If you’re shopping for a home generator, you’ve probably seen advice like “get a 5,000-watt generator for an average house” or “a 2,000-square-foot home needs a 10,000-watt generator.”

That’s not a reliable way to size a generator.

The right size depends on what you actually need to power — not your home’s square footage. Getting this right matters before you commit to buying anything. A generator’s job is to run specific appliances and circuits, not “the house” as an abstract unit. Two homes of the same size can have very different power needs depending on what’s actually running.

What does wattage actually mean?

Generators are rated in watts, which measure electrical power output. You’ll typically see two numbers:

• Running watts (or rated watts) — the continuous power the generator can supply
• Starting watts (or surge watts) — a higher, brief burst of power some equipment needs to start up, particularly anything with a motor or compressor

This distinction matters because appliances with motors — refrigerators, well pumps, air conditioners — often need two to three times their running wattage just to start. A generator sized only for running watts can fail to start equipment that would otherwise run fine once going.

Why square footage isn’t enough

Square footage tells you nothing about what’s actually drawing power in a home.

One home might have:

• A gas furnace (low electrical draw, just needs power for the blower and controls)
• A standard refrigerator
• Basic lighting and outlets

Another home of identical size might have:

• Central air conditioning (a much bigger electrical load)
• An electric water heater
• A sump pump
• A well pump

The second home needs substantially more generator capacity, even though the square footage is identical. This is why “square feet per watt” rules of thumb produce unreliable results.

What is a load calculation, and do you need one?

The accurate way to size a generator is to add up the actual wattage of what you want to power during an outage.

For a portable generator running select circuits, this means listing out specific appliances: refrigerator, some lights, maybe a window AC unit or space heater, phone chargers, and a few other essentials. You add up their running watts, account for the highest starting watts among them, and size accordingly.

For a whole-house standby generator, an electrician typically performs a more complete load calculation based on your home’s electrical panel and what you want backed up — sometimes everything, sometimes just critical circuits.

Ask a generator installer or electrician: “Can you walk me through the load calculation for my home?” That’s a better question than “What size generator do I need?” on its own.

What can a small portable generator actually run?

A smaller portable generator (roughly 2,000-3,500 running watts) can typically handle:

• A refrigerator
• Some lights
• Phone and device charging
• A window fan or small space heater
• A laptop or TV

It generally cannot run central air conditioning, an electric water heater, or a well pump simultaneously with those other items — the math often doesn’t work.

What can a larger portable generator run?

A larger portable generator (roughly 5,000-8,500 running watts) can typically handle more simultaneous loads:

• A refrigerator and freezer
• Multiple rooms of lighting
• A sump pump
• A window AC unit
• Some kitchen appliances (not all at once)

Running a central air conditioner or an electric range at the same time as everything else may still exceed capacity, depending on the specific units involved.

What about a whole-house standby generator?

Whole-house standby generators are typically sized much larger (often 14,000-22,000+ watts) and are installed to automatically power some or all of a home’s electrical panel during an outage.

Even here, sizing isn’t automatic. A standby generator can be sized to run everything in the home simultaneously, or sized more modestly to run essential circuits only (heating, refrigeration, some lighting, a well pump) while some non-essential circuits stay off during an outage. This is a real decision with real cost implications, not a one-size-fits-all default. See Portable vs Standby Generator for a closer look at which category fits your situation.

Does a bigger generator just make more sense?

Not automatically. A generator sized well beyond your actual needs means:

• A higher upfront cost
• More fuel consumption to run
• Potentially oversized equipment that’s less efficient at partial loads

The goal isn’t maximum capacity. It’s capacity that matches what you actually intend to power.

What if you only want to power the essentials?

Many homeowners don’t try to power an entire house during an outage — they prioritize:

• Refrigerator and freezer
• Some lighting
• A sump pump, if flooding is a risk
• Heating system controls (even a gas furnace needs some electricity to run the blower and thermostat)
• Maybe a window AC unit or space heater depending on season

Add up the running watts for just these items, plus the highest starting watts among them, and you’ll typically need far less capacity than a “power everything” approach.

Questions to ask before buying

Before purchasing a generator, ask:

• What specifically do I want to power during an outage?
• What are the running and starting watts for each of those items?
• Am I trying to power everything, or just essential circuits?
• Do I need a transfer switch, and is that included in the price?
• For a whole-house system, has an actual load calculation been done for my panel? This is also where knowing what to ask before hiring an installer pays off. See How Much Does a Whole-House Generator Cost? for what a full system typically runs.
• What fuel type makes sense for how long I might need to run this?

Should you plan for future needs?

If you’re considering adding an EV charger, upgrading to central air, or making other major electrical changes, actor that into your generator sizing conversation now rather than discovering later that your generator can’t keep up with a home that’s changed since you bought it. See How Much Does It Cost to Install a Generator? to understand installation costs alongside sizing.

The bottom line

There’s no reliable generator size you can determine from square footage alone. The right approach is to decide what you actually want to power during an outage, add up the running and starting watts for that specific equipment, and size the generator — or have an electrician size a whole-house system — based on that real number.

Before You Fix, know what you’re actually trying to power before you buy a generator sized for guesswork.