What Size Generator Do I Need? (Calculator + Complete Sizing Guide)

"The power just went out, the fridge is warming up, the sump pump stopped, and you're staring at a wall of generators at the hardware store. The 3,500-watt model is $400. The 10,000-watt model is $1,200. You have no idea which one you actually need."

That moment of confusion is exactly why most people either buy a generator that's too small — and trip the overload breaker the first time they plug in the AC — or overspend on a unit twice the size they need.

The core problem is the difference between running watts and starting watts. Every appliance with a motor (your refrigerator, AC, sump pump) needs a massive burst of power for the first 1–3 seconds of startup. If your generator can't handle that burst, nothing starts.

Here's what we'll give you: an interactive generator sizing calculator, a master appliance wattage table with 30+ items, per-tonnage AC sizing, reverse lookup charts, and 6 worked examples. By the end, you'll know your exact wattage needs down to the appliance.

Before you start, you'll need two things:

1. A list of every appliance you want to run simultaneously during a power outage.
2. Your AC tonnage (check the nameplate on your outdoor condenser unit — it's listed in tons or BTU).

How Generator Sizing Works: Running Watts vs Starting Watts

What Are Running Watts?

Running watts (also called rated watts) are the continuous power an appliance needs to operate. A refrigerator hums along at 100–400 running watts. A 3-ton central AC draws 2,800–3,500 running watts while cooling your house.

These are the numbers you see on the appliance nameplate or in the owner's manual. They're the easy part.

What Are Starting Watts (Surge Watts)?

Starting watts are the extra power needed for the first 0.5–3 seconds when an electric motor kicks on. At that instant, the motor's rotor is stationary — electricians call this Locked Rotor Amps (LRA) — and the current draw spikes to 3–5 times the running amperage.

That same refrigerator pulling 400 running watts? It needs 1,200–2,200 watts to start the compressor. A 3-ton central AC running at 3,000 watts can surge to 9,000+ watts at startup.

This is why a 3,000-watt generator can't start a 3-ton AC even though the running watts seem to fit. The starting surge overwhelms the generator and trips the overload protection.

The Generator Sizing Formula

The industry-standard formula used by Generac, Honda, and Champion is straightforward:

Minimum Generator Size = (Total Running Watts of All Appliances) + (Highest Single Starting Watts) × 1.20

Here's the logic. You add up the running watts of everything you'll power at the same time. Then you add only the single highest starting-watt appliance — because motors don't all start simultaneously. That 1–3 second surge happens to one appliance at a time. The 1.20 multiplier gives you 20% headroom for safety.

Appliance Wattage Reference Table (Running + Starting Watts)

This is the master reference. Find your appliances, check both columns, and use these numbers in the calculator above. Values represent typical ranges across major manufacturer data from Generac, Honda, and Lowe's reference charts.

Note: Resistive loads (heaters, lights, coffee makers) have no starting surge. Motor-driven loads (compressors, pumps, blowers) surge 2–5× their running watts. Always check the nameplate on your specific appliance — these are typical ranges.

What Size Generator to Run a House? (Sizing by Scenario)

The answer depends entirely on what you want to keep running. Here are the three most common scenarios, from minimal to full coverage.

Scenario 1: Essentials Only — Fridge + Lights + Sump Pump

This is the "keep the food cold and the basement dry" approach. Most people's first portable generator purchase falls here.

Recommended generator: 3,500–5,000 watts (portable). A Champion 3500W (4,375 starting watts) or Honda EU3000iS handles this comfortably.

Scenario 2: Essentials + Air Conditioning

Adding AC changes everything. The compressor starting surge dominates the entire calculation. Let's use a 3-ton central AC, which covers most 1,500–2,500 sq ft homes.

Recommended generator: 14–20 kW standby, or 12,000+ watt portable. This is why Generac recommends a minimum 14 kW standby generator for homes with 3-ton AC. A portable 10,000W unit is marginal here — it might not handle the AC startup surge.

Scenario 3: Whole House Generator Sizing

Whole-house coverage means running AC, the electric water heater, major appliances, and every circuit. According to the U.S. Energy Information Administration (EIA), the average American home uses about 865 kWh per month, which works out to roughly 1,200 watts average continuous draw. But that's a 24-hour average including sleeping hours.

During peak daytime usage — AC running, cooking dinner, dryer spinning — a typical home draws 5,000–10,000 watts simultaneously. Add the AC compressor starting surge on top of that, and you need significant capacity.

The Generac Guardian line covers these sizes: 10 kW, 14 kW, 18 kW, 22 kW, 24 kW, and 26 kW. All include automatic transfer switches that kick the generator on within seconds of an outage.

What Size Generator to Run AC? (By Tonnage)

Air conditioning is the single largest electrical load in most homes. The AC tonnage determines the generator you need. One ton of cooling capacity equals 12,000 BTU per hour.

The critical factor is starting watts. AC compressors draw 3–5× their running watts during the first 1–3 seconds of startup. This is caused by the motor's Locked Rotor Amps (LRA) — the massive current needed when the compressor rotor is stationary.

Generator for 1.5 Ton AC (18,000 BTU)

Running watts: 1,500 W. Starting watts: 4,500 W. Minimum generator for AC alone: 5,000 W. With essentials added: 7,500–10 kW.

Generator for 2 Ton AC (24,000 BTU)

Running watts: 2,000 W. Starting watts: 6,000 W. Minimum generator for AC alone: 7,500 W. With essentials added: 10–11 kW.

Generator for 3 Ton AC (36,000 BTU)

Running watts: 2,800–3,500 W. Starting watts: 8,400–10,500 W. Minimum generator for AC alone: 10,000 W. With essentials added: 14 kW.

This is the most common residential AC size, covering homes from 1,500–2,500 sq ft. A 7,500-watt portable generator is marginal here. Most HVAC professionals recommend a 14 kW standby generator as the minimum for a 3-ton system.

Generator for 4 Ton AC (48,000 BTU)

Running watts: 3,500–5,000 W. Starting watts: 10,500–15,000 W. Minimum generator for AC alone: 14 kW. With essentials added: 17–20 kW.

Generator for 5 Ton AC (60,000 BTU)

Running watts: 5,000–6,000 W. Starting watts: 15,000–18,000 W. Minimum generator for AC alone: 17.5 kW. With essentials added: 20–24 kW.

At 5 tons, you're firmly in standby generator territory. No portable generator under 12,000 watts can handle the starting surge. The heat pump equivalent has similar requirements.

Pro tip: Installing a hard start kit (soft starter) on your AC compressor can reduce starting watts by 50–70%. A 3-ton unit with a soft starter might only surge to 4,000–5,000 watts instead of 10,000. This can drop your generator requirement by one or two size categories.

What Size Generator for a Window AC? (By BTU)

Window ACs are far easier on generators than central systems. They run on 120V (not 240V) and their compressors are much smaller. Here's what you need, including the starting surge:

A Honda EU2200i (2,200 starting watts, 1,800 running watts) can run a 5,000 BTU window AC and not much else. To run an 8,000 BTU window AC plus a fridge and lights, you need at least 3,500 watts.

What Size Generator for a Refrigerator?

This is one of the most-searched generator questions. A standard full-size refrigerator uses 100–400 running watts but surges to 1,200–2,200 starting watts when the compressor cycles on.

The running watts are low — a fridge barely sips power. The starting surge is the problem. A 2,000-watt generator can start and run a standard refrigerator. A Honda EU2200i (2,200 peak watts) handles it with room to spare for lights and phone chargers.

For a fridge plus a chest freezer, add another 50–100 running watts and 500–1,000 starting watts. A 3,500-watt generator covers both comfortably, since they won't cycle their compressors at the exact same moment.

Mini fridges are much easier: 75 running watts and 400–600 starting watts. Even a 1,000-watt generator handles a mini fridge.

Portable vs Standby Generator Sizing Comparison

The crossover point: If you need more than 7,500 watts — basically any home with central AC — a standby generator makes more sense. A 14 kW Generac Guardian starts at about $4,500 for the unit. When you consider that a high-end 10,000-watt portable costs $1,500–$2,500, the price gap narrows quickly once you factor in the automatic transfer switch and convenience.

What Can a _____ Watt Generator Run? (Reverse Lookup)

Sometimes you already own a generator and want to know what it can handle. Here's the reverse lookup:

What Can a 3,500 Watt Generator Run?

With 3,500 running watts and ~4,375 starting watts, you can run: refrigerator + sump pump + 6 LED lights + ceiling fan + TV + Wi-Fi router + phone chargers. That's the essentials-only package.

You can also add a 5,000 BTU window AC if you stagger your loads (don't start the AC while the sump pump is cycling). You cannot run central AC or an electric heater at full power.

What Can a 5,000 Watt Generator Run?

Everything above, plus an 8,000 BTU window AC OR a well pump (not both starting at the same time). This is the sweet spot for a power outage essentials kit. You'll have cool air in one room, cold food, working lights, and your sump pump.

What Can a 7,500 Watt Generator Run?

Now we're talking. All the essentials plus a 2-ton central AC with careful load management. Or a 12,000 BTU window AC plus all your essentials running at once. This is also enough for a small workshop: circular saw (1,400 W running) + lights + drill.

At 7,500 watts, you're at the upper edge of what portable generators can practically deliver. Above this, consider stepping up to a standby unit.

What Can a 10,000 Watt Generator Run?

Full essentials plus a 3-ton central AC and furnace blower. This is whole-house coverage for a typical single-story home, minus the electric dryer and electric stove. If your area experiences frequent outages and you can't install a standby generator, a 10,000-watt portable with a manual transfer switch is a solid compromise.

Generator Sizing Worked Examples

Example 1: Power Outage Essentials Only

Scenario: Winter ice storm, 2-day outage. No AC needed. You want the fridge, lights, sump pump, furnace blower (gas furnace), TV, and phone chargers.

Answer: A 5,000-watt portable generator handles this easily. A Champion 5000W (6,250 starting watts) is the right match.

Example 2: Running a 3-Ton Central AC

Scenario: Summer heat wave in Texas. You need the AC, fridge, and basic loads. You have a 3-ton, 16 SEER central AC system.

Answer: A 14 kW standby generator or 12,000+ watt portable. This is why HVAC pros say "if you want AC on a generator, go standby." A Generac 14 kW Guardian (starting at $4,569) is purpose-built for this scenario.

Example 3: Window AC + Fridge + Lights (Apartment/Small Home)

Scenario: Studio apartment, summer outage. You have an 8,000 BTU window AC and a standard fridge.

Answer: A 3,500-watt portable generator. A Champion 3500W (4,375 starting watts) is the perfect match. Affordable, portable, and enough power to keep one room comfortable.

Example 4: Whole House Standby Sizing

Scenario: 2,800 sq ft home in Florida with a 4-ton AC, electric water heater, and a full appliance load. You want everything to run seamlessly during hurricane season.

Answer: A 24–26 kW standby generator. Note that the electric water heater alone draws 4,000 W — that's why whole-house sizing climbs fast. If you have an electric tankless water heater, add 18,000–36,000 W. At that point, you may need load management or a much larger generator.

Example 5: RV/Camping Generator

Scenario: You're camping with a travel trailer. You need to run the 15,000 BTU RV air conditioner, a mini fridge, lights, and charge devices.

Answer: A 3,500-watt inverter generator (like the Champion 3500W at 4,375 starting watts) starts the RV AC and runs everything. Honda's EU2200i is too small — the 2,200-watt starting capacity can't handle the 4,500-watt RV AC surge. If you want whisper-quiet operation, the Honda EU3000iS (3,000 starting watts, 2,800 running) is tight but workable with a soft-start kit on the AC.

Example 6: Workshop With Power Tools

Scenario: Detached workshop during a power outage. You want to run a circular saw, a drill, shop lights, and a radio.

Answer: A 5,000–7,500 watt portable generator. Don't try to run a table saw (1,800 W running, 4,500 W starting) on anything under 7,500 watts. Air compressors are even worse — a ½ HP unit needs 1,000 running watts but surges to 2,000+ watts.

Running Watts vs Starting Watts Explained

If you skipped here for a quick definition:

Running watts = the steady, continuous power an appliance needs while operating. Think of it as cruising speed on the highway.

Starting watts = the brief power surge needed during the first 0.5–3 seconds of motor startup. Think of it as flooring the gas pedal from a dead stop.

Only motor-driven appliances have significant starting surges: refrigerators, AC compressors, pumps, furnace blowers, washing machines, and power tools with motors. Resistive loads — space heaters, light bulbs, coffee makers, toasters — draw the same wattage whether starting or running.

The starting surge happens because of something called Locked Rotor Amps (LRA). When an electric motor first starts, the rotor isn't spinning. It acts like a short-circuited transformer, drawing maximum current. As the rotor accelerates to operating speed (within 1–3 seconds), the current drops back to normal running amps.

A general rule of thumb: LRA is approximately 5–6 times the Full Load Amps (FLA). For AC compressors, the starting watts are typically 3–5× the running watts. You can find the exact LRA on your AC compressor's nameplate — multiply LRA by voltage to get the starting wattage.

Generator Sizing FAQ

How many watts does a house use?

The average U.S. home uses about 865 kWh per month, which averages to roughly 1,200 watts at any given moment. However, during peak usage (AC running, cooking, laundry), a home can draw 5,000–10,000 watts simultaneously. That's why a 10–22 kW standby generator is recommended for whole-house coverage.

Can a portable generator run central air conditioning?

It depends on the AC size. A 7,500-watt portable can handle a 1.5–2 ton AC with careful load management. A 10,000+ watt portable can start a 3-ton AC. For anything larger, you need a standby generator. The AC compressor starting surge is the limiting factor — not the running watts.

What is the difference between running watts and starting watts?

Running watts are continuous operating power. Starting watts are the 2–5× higher surge needed for the first 1–3 seconds when a motor starts. Your generator must handle the starting watts or the motor won't start. See the section above for a full explanation.

What size generator do I need for a 2,000 sq ft house?

A 2,000 sq ft home typically has a 2.5–3 ton AC system. For whole-house coverage with AC, you need an 18–20 kW standby generator. For essentials only (no AC), a 5,000–7,500 watt portable is sufficient.

Do I need a generator big enough to handle starting watts?

Yes — absolutely. If your generator can't deliver the starting watts, motor-driven appliances simply won't start. The motor will stall, and the generator's overload protection will trip. Always size based on total running watts + highest starting watts, not running watts alone.

Will a 3,500-watt generator run a refrigerator?

Yes, easily. A refrigerator needs only 400 running watts and 2,200 starting watts. A 3,500-watt generator (typically 4,375 starting watts) starts and runs a fridge with over 1,000 watts to spare for lights, fans, and chargers.