What Size Generator Do You Need? A Load Worksheet With Surge, Fuel and Connection Rules

Size a generator by adding the running watts of everything that must run at the same time, then adding the largest extra surge any single motor needs to start, and choosing a generator whose running rating covers the first number and whose starting rating covers the total. For an example essentials list (refrigerator, freezer, gas furnace blower, half-horsepower sump pump, lights and internet) that works out to 3,636 running watts and about 6,000 watts at the moment the sump pump starts, which a generator rated around 5,000 running watts with a starting rating of at least about 6,000 watts can handle if the motors are not started all at once.

Those figures are an illustration built from example nameplates, not a rule of thumb for your house. Generator sellers publish "a typical home needs 5 to 7 kW" tables; the trouble is that the right answer for your house can be half or double that depending on three things the tables skip: whether your heat and water come from motors that run on 240 volts, how many motors can start at the same moment, and how long you need to run. This page is a worksheet for all three, plus the connection and carbon monoxide rules that decide whether a generator is safe to use at all. It is part of our backup power hub. It is general information, not electrical or engineering advice for your home; a licensed electrician should design any connection to house wiring.

Decide what has to run, not what the house has

A sizing mistake worth avoiding is starting from the main panel. A 200-amp service can deliver about 48,000 watts at 240 volts (200 × 240), far more than an essentials plan needs during an outage. What you need is the set of loads that must run at the same time to keep the house safe and livable for the length of outage you plan for.

A useful way to sort loads is into three tiers:

How long you need to run matters as much as how much. The EIA reports that U.S. electricity customers averaged about 11 hours without power in 2024, nearly twice the decade average, and that about 9 of those hours came from major events such as hurricanes; outside major events, interruptions routinely average about 2 hours a year. South Carolina customers averaged nearly 53 hours in 2024. Averages also hide the long tail. EIA says major events accounted for 80% of hours without electricity in 2024, and a multi-day outage is the case that drives the fuel planning below.

Read the nameplates

Every appliance and motor has a label (a nameplate or data plate) that lists its electrical rating. That label is the number to size from.

• Volts and amps. For a 120-volt load, running watts are roughly amps × 120. For a 240-volt load, amps × 240. Nameplate amps are a maximum rating, so this gives a conservative (high) running figure. Some labels list watts directly.
• Starting current for motors. Compressors (refrigerators, freezers, air conditioners, heat pumps) and pumps draw a large current for a fraction of a second as they start. Compressor nameplates often list it as LRA (locked rotor amps). That surge is what trips undersized generators. If the label gives no starting figure, the manufacturer's manual or support line can.
• Furnaces. A gas or oil furnace's data plate lists the total amps for the blower, inducer and controls. The burner uses fuel, not electricity, so a gas furnace's electrical load is small compared with an electric furnace or a heat pump.
• EnergyGuide labels are not sizing labels. The yellow EnergyGuide label gives annual kWh, which tells you how much energy the appliance uses over a year, not its peak draw. It is useful for fuel and battery runtime math, not for choosing generator capacity.

When in doubt, a clamp meter reading taken by an electrician during normal operation gives the real running current. For corded appliances, a plug-through meter rated for the load gives the same reading, where the appliance maker allows one; see our smart plug and energy monitoring guide for which loads should not go on one.

The worksheet, filled in

Here is the example essentials list as a worksheet. The amps are example nameplate values; the starting surge is an assumption (three times running watts for each motor) used only to show the method. Replace both with your own figures.

Then work out the peak:

1. Running total: 3,636 watts. The generator's continuous (running) rating must exceed this, with margin; generator manuals generally advise against running continuously at full rated output, so check yours for its recommended continuous load.
2. Largest single surge: the sump pump, at an assumed 2,352 extra watts while starting. If the sump pump starts while everything else is running, the peak is 3,636 + 2,352 = 5,988 watts. The generator's starting (surge) rating must cover that.
3. Worst case: if all four motors start at the same instant, which happens when power is switched over to a generator with every load already on, the peak is 3,636 + 6,672 = 10,308 watts.

The gap between the second and third bars is the cheapest capacity you will ever buy. Switching loads on one at a time, a few seconds apart, after the generator is running turns a 10 kW problem into a 6 kW one. A transfer switch with load management, or simply a habit of turning circuits on in sequence, does the same job.

120 volts, 240 volts and the two legs

A generator with a 120/240-volt outlet supplies 240 volts across two 120-volt legs, like the split-phase service to a house. That matters in two ways.

Each 120-volt leg carries only part of the generator's capacity. If a 7,500-watt generator's output is split evenly, each 120-volt leg can carry about 3,750 watts (7,500 ÷ 2); the manual lists the actual per-leg and per-receptacle limits. If all your must-run loads sit on the same leg, you can overload it while the generator's total is nowhere near its rating. An electrician wiring a transfer switch should balance the circuits across both legs.

Some must-run loads are 240-volt only. Central air conditioners and heat pumps, electric furnaces, electric water heaters, electric dryers and many well pumps are commonly wired for 240 volts; the nameplate tells you which yours is. A generator without a 240-volt outlet, or an inverter generator that only produces 120 volts, cannot run them at all, regardless of its wattage.

Heat is the load that decides the class

What heats your house can change the answer more than any other single load. The table is general guidance, not data from a source; your equipment's nameplate and manuals override it.

For heat pump homes, the heat-strip rating on the indoor unit's label is the number to look for; it is often larger than everything else in the essentials list combined. Our guide to dual-fuel and backup heat for heat pumps explains how those strips are staged.

Starting surge: three ways to shrink it

• Stagger the starts. Bring loads online one at a time once the generator is running and stable. It costs nothing, and in the worksheet example it cuts the peak from 10,308 to 5,988 watts.
• Soft starters on compressors. A soft-start kit for an air conditioner or heat pump lowers the starting current, which can let a smaller generator start the compressor. Have an HVAC technician fit and check it; it must suit the specific compressor.
• Inverter-driven equipment. Variable-speed (inverter-driven) compressors are designed to ramp up rather than start at full speed. Ask the manufacturer for the starting current of your exact model; a low figure makes the unit far easier to run from a generator or battery.

Inverter generators are a different thing: they regulate their output electronically, and their spec sheets list running and starting ratings that you should compare with your worksheet, not with the headline wattage. Check the starting watts they list, not only the running watts.

Fuel: how long it runs, and how much you need to store

A generator's fuel consumption depends on its size and how heavily it is loaded. The manufacturer's specification sheet lists runtime at a given load, usually at 25% or 50% of rated output, for a full tank. Convert that to gallons per hour, then multiply by the hours you plan for.

Worked example (illustration with stated inputs). A gasoline portable generator's spec sheet lists 10 hours of runtime at 50% load from a 6-gallon tank, or 0.6 gallons per hour. Your example essentials list runs at around half of its rating most of the time.

Thirty to forty gallons of gasoline is a lot to store safely at home, and the generator manual sets out how fuel should be stored and rotated. Fuel stations can be affected by the same outage. That is why households planning for multi-day events often choose dual-fuel generators that can run on propane, or a permanently installed standby generator on natural gas or a large propane tank. Our standby generator cost guide covers that class, and our propane tank installation cost guide covers the fuel side.

Running the generator in cycles also stretches fuel. A refrigerator and freezer hold their temperature for hours with the doors closed. Ready.gov's guidance is that a refrigerator keeps food cold for about four hours and a full freezer holds its temperature for about 48 hours. Running the generator in shifts, for example on for a few hours and then off for no more than about four hours at a time so the refrigerator stays within Ready.gov's four-hour limit, with the sump pump on its own schedule if the water table allows, cuts the hours the generator burns fuel. Running it half the time cuts fuel use roughly in half, as long as the load while it runs stays similar.

Connecting it to the house: the part that hurts people

How the generator connects matters more than its size. There are three acceptable approaches and one dangerous one.

Backfeeding is the one to understand. The New Jersey Department of Community Affairs warned during Hurricane Sandy that a generator connected to a home's wiring or plugged into a regular household outlet without a functional transfer switch can feed power back onto utility lines and electrocute anyone who touches them, including lineworkers making repairs. The same advisory said connecting a generator to a home's circuits must be done by a qualified, licensed electrician who installs a transfer switch. FEMA's U.S. Fire Administration gave the same warning after Hurricane Irene, in an advisory reposted by the Maryland Department of Emergency Management: backfeeding happens when a portable generator is connected directly to a home's wiring without a functional transfer switch, and it poses an electrocution hazard for first responders, utility workers and neighbors.

For the cost of that work, see our generator transfer switch installation cost guide. If you are weighing a larger electrical upgrade at the same time, our panel capacity guide for electrification explains how a generator inlet fits into a panel plan.

Carbon monoxide: where the generator sits

CPSC states that an average of nearly 100 people in the U.S. die each year from CO poisoning caused by portable generators, citing its report on non-fire carbon monoxide deaths associated with consumer products.

CPSC's February 2026 guidance is specific:

• Never operate a portable generator inside a home, garage, basement, crawlspace, shed or other enclosed space. Opening doors or windows will not provide enough ventilation.
• Operate it outside only, at least 20 feet from the house, and not on a porch or in a carport.
• Direct the exhaust away from the home and from other buildings' entrances.
• Install CO alarms; interconnected alarms are best, so that when one sounds, they all sound.
• Look for portable generators that have a CO shut-off safety feature.

Twenty feet of separation has a sizing consequence: cord runs get long, and long cords drop voltage. Use cords rated for outdoor use and for the current they carry, and keep the total length as short as the 20-foot rule allows. A transfer switch with an outdoor inlet box placed for that distance solves the cord problem permanently.

Generator classes, and what each means for the worksheet

The worksheet result points to a class of machine before it points to a model. Each class reads the same two numbers (running and starting watts) differently. As with the heating table, this is general guidance; the spec sheet of the model you are considering is the source to trust.

Two numbers on a spec sheet deserve a second look. The first is the fuel used for the rating: a dual-fuel machine's propane output is the one that matters if your plan for a multi-day outage is a propane cylinder rather than gas cans. The second is the outlet layout: a generator with a 30-amp, 240-volt outlet can feed a transfer switch inlet; one with only 120-volt household receptacles cannot run any hard-wired 240-volt load, whatever its wattage.

Rehearse the plan before the outage

A generator bought in October and first started during an ice storm is an untested plan. A short rehearsal on a mild day exposes most problems while the stores are open:

1. Start it and run it under load. Plug in or transfer the must-run list, in your staggered order, and leave it for an hour. Watch for breakers tripping on the generator or the transfer switch.
2. Check the furnace and pumps actually run. If the furnace locks out or a pump will not start, call the installer before winter rather than during it.
3. Time the fuel. Note the fuel level before and after the hour. Your measured consumption at your real load is a better runtime figure than the spec sheet's 50% line.
4. Walk the cord or inlet route. Confirm the 20-foot placement works in snow or rain, that the exhaust points away from every door and window, and that the CO alarms on each floor are working.
5. Write the sequence down. A one-page card taped inside the panel door with the switching order and the loads to leave off is what lets someone else in the household run the system if you are not home.

Repeat the rehearsal each fall, and rotate stored gasoline with a stabilizer as the manufacturer recommends. If your generator has a CO shut-off feature and it trips during a test, treat that as a placement problem, not a fault in the generator.

Generator or battery?

Batteries change the sizing logic. A battery system is rated by power (kW, what it can supply at once) and energy (kWh, how long it lasts), and its inverter decides what it can start. It makes no noise or exhaust, but it runs out unless solar or a generator recharges it. A battery sized for a few hours covers the typical non-major-event year (about 2 hours, per EIA). Major events, which EIA says accounted for 80% of 2024 outage hours, can run for days, and a battery then needs a large capacity or a way to recharge, such as solar or a generator. Our battery critical-load sizing guide uses the same must-run list method as this worksheet, so the two can be compared directly.

A sizing checklist you can fill in tonight

1. List every must-run load and its nameplate volts and amps. Note which ones are 240 volts.
2. Add running watts. This is your minimum continuous rating, before margin.
3. Find the single largest starting surge (from LRA, the manual, or the manufacturer).
4. Decide whether you will stagger starts. If not, add every motor's surge.
5. Check 120-volt leg balance if you use a 240-volt generator.
6. Pick a runtime target from your outage history, and turn it into gallons or pounds of fuel.
7. Choose the connection method, and budget for an electrician if anything is hard-wired.
8. Plan the generator's location 20 feet from the house before you buy cords or an inlet.

Next up: if the worksheet shows a whole-house load or a heat pump you need to run, compare the result with the installed price in our standby generator cost guide, because at that size a permanently installed unit with an automatic transfer switch often makes more sense than a large portable.

Frequently Asked Questions

What size generator do I need to run a refrigerator and a furnace?

Add the running watts of both from their nameplates, then add the larger of the two starting surges. In the example worksheet on this page, a refrigerator (720 W) and gas furnace (960 W) together run at 1,680 watts, and the furnace blower's assumed start-up surge brings the peak to about 3,600 watts. Your appliances' labels give the real figures.

Can a portable generator run a heat pump?

Sometimes, but it is a demanding load. A heat pump's outdoor compressor runs on 240 volts and has a large starting surge, and its backup heat strips can draw several kilowatts. A soft starter or an inverter-driven heat pump reduces the surge. Without one, many heat pumps need a generator in the standby class.

How many watts does it take to run a whole house?

Running every circuit at once is rarely the goal. A 200-amp service can deliver about 48,000 watts, but a household's real simultaneous load during an outage is far smaller if large loads such as central air, electric cooking, electric water heating and EV charging are run one at a time or not at all. Size to the loads that must run together, not to the panel.

What is the difference between running watts and starting watts?

Running watts are what a load draws once it is going. Starting (surge) watts are the extra a motor needs for a moment as it starts. A generator lists both. Its running rating must cover the total of everything running, and its starting rating must cover that total plus the largest surge you allow to happen at one time.

How far from the house should a generator be?

CPSC says to operate portable generators outside only, at least 20 feet from the house, never in a garage, porch or carport, with the exhaust directed away from doors and windows. Opening windows does not make indoor or garage use safe.

Is it legal to plug a generator into a wall outlet?

Plugging a generator into a household outlet is backfeeding. The New Jersey Department of Community Affairs and FEMA's U.S. Fire Administration both warn that it can energize utility lines and endanger utility workers. The New Jersey advisory adds that connecting a generator to a home's circuits must be done by a licensed electrician who installs a transfer switch. Local codes and permit rules govern the installation.