit plus $3,000-5,000 in installation. That's a real difference for a capability you might use three or four times a year.

load management: the smarter alternative to buying bigger

Here's something most sizing guides won't tell you, because it doesn't help anyone sell you a bigger generator.

Load management — sometimes called load shedding or smart load control — lets a smaller generator handle a larger total load by making sure your high-draw appliances don't all run at the same time.

The concept is simple: you never actually run everything in your house simultaneously. Your AC compressor cycles on and off. Your well pump runs for a few minutes, then stops. Your water heater heats up and shuts off. At any given moment, only some of your loads are active.

A load management system automates this. It monitors your generator's output and strategically delays or cycles certain loads so you stay within capacity. The AC compressor wants to kick on while the well pump is running? The load manager makes the AC wait 30 seconds until the well pump is done.

What this means in practice:

• A 16kW generator with load management can cover the same loads as a 22kW generator without it.
• You save $2,000-4,000 on the generator itself.
• You use less fuel because a smaller generator running at 70-80% is more efficient than a larger one running at 50%.
• You get the same effective coverage with lower upfront and ongoing costs.

Most modern standby generators from Generac, Kohler, and Briggs & Stratton offer load management modules either as standard equipment or affordable add-ons. Generac's Smart Management Modules run about $100 each and are installed on the circuits you want to manage.

The trade-off? Occasional brief interruptions to managed loads. Your AC might lag 30-60 seconds during a particularly heavy load period. Your water heater might pause for a few minutes. In practice, most people never notice.

putting it all together: a real example

Let's walk through the five steps with a realistic scenario.

The house: 2,200 sq ft, 3-bedroom, gas furnace, 3-ton central AC, well water, unfinished basement with sump pump. Family of four. No medical equipment. Located in North Carolina (hot summers, occasional ice storms).

Step 1 — Essential loads:

• Refrigerator
• Well pump (1/2 HP)
• Furnace blower
• Sump pump
• LED lights (kitchen, bathrooms, hallway)
• Modem + router
• Phone chargers (x3)

Step 2 — Running watts:

• Refrigerator: 150W
• Well pump: 1,000W
• Furnace blower: 500W
• Sump pump: 800W
• Lights: 300W
• Modem + router: 30W
• Phone chargers: 75W
• Total running: 2,855W

Step 3 — Highest starting surge: Well pump (3,000W starting - 1,000W running = 2,000W surge). Total: 2,855 + 2,000 = 4,855W

Step 4 — 20% safety margin: 4,855 x 1.2 = 5,826W

Step 5 — Generator match: A 7,500W dual-fuel portable covers this with room to spare. If they want AC during summer outages (add 3,500W running + 1,750W surge for the 3-ton unit), they're looking at roughly 12,000W — which means a standby generator in the 16-22kW range, ideally with load management.

Decision: They bought a 7,500W portable for $1,100 and a manual transfer switch for $300 (plus $400 installation). Total cost: $1,800. During summer outages, they run a window AC in the bedroom instead of central air — adds 1,200W running, well within capacity. They've been through three multi-day outages with this setup and haven't had a single issue.

Want to run these numbers for your own house? The sizing calculator does exactly this.