Sizing and cost calculators

What size generator do I need

Blank notebook and tape measure on gravel.

The short answer

What size generator do I need? Add all running watts that will run at the same time. Then add starting gaps with the correct method: only the largest gap if motors start one at a time, or all gaps if they can start together. Use the rated output for your actual fuel, and confirm voltage and connection limits. Treat that result as a screening number, not a full compatibility check.

Last updated October 2026

What's covered15

Generator size starts with your load, not your square footage. A load is everything you want powered at the same moment. Floor area can hint at a home's systems, but it cannot list them.

This guide explains the sizing steps. It covers running watts, motor surges, fuel ratings, voltage and connection limits.

To size a generator, add the running watts of everything that will run at once. Then add the extra starting watts for motors. Add only the largest gap if motors start one at a time, or every gap if they may start together. Next, use the rated output for your fuel. Finally, check voltage and how the unit connects.

Generator sizing factors at a glance

Factor What to check Why it matters
Running load Sum of running watts for all loads on at once Must stay at or under rated output
Starting gap Starting watts minus running watts, per motor Covers the brief surge when a motor starts
Start pattern One at a time, or possibly together Changes how many gaps you add
Fuel Rated output for the fuel you will use Listed models rate fuels differently
Voltage 120 V only, or 120/240 V An adapter cannot create a missing voltage
Connection Receptacle, inlet, switch and panel match Output alone does not prove compatibility
Placement Outdoor rules or the standby manual Engine exhaust contains carbon monoxide

How to work out what size generator you need

Start with R. R is the total running watts of everything on at the same time. Include every motor in R, using its running watts.

Next, find each motor's starting gap. The gap equals its total starting watts minus its running watts. If that number is negative, treat it as zero.

If only one motor will start at a time, use this rule. Required size S equals R plus the largest single gap. If motors may start together, add every gap instead. Count each motor that could start at that moment.

Two mistakes cause wrong numbers. One is adding a motor's full starting watts on top of R, which counts that motor twice. The other is leaving its running watts out of R, then adding only its gap.

Do not assume motors never start together. After an outage, power may return to several loads at once. If you cannot control that, use the simultaneous-start method.

Why floor area is only context

Two homes can share a footprint and still have very different loads. One may heat with gas. The other may use an electric heat pump.

Floor area tells you nothing about which devices run together. It also says nothing about motor surges or voltage. Use it as background only, then build a real load list.

Running watts and rated output

Running watts are what a device draws once it is already on. A generator's rated output is what it can supply over time. Compare your R total with that rated figure.

Maximum output is a separate, higher number. Compare starting needs with it, not your steady running load. Honda's published US specs show the gap between the two ratings.

The Honda EU2200i lists 1800 W rated and 2200 W maximum output at 120 V. The EU3200i lists 2600 W rated and 3200 W maximum. The EU3000iS lists 2800 W rated and 3000 W maximum.

These numbers describe generator output only. They do not prove any appliance will run. You still need that appliance's running watts, starting watts, voltage, plug type and expected run time.

Run time also differs by load. Honda lists the EU2200i at 3.2 hours at rated load. It lists 8.1 hours at quarter load. A bigger load drains fuel faster on that model.

Starting watts and motor surges

Many motors draw a short surge when they start. The generator must cover that surge at that moment. If it cannot, the motor may stall or a breaker may trip.

Use the maker's starting watt figure for each motor when it is listed. If no figure is listed, your number is incomplete. A missing starting figure is a gap, not a zero.

Inverter generators still use an alternator. They add power electronics that shape the output. That design does not prove a given motor will start. Use the same arithmetic for every generator type.

For more detail on the two ratings, read running wattage vs starting wattage.

Watts, volt-amps and watt-hours

Three units often get mixed up. The Department of Energy style guide separates them clearly.

Watts measure power. One thousand watts equals one kilowatt. Watt-hours measure stored or used energy. Watts times hours gives watt-hours.

Volts times amps gives volt-amps, or VA. That is apparent power. You can turn VA into watts only when the power factor is known. Without it, do not treat a VA figure as watts.

This matters for labels. Some devices list amps instead of watts. Multiplying amps by volts gives VA, not a confirmed watt figure.

Fuel derating on listed models

Some generators run on more than one fuel. The listed models below rate each fuel differently. Always check the figure for the fuel you will use.

The Champion 201176 lists 7250 W running on gasoline. It lists 6525 W on propane and 5875 W on natural gas. It offers 120/240 V output.

The Westinghouse WGen11500TFc lists 11,500 W running on gasoline. It lists 10,500 W on propane and 9500 W on natural gas. Its peak ratings are 14,500 W, 13,500 W and 12,000 W, in the same fuel order.

The current DuroMax XP13000HXT manual lists 10,500 W running on gasoline, 10,000 W on propane and 9,000 W on natural gas. Use the matched set of current running and starting ratings below after checking your unit revision.

DuroMax rating sources. The current XP13000HXT manual, page 80 lists running/starting watts of 10,500/13,000 on gasoline, 10,000/12,500 on propane and 9,000/11,250 on natural gas. The current US product table agrees.

The manual filename marks revision 02072025. The older sales sheet, page 2 gives lower propane and natural-gas ratings. These are current document figures, not proof that every unit has those ratings.

Match your unit's label and manual revision before sizing loads. Starting watts do not promise a motor will start or state how long peak output lasts.

Smaller dual-fuel units follow the same pattern. The Westinghouse iGen2550DFc lists 1900 W running and 2550 W peak on gasoline. On propane it lists 1700 W running and 2300 W peak.

The Champion 201278 lists 3500 W running and 4500 W starting on gasoline. On propane it lists 3150 W running, with the same 4500 W starting figure.

These ratings apply to these models only. Other models may rate fuels differently. Fuel availability also varies by location, so plan around what you can actually get.

Natural gas use brings its own checks. Fuel pressure and flow, exact instructions and voltage are separate checks. You cannot work out gas flow from electric watts. Gas connections belong to licensed trades.

Voltage and connection limits

Voltage is a hard limit. A 120 V generator cannot supply a 240 V load. Generac states that an adapter cannot create a missing output voltage.

The Honda models above list 120 V. The Champion 201176 and the Westinghouse WGen11500TFc list 120/240 V. Match the output to your loads and connection plan.

A transfer switch separates home wiring from the utility. That stops power from flowing back onto utility lines. Generac notes that the generator, switch, inlet, panel and circuit design must all match.

The Reliance Controls 306CRK is a six-circuit, 30 A manual kit. It includes a 306C switch, PB30 inlet and PC3010 ten-foot cord. That list shows the contents, not whether it suits your home.

The Generac HomeLink 6853 is a 30 A pre-wired manual kit. Generac says upgrades depend on system requirements.

The Reliance TF152W handles one circuit at 125 V and 15 A. It has a NEMA 5-15 inlet. Reliance lists a maximum generator running rating of 1875 W. That 125 V figure is a component rating, not a voltage for all appliances.

Receptacles matter too. The iGen2550DFc has an RV TT-30R outlet. Westinghouse says that model is not transfer-switch-ready. An RV outlet alone does not prove approved home use.

Still, 120 V output alone does not rule out every transfer setup. The exact product documents decide. Interlocks must be listed and accepted for your exact panel and local rules.

Westinghouse's current WGen11500TFc page also describes pairing with its WATS200 automatic transfer system. This guide does not compare that system's full requirements. Whether automatic operation suits your home stays open until you have the exact documents.

Installation and service belong to licensed electrical trades. Never modify wiring, neutrals or gas lines yourself. Never bypass protective devices.

Placement and safety limits

Engine generators make carbon monoxide. The CPSC says to run portable engine generators outdoors. Keep them at least 20 feet from homes, with exhaust pointed away. Never run them indoors or in a garage, even with doors open.

The CPSC safety alert adds more steps. Use a dry surface. For cover, use an open, canopy-like structure. Follow the maker's exact rules for wet weather.

Never touch the unit with wet hands. Shut it off and let it cool before refueling. Never plug a generator into a wall outlet to backfeed the house.

Install carbon monoxide alarms. If one sounds, or anyone feels sick, get to fresh air at once.

Standby units follow their own installation manual and local rules for clearance. Any engine generator still needs safe outdoor placement, even if it runs automatically.

Battery power stations

Battery power stations store energy instead of burning fuel. Size them with two numbers. Rated output in watts limits what runs at once. Capacity in watt-hours limits how long it runs.

The BLUETTI AC200L lists 2048 Wh capacity and 2400 W rated AC output. It has 120 V US outlets and can recharge from solar or AC power.

The Jackery Explorer 2000 v2 lists 2042 Wh and 2200 W rated AC output, at 120 V. The EcoFlow RIVER 2 Pro lists 768 Wh and 800 W output. A separate 700 variant has different specs, so match the exact model.

These sources do not list surge ratings for every unit. Follow each product's own instructions for use and placement.

Standby generators and larger loads

Standby generators are usually rated in kilowatts. The listed models below rate propane higher than natural gas.

The Generac Guardian 26 kW lists 26 kW maximum continuous on liquid propane. It lists 22.5 kW on natural gas, rated at 240 V.

The KOHLER 20RCA lists 20 kW on propane and 18 kW on natural gas. It offers a US single-phase 120/240 V option. On the listed option, the automatic transfer switch is sold separately.

The Cummins QuietConnect RS20A brochure lists 18 kW on natural gas and 20 kW on propane. It is 120/240 V, single phase. Confirm the exact order setup in the current manual.

Check installation scope before tying a size choice to a budget. Generac's US guidance puts a complete standby system at an indicative $8,000 to $16,000, including equipment, materials and installation. It does not price the specific circuits or fuel route in your home. Ask the installer to show those items in the quote.

A larger output label alone cannot explain the gap between two installed bids.

Building your load list

Write down each device you want powered. Record its running watts, starting watts, voltage, plug type and needed run time. Mark which motors could start together.

Then run the numbers in our generator size calculator. It applies the startup rules above.

Some loads need a closer look. Read what size generator to run heat pump for compressor loads. For cooling food, read what size generator to run refrigerator and freezer.

If you are weighing one output class, read what can you run on a 5000 watt generator. A generator size chart can help you compare classes. For general buying advice, start at our best generator for home guide.

Sources, method and limitations

We compared figures with maker documents and government guidance. We used each model's exact US specification where one was listed.

When maker documents differ, identify the exact source used. The DuroMax note above gives its current manual ratings, and an older sheet differs. Match that revision to your unit.

This guide has limits. It does not list appliance wattages, noise comparisons or lifespans. It does not test whether a generator starts a specific motor. Specs can change, so check the current page for your exact model.

What size generator do I need FAQs

How do I figure out generator size?

Add the running watts of everything on at once. Then add starting gaps for motors. Use only the largest gap if motors start one at a time. Add every gap if they may start together.

Compare the total with the rated output for your fuel, then check voltage and connection.

Can I size a generator by square footage?

No. Floor area is only background. Two homes of the same size can have very different loads. Heating type, motors and voltage all change the answer.

Build a list of devices that will run together. Then use their running and starting watts to find your number.

Why does fuel change generator output?

Some multi-fuel generators list a different rating for each fuel. For example, the Champion 201176 lists 7250 W on gasoline and 5875 W on natural gas. Other models may differ, so never assume. Always size with the rating for the fuel you will actually use.

Can an adapter fix a voltage mismatch?

No. Generac states that an adapter cannot create a missing output voltage. A 120 V generator cannot run a 240 V load. Check the generator's output voltage against every load and connection part.

A licensed electrician should handle any home connection under local rules.

Primary documents

What size generator do I need Verdict

What size generator do I need? Size your generator from a real load list. Include all running watts in R. Then add starting gaps the right way for your start pattern.

If motors start one at a time, add the largest gap. If they may start together, add every gap. Use the rated output for your actual fuel.

If your loads need 240 V, choose a model that lists 120/240 V. If you plan a home connection, match every part and use licensed trades. Treat your result as a screening number, not proof of fit.

Continue your backup plan

For an installed backup plan, compare the best whole house generator options after completing the load check. The package and transfer equipment need their own review.

Compare the backup options

With your load and cost limits in hand, compare home backup options. Match the listed limits to your figures before choosing a model or planning an installation.