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) / 1000On 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
| Input | What 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 labelled 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 energises 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 enquiry form is not a storm line.
Related on this site
- Backup power as a service on the homepage Portable, inverter or standby, and why the transfer equipment is the safety-critical part.
- Frequent or long outages Which circuits must stay live is the question underneath the product.
- Warning signs Burning smells and heat at a panel are a stop, generator planning or not.
- Find electrical help An enquiry, not a dispatch, and not an emergency service.
Questions this calculator answers
- What size generator do I need for my house?
- The size of the loads that must stay up, not the size of the house. Add nameplate running watts for those circuits, add the starting bump of the hardest motor, then divide by a derate so the set is not planned at 100% of its sticker. A refrigerator, some lighting and a furnace fan is a different job from a well pump, an electric range and a 240 V charger. This tool only does that arithmetic on the watts you type.
- Why is a 5,000 W load not a 5,000 W generator?
- Two reasons, both on this form. Motors draw more while they start than while they run, so the starting look can be the number that sizes the set. And a generator that is planned at 100% of its rated watts has no headroom for heat, altitude, fuel quality or a load you forgot. The default derate here is 0.8, a planning placeholder, not a manufacturer derating 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 the thing sits in. A well pump and a sump pump can look identical on a floor plan and differ by thousands of watts on the plate.
- What is the starting adder?
- The extra watts a motor wants for a few seconds when it starts. Compressors and pumps are the usual culprits. You type it because we will not invent a locked-rotor table for your pump. If you do not know it, ask the electrician who will connect the set, or read the manufacturer starting-watt figure, rather than leaving it at zero and hoping.
- Can I plug a generator into a dryer outlet?
- No. That arrangement backfeeds the service and can energise lines that utility workers believe are dead. This page will not size, draw or describe that cord. The legal path is a transfer switch or an interlock kit listed for your panel, installed as licensed electrical work and inspected. Portable generators also produce carbon monoxide; placement is a life-safety question, not a convenience question.
- Where can I run a portable generator?
- Outdoors only, well away from openings. The U.S. Consumer Product Safety Commission says never to run a portable generator inside a home or garage, even with doors and windows open, and to keep it at least 20 feet from the house with the exhaust pointing away. The CDC says the same 20-foot figure. Carbon monoxide is colourless and has no smell. A garage with the door up is still indoors for this purpose.
- Do I need a transfer switch?
- If the generator will feed house circuits through the panel, you need a transfer means that prevents the set and the utility from being connected at the same time. That is licensed work. A listed transfer switch or a listed interlock is the conversation. An extension cord to a single appliance, outdoors, is a different (and smaller) use, and still does not make the garage a legal place to run the engine.
- 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 enquiry 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 colourless 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.)
Related
- Outage cost Hours and lumps you type. The dollar is not the kilowatt class.
- Electrical load calculator Whole-house demand is a different question from which circuits stay up on a set.
- Panel space checker A transfer switch or interlock still needs a panel that can take it.
- Electrical project cost Type the transfer-equipment line from a bid. We will not invent it.
- Electrical safety check A burning smell or a breaker that will not hold is a stop, generator or not.
- Electrification readiness If the house is going all-electric, the standby conversation changes with it.