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Generator size from the watts you type, not a SKU

A 5,000 W running load is not a 5,000 W generator. Starting current and a derate sit between the nameplates and the class. This page will not pick a model, will not walk a cord into a dryer outlet, and will not treat a transfer switch as a weekend project.

8.8 kW class on a 5,000 W running example 5,000 W running plus a 2,000 W starting adder at a 0.8 derate. Planning watts, not a spec sheet, and not a reason to run a generator in a garage.

Add the running watts of every load that must stay up, then a starting adder for the motor that kicks hardest, then a derate (0.8 is the placeholder on this form).

On 5,000 W running and 2,000 W starting at 0.8, the class is 8.8 kW, not a 5 kW carton. Carbon monoxide and backfeed kill people. A transfer switch or interlock is licensed work. HyreElectrical does not sell or install generators. Live work is not DIY.

Running watts, starting adder, derate

Nameplates, not a room count. Nothing is emailed. This will not pick a model.

Sum of nameplate running watts on the circuits you actually intend to keep alive.

Extra watts for the hardest motor start. 0 only if nothing on the list is a motor.

0.8 is a placeholder so the set is not planned at the sticker. Not a spec sheet.

— Kilowatt class
— Starting look vs running
— What this is not

What this assumed —

Planning watts, not a spec sheet and not a SKU. Interlock or transfer switch is a licensed job. Portable generators kill people with CO and with backfeed. Live work is not DIY. HyreElectrical does not install generators.

The generator is not the hazard. The exhaust is, and it kills people every year

The exhaust warning has to come before the arithmetic, because sizing a machine correctly and then placing it wrongly is a fatal combination and the placement error is the common one.

CPSC states the placement rule plainly. “Operate generators outdoors only, at least 20 feet from homes and buildings, with exhaust directed away from windows, doors and vents.” 20 feet, outdoors, with the exhaust pointed away from every opening in the building.

And the prohibition, without exceptions. “Never use a portable generator inside homes, garages, basements, crawlspaces, sheds or other enclosed areas, even if doors or windows are open.” Note the last clause. Opening the garage door does not make a garage an outdoor space, and it is precisely the reasonable-sounding compromise that recurs in these cases.

Why carbon monoxide is different from other hazards. It has no smell, no color and no taste.

There is no sensory warning at all, and its early effects — headache, nausea, drowsiness, confusion — are both unremarkable and specifically the kind of symptoms that reduce a person’s ability to recognize that something is wrong and act on it.

The circumstances make it worse. These events happen during storms and cold snaps, when the natural instinct is to move the machine somewhere sheltered and to keep windows and doors shut. Every one of those instincts pushes in the direction of the hazard.

Which is why a working carbon monoxide alarm belongs in the kit, alongside the fuel and the cords. Not instead of the 20 feet — the alarm is the last line, not the first — but because the first line depends on judgment made by cold, tired people in the dark.

And it is why this page will not help you improvise. Live electrical work is not a do-it-yourself subject here. Nothing on this site instructs a reader to open, probe or modify energized equipment.

There is no version of running a generator in a partially enclosed space that this site will describe as acceptable, and CPSC’s wording above is why.

The numbers worth knowing before you size anything

20 ftminimum distance from homes and buildings, exhaust directed away from windows, doors and ventsUS Consumer Product Safety Commission
11 hoursaverage electricity interruption per US customer in 2024US Energy Information Administration
2 hoursof that which is routine, non-major-event interruption in a typical yearUS Energy Information Administration
9 hoursattributed to major events in 2024, against roughly four in the previous decadeUS Energy Information Administration

EIA figures from Hurricanes in 2024 led to the most hours without power in the United States in 10 years (Today in Energy), 1 December 2025.

They are national averages: your own utility’s record is the number that describes your risk, and it varies enormously by region and by whether your supply is overhead or underground.

Running watts and starting watts are two different constraints, and only one of them is on the box

The sizing failure that catches people is not the total. It is the instant a motor starts.

Where the capacity goes in a modest backup set — illustrativeRefrigerator, running — 200 W21.3%Well pump, running — 1,000 W12.8%Furnace fan, running — 600 W8.5%Lighting and electronics — 400 W53.2%Headroom for the largest motor starting — 2,500 WIllustrative. The segment values are placeholders chosen to show the shape of the problem — the starting allowance dominating the running load— and are not measurements of any equipment. Use your own nameplates.
Illustrative, stated here rather than in a footnote. The point is the proportion: the surge allowance is frequently larger than everything running combined.Our own illustration of the running-versus-starting problem. No figure in it is a measurement.

Running watts is a steady-state sum. Add up what the things you want to keep alive draw while they are simply operating. This is the arithmetic everybody does, and it is the easy half.

Starting watts is a transient. Motors — compressors, pumps, fans — draw a large multiple of their running current for a fraction of a second while the rotor gets moving. The generator has to ride through that moment without its voltage collapsing.

Only the largest one matters, not the sum of them. Two motors starting in the same instant is improbable and is usually designed against anyway.

So the rule of thumb is: the total running load, plus the starting surge of the single largest motor. This tool asks you to type that adder rather than assuming it.

Why we make you type it. The multiple varies substantially with motor type and starting method, and a modern variable-speed compressor behaves nothing like an old single-speed one. Printing a single multiplier would be inventing a number that applies to nobody. The equipment nameplate or its data sheet is where that figure comes from.

What undersizing feels like. Not a clean failure. Lights dip when the pump starts, the generator labors, and motors that are fed low voltage while trying to start draw more current and get hot.

Sustained brownout conditions are harder on equipment than having no power at all, which is why "it mostly works" is not a satisfactory result.

And what oversizing costs. Fuel, noise, purchase price, and — on some engines — running lightly loaded for long periods, which is not what they are happiest doing. Bigger is not automatically safer here either.

Deciding what you are actually trying to keep alive

Deciding what to keep running sizes everything else, and it is worth making deliberately rather than by adding up the whole house. The last column is what people wish they had thought about first.

LoadWhy it makes the listWhat it does to the sizingThe thing people miss
Refrigeration and freezingThe single most defensible item: food spoils on a clock and cannot be recovered.Modest running load, real starting surge from the compressor.A full freezer holds temperature for a surprisingly long time unopened. Cycling a generator for a few hours a day may be sufficient, which changes the sizing question entirely.
Heating in a cold climateA house below freezing is a plumbing catastrophe waiting to happen, quite apart from comfort.Often small if it is only a furnace fan and controls; very large if the heat itself is electric.A gas furnace usually needs only its fan and controls backed up. That is a tiny load, and it is the single highest-value item on many lists.
Water — well pump or boosterNo power, no water, and no sanitation with it.Usually the largest starting surge in a residential set.The pump is what sets the size. Households on a well frequently discover that this one load doubles the machine they needed.
Sump pumpStorms cause outages and water at the same time, by the same mechanism.Small running load, meaningful surge.This is the load whose absence costs the most, because a flooded basement during the outage is a restoration job afterwards.
Medical equipmentNon-negotiable where it applies.Usually small.It needs its own plan that does not depend on a generator starting: batteries, and a conversation with the utility about a medical registry, which many operate.
Lighting, phones, internetCheap, and disproportionate to morale and to being able to get information.Almost nothing.The router and the modem are trivially small and routinely forgotten in favor of large appliances.
Air conditioning, electric water heating, ovens, dryersComfort, and in a heatwave sometimes more than comfort.Very large. These are what turn a modest set into an expensive one.These are the loads that determine whether you are buying a portable set or a standby installation, and they should be a conscious decision rather than a default.

Our own framing of the priorities, written to shrink the list rather than grow it. It prices nothing and recommends no equipment. HyreElectrical holds no dataset of bids, invoices, inspections or jobs. No figure on this site is our own measurement of the market.

How the power gets into the house is the part that matters most, and it is not a homeowner job

Sizing is arithmetic. Connection is the part with the two genuinely serious failure modes, and one of them endangers people who are nowhere near your house.

The safe options exist and they are ordinary. Running cords from the machine to individual appliances is legitimate and needs no installation at all.

Beyond that, permanently installed transfer equipment is what allows a generator to feed the house wiring itself.

That equipment is installed by a licensed electrician, under permit and inspection where the jurisdiction requires it.

What transfer equipment does. It guarantees that the generator’s output and the utility’s supply can never be connected at the same time. That guarantee is mechanical or electrical rather than procedural, which is exactly the point: it does not depend on anybody remembering anything at 3am in the rain.

The failure mode it prevents. Backfeeding — energizing the house wiring in a way that pushes power back out onto the utility supply.

That can put a lethal voltage onto conductors that line workers have every reason to believe are dead, and it can do it through a transformer at a very much higher voltage than the generator produces.

This is the reason the improvised double-male-ended cord is treated as an emergency-level hazard rather than as a shortcut.

And the second failure mode. When utility power returns, an unswitched generator meets it. That is destructive to equipment and dangerous to whoever is standing next to it.

Which is why this page stops here. HyreElectrical does not perform, supervise or warrant electrical work, does not open panels, and does not pull permits. We will not describe an interlock arrangement, an inlet installation or any part of the connection as something to attempt.

The correct content on this subject is: get an electrician. NEC Article 702 (optional standby systems) and Article 705 (interconnected power production sources, including the busbar sizing allowance often called the "120 percent rule") are directly relevant to generator and battery work.

We did not retrieve verbatim text for either on the retrieved date, so no numeric rule from them is quoted anywhere on this site. Ask the electrician which article they are working to.

One consequence for the sizing conversation. Because the connection is a permitted, inspected job, it is also the part of the project that takes time. If you are reading this because a storm is forecast, cords to individual appliances outdoors is the option that exists this week, and the installation is a conversation for afterwards.

What a good standby proposal says — and what to have ready before the storm

The first six are questions for a contractor. The last three are for you, this week, whatever you decide about equipment.

  • Which circuits are being backed up, listed individually

    Not "essential circuits". The actual list, agreed in advance, because that list is what sizes the machine and what you will live with in the dark.

  • The running load and the largest starting surge, written down

    With the nameplate figures they came from. If the surge allowance is missing, the sizing has skipped the constraint that actually binds.

  • What transfer equipment is proposed, and who permits and inspects it

    A professional will answer this immediately for your jurisdiction. Vagueness about the permit on this particular job is a serious warning sign, because this is the safety-critical element.

  • Fuel: what, how much, and how long it lasts at the modeled load

    Runtime figures are quoted at a stated load and fall sharply above it. A tank that lasts a day at half load may not last a night at three quarters.

  • Where the machine sits, and how far that is from every opening

    CPSC: “Operate generators outdoors only, at least 20 feet from homes and buildings, with exhaust directed away from windows, doors and vents.” A spot near a window or under a deck is a design problem to solve now, not a rule to bend later.

  • What maintenance it needs, and what happens if it is skipped

    A standby set that has never been exercised is a decoration. Ask what the exercise schedule is, who does it, and what the fuel does while it sits.

  • A working carbon monoxide alarm on every level, tested

    Cheap, immediate, and the one item on this page that is useful whether or not you ever buy a generator.

  • A plan that does not require electricity

    Water, blankets, torches with charged batteries, a way to charge a phone, and knowing which neighbors may need checking on. Most outages are short: EIA puts routine interruption at about 2 hours a year.

  • An improvised connection to the house wiring

    Never. This is the backfeed hazard described above, it endangers utility workers who cannot know you have done it, and no saving justifies it.

What this tool cannot do

The calculator does not name a machine. No brand, no model, no affiliate link. It produces a watt figure from the loads you typed and stops there.

It does not know your starting surges. You type the adder because the multiple varies by motor and starting method, and we will not print a universal multiplier that describes nobody’s equipment.

It says nothing about the connection. Transfer equipment is the safety-critical part of the installation, it is licensed work, and NEC Article 702 (optional standby systems) and Article 705 (interconnected power production sources, including the busbar sizing allowance often called the "120 percent rule") are directly relevant to generator and battery work.

We did not retrieve verbatim text for either on the retrieved date, so no numeric rule from them is quoted anywhere on this site. Ask the electrician which article they are working to.

It does not model runtime or fuel. Those depend on the engine, the load fraction and the fuel supply, none of which is on this form.

It does not price anything. There is no national average price for a panel upgrade, a service change, an EVSE circuit or a rewire on this site, because we hold no dataset of bids and the figures that circulate in roundups come from lead-generation sites with unpublished methods.

Nor does it model whether a generator pays for itself; that question belongs on the outage cost page, where you type your own losses.

And it is not a substitute for the exhaust rule. “Never use a portable generator inside homes, garages, basements, crawlspaces, sheds or other enclosed areas, even if doors or windows are open.” No sizing result changes that sentence.

How this calculator works

The calculator takes the running watts you type, adds the starting adder you type, and divides both by the derate you type (clamped between 0.5 and 1, default 0.8):

needRun   = running watts / derate
needStart = (running watts + starting adder) / derate
class kW  = max(needRun, needStart) / 1000

On the default example, 5,000 / 0.8 = 6,250 W running, (5,000 + 2,000) / 0.8 = 8,750 W starting, so the class is 8.8 kW.

That is the figure the engine is tested against. It is not a Honda, a Generac, a Champion or any other carton, and it is not permission to connect the set to the house.

The derate is a planning placeholder. Manufacturers publish altitude, temperature and power-factor derates of their own. If you have those, type a derate that reflects them. If you do not, leave 0.8 and treat the result as a conversation with the person who will install the transfer equipment, not as an order.

What each input means

Inputs on this tool, in the order they appear on the form.
InputWhat it is actually asking
Running watts The sum of nameplate running watts on the loads that must stay up. Not every circuit in the house. Not a square-footage rule of thumb.
Starting adder The extra watts of the single hardest motor start you expect while other loads are already running. Typed by you, because a national locked-rotor table would be fiction for your pump.
Derate A number between 0.5 and 1. Default 0.8. Divides the watts so the set is not planned at 100% of its rating. Not copied from a spec sheet we do not have.

Worked examples

Including one where the naive answer misleads, which is the example most calculators leave out.

A refrigerator, lighting and a furnace fan

Say the plates add to 1,800 W running, and the furnace-fan starting bump is 800 W, at the default 0.8 derate. needRun = 2,250 W. needStart = 3,250 W. Class 3.3 kW.

That is a small inverter conversation, still with outdoor placement and still without a cord into the dryer outlet. The transfer question does not go away because the number is small.

The same house plus a well pump

Keep 1,800 W running of house load and add a well pump whose plate is 2,200 W running with a 3,500 W starting adder, because the pump is the motor that actually sizes the set.

Running total 4,000 W, starting look 4,000 + 3,500 = 7,500 W, both divided by 0.8: 5,000 W running, 9,375 W starting. Class 9.4 kW. The pump, not the floor area, moved the class.

Leaving the starting adder at 0 because "it only runs a few seconds" is how a 5 kW set stalls the first time the well calls.

The one where a 5 kW sticker misleads

Loads that add to 5,000 W running, a 2,000 W starting adder ignored, derate ignored, carton labeled 5,000 W purchased. This engine on those watts at 0.8 is an 8.8 kW class.

The naive answer is a set that is already at its sticker before anything starts, and that is the example most product finders leave out.

A second naive answer is worse: feeding the house through a dryer outlet or a similar backfeed so the panel is live while the utility is not. This page will not size that cord. The transfer switch is the job.

What changes the result

The starting adder only moves the class when it is large enough that (run + start) / derate beats run / derate. A 200 W bump on a 5,000 W running total does almost nothing at 0.8. A well pump or an air conditioner compressor can be the whole conversation.

The derate is a straight divider. At 1.0 the class equals the larger of running and starting, in kilowatts. At 0.8 it is 25% larger. At 0.5 it doubles. Do not type 1.0 to make a carton you already bought look sufficient.

What you include in running watts changes the class more than any of the above. Whole-house standby (range, dryer, EVSE, electric water heater) is a different order of set from "fridge, lights, furnace." If the house is electrifying, run the electrification tool and the load calculator before you shop a generator.

Local considerations

Placement and transfer equipment are both local as well as national. Carbon monoxide rules from CPSC and CDC apply everywhere in the United States: outdoors, more than 20 feet from openings, never in a garage even with the door up.

How your building department permits a standby set, whether an interlock is accepted on your panel type, and what the utility requires for interconnection are local. Ask those two offices. A 50-state dropdown on this page would be invented law.

The National Electrical Code is a model code. Your jurisdiction adopts an edition and may amend it. This page does not name your edition and does not say your current setup is illegal. It does say that a generator which can backfeed the utility is a hazard to people who did not buy the generator.

When not to use this

Do not use this as a shopping list, a model number, or a reason to connect a generator to the house yourself. It cannot see altitude, fuel, the starting method, whether the panel will take a listed interlock, or whether a standby pad will pass inspection.

Do not use it during an outage to invent a connection. If the power is already out and you have a portable set, the only use this site will describe is a cord to a single appliance, outdoors, with the generator itself outdoors and 20 feet from the house.

Anything that energizes house wiring is licensed work, and it is not something to learn from a web page in the dark.

Do not run the set in a garage, a basement, a porch with walls, or next to a window because of rain. Carbon monoxide is the reason, not fussiness. Get to fresh air and call emergency services if anyone has headache, dizziness, nausea or confusion around a running generator.

HyreElectrical does not perform electrical work, sell generators, or dispatch technicians. Matching is still being built. The inquiry form is not a storm line.

Related on this site

Questions this calculator answers

What size generator do I need for my house?
Sized to the loads that must stay on, not the house. Add the running watts of those circuits, add the starting surge of the hardest motor, and apply a derate so the generator is not planned at 100%. A refrigerator and furnace fan is a smaller job than a well pump and electric range.
Why is a 5,000 W load not a 5,000 W generator?
Two reasons. Motors draw more when starting than when running, so the starting surge can decide the size. And a generator planned at 100% of its rating has no headroom for heat, altitude, fuel or a forgotten load. This form's default derate of 0.8 is a planning figure, not a manufacturer's table.
Where do I find running watts?
On the nameplate or in the appliance manual, as watts or as amps times volts: a 120 V device at 10 A is 1,200 W. Do not guess from the room. A well pump and a sump pump look alike on a floor plan and can differ by thousands of watts on the plate.
What is the starting adder?
The extra watts a motor draws for a few seconds when it starts; compressors and pumps are the usual culprits. You type it because we will not invent a starting table for your pump. If you do not know it, use the manufacturer's starting-watt figure or ask the electrician who will connect the generator.
Can I plug a generator into a dryer outlet?
No. That backfeeds the service and can energize lines that utility workers believe are dead. The legal path is a transfer switch or an interlock listed for your panel, installed as licensed work and inspected. Portable generators also produce carbon monoxide, so where one sits is a life-safety question.
Where can I run a portable generator?
Outdoors only, well away from openings. The CPSC says never run one inside a home or garage, even with doors and windows open, and keep it at least 20 feet from the house with the exhaust pointed away; the CDC gives the same distance. Carbon monoxide is colorless and odorless.
Do I need a transfer switch?
Yes, if the generator will feed house circuits through the panel. You need a listed transfer switch or interlock that stops the generator and the utility from connecting at the same time, installed as licensed work. An extension cord to one appliance, outdoors, is a smaller use and needs no transfer equipment.
Is this the generator I should buy?
No. The output is a kilowatt class on the watts you typed, after the derate you typed. Fuel, runtime, starting method, whether the set is portable or standby, and whether it is listed for the transfer equipment you will use are all off this page. HyreElectrical does not sell generators and does not rank brands.
Should I size from dollars I lose in an outage?
No. The outage-cost tool on this site adds hours and lumps you type. That number can tell you whether backup is worth talking about. It cannot tell you whether a refrigerator and a furnace fan fit on a 3 kW inverter. Size from watts here. Price the conversation separately.
Does HyreElectrical install generators?
No. HyreElectrical does not perform electrical work, sell equipment, or dispatch anyone in a storm. Matching is still being built. The inquiry form is not an emergency line. If you can smell exhaust indoors, get to fresh air and call emergency services.

Sources and methodology

Figures dated 26 August 2026. Last reviewed .

  • Carbon monoxide information center (portable generators) (U.S. Consumer Product Safety Commission, retrieved 2026-08-26. Never run a portable generator in a home or garage, even with doors and windows open. Keep it at least 20 feet from the house, exhaust pointing away. CPSC reports more than 100 CO deaths a year linked to portable generators.)
  • Carbon monoxide poisoning basics (Centers for Disease Control and Prevention, retrieved 2026-08-26. CO is colorless and odorless. CDC repeats the outdoor, more-than-20-feet placement rule for generators, and the same "not in the garage even with the door open" warning.)
  • NFPA 70, National Electrical Code (National Fire Protection Association, retrieved 2026-08-26. Cited as the model code that jurisdictions adopt. Transfer equipment that prevents backfeed lives in the adopted edition. This page does not reprint those articles and does not declare your install illegal.)
  • Home electrical safety guidance (Electrical Safety Foundation International, retrieved 2026-08-26. Cited for the standing rule that live work and panel work are not homeowner walkthroughs.)

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