Generator Size Calculator
Generator Size Calculator
Tick what you want to run in an outage, on the job or at the campsite, and see the generator size you need: the running watts of everything together, the extra surge a motor needs to start, and the smallest common generator that covers both with room to spare.
Running load by appliance
Running and starting against the generator
Appliance watts are typical values within the ranges published by generator makers such as Champion Power Equipment; the label on your own appliance, or its manual, is better. Starting watts are the brief peak a motor draws as it starts. The page assumes only one motor starts at a time, so the starting peak is everything running plus the largest single motor’s extra. Headroom keeps the generator below full load; 25 percent means running it at no more than 80 percent. Portable generators are listed up to 15,000 W and standby units above that; a portable’s starting rating is taken as 1.25 times its running rating, so check the real figure on the model you buy. Connect a generator to house wiring only through a transfer switch or interlock fitted by an electrician. The CSV, image and printed report contain the values you entered, nothing preloaded.
How to use this tool
Open each group and tick what you want to run at the same time, setting how many where you have more than one. If something is not listed, enter its running and starting watts from the label as another load. The running load, the starting peak and the generator to look for appear at the top, and the common setups on the right open with one click.
Headroom is set to 25 percent, so the generator runs at no more than 80 percent of its rating; lower it if you will switch loads on and off carefully. The first chart shows what each load draws, and the second compares the running and starting peaks with the suggested generator.
How the size is worked out
Every appliance draws its running watts while it works. Anything with a motor, a fridge compressor, a pump, a furnace blower, an air conditioner or an air compressor, also needs a brief surge as it starts, often two or three times its running draw. A generator has two ratings to match: running watts it can supply all day, and starting watts it can supply for a few seconds.
The page adds up the running watts of everything ticked, then adds the extra starting watts of the single biggest motor, on the basis that motors rarely start in the same instant. The running total times the headroom gives the running rating needed, and the starting peak the starting rating. The suggestion is the smallest common size that covers both, taking a portable's starting rating as 1.25 times its running rating.
Running and starting watts of common appliances
| Appliance | Running W | Starting W |
|---|---|---|
| Home essentials | ||
| Refrigerator | 300 | 1,000 |
| Chest or upright freezer | 300 | 800 |
| Sump pump, 1/2 hp | 950 | 1,800 |
| Well pump, 1/2 hp | 900 | 1,800 |
| Well pump, 1 hp | 1,500 | 3,000 |
| Furnace blower, 1/2 hp | 550 | 1,200 |
| LED bulb | 10 | same |
| TV | 200 | same |
| Internet router and modem | 15 | same |
| Phone and laptop charging | 60 | same |
| CPAP with heated humidifier | 90 | same |
| Security system | 25 | same |
| Kitchen and laundry | ||
| Microwave | 1,000 | same |
| Electric range, one burner | 1,500 | same |
| Dishwasher | 1,500 | same |
| Washing machine | 750 | 1,650 |
| Electric dryer | 5,000 | same |
| Electric water heater | 4,000 | same |
| Heating and cooling | ||
| Window AC, 5,000 BTU | 525 | 1,050 |
| Window AC, 10,000 BTU | 1,050 | 2,100 |
| Central AC, 2 ton | 2,250 | 4,000 |
| Central AC, 3 ton | 3,250 | 5,500 |
| Central AC, 4 ton | 4,500 | 7,250 |
| Space heater | 1,500 | same |
| Ceiling or box fan | 75 | same |
| Tools and outdoors | ||
| Circular saw, 7 1/4 in | 1,500 | 3,000 |
| Air compressor, 1 hp | 1,500 | 4,500 |
| Pressure washer, 2,500 psi | 2,250 | 4,500 |
| RV air conditioner, 13,500 BTU | 1,750 | 3,150 |
| RV refrigerator | 350 | 700 |
Typical values within the ranges generator makers publish. Starting watts are the total draw for the moment a motor starts.
A worked example you can check on paper
For an outage, a home wants a fridge at 300 W, a 1/2 hp sump pump at 950 W, a furnace blower at 550 W, six LED bulbs at 10 W, a TV at 200 W, the router at 15 W and phone and laptop charging at 60 W. The running total is 2,135 W.
The sump pump is the biggest motor: 1,800 W to start against 950 W running, 850 W extra. The starting peak is 2,135 plus 850, 2,985 W. With 25 percent headroom the running rating needed is 2,135 times 1.25, 2,669 W, so a 3,000 W portable, with about 3,750 W to start, covers both. Add a 1/2 hp well pump and the answer rises to 4,000 W.
Portable, inverter or standby
Inverter generators, usually 2,000 to 4,000 W, give clean, quiet power for electronics, RVs and camping, and some can be paired for more output. Open frame portables, 3,000 to 15,000 W, cost less per watt and run essentials or tools. Standby generators, about 10 to 26 kW for homes, are installed permanently on natural gas or propane and start by themselves through an automatic transfer switch.
Mistakes that catch people out
Ignoring starting watts. A generator that runs the load can still stall or trip when the well pump or air conditioner starts.
Forgetting the voltage. Well pumps, central air, water heaters and dryers are often 240 V loads; the generator and transfer switch must supply 240 V.
Buying the biggest you can. A generator far larger than the load burns more fuel for nothing; the table shows when a size is bigger than needed.
Running it too close. Carbon monoxide from a generator in a garage or near a window kills people every year. Keep it outside, well away from openings.
Popular calculations
Each opens the calculator with its own address, and every value can be changed once it is open.
Outage essentials
Well pumps
Air conditioning
RVs
Job site
Headroom
How this was checked
The engine was tested in Node against the hand calculation above, 2,135 W running, 2,985 W starting and a 3,000 W generator, and against cases where the starting peak rather than the running load decides the size: a circular saw with a 1 hp compressor runs on 3,000 W but needs 6,000 W to start, so the page suggests 5,000 W, not 4,000 W. Appliance figures were checked against published manufacturer wattage charts and fall within their ranges.
Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.
Common questions
What size generator do I need to run a refrigerator?
A refrigerator runs on about 300 W but can need around 1,000 W for a moment to start its compressor, so even a 2,000 W portable covers one. A fridge, freezer and 1/2 hp sump pump together come to about 1,550 W running and 2,400 W starting, still within a 2,000 W unit but with little to spare; 3,000 W leaves room for lights and a furnace fan.
What size generator do I need for a house?
For outage essentials, a fridge, freezer, sump or well pump, furnace blower, lights and electronics, most homes need 3,000 to 6,500 W. Running central air, an electric water heater or an electric range pushes it to 10,000 W or more, usually a standby unit.
What is the difference between running and starting watts?
Running watts are what an appliance draws while it works. Motors, in fridges, pumps, compressors and air conditioners, often draw two to three times that for a second or two as they start. A generator lists both ratings, and it must cover the running total and the starting peak.
Can a 3,000 W generator run a central air conditioner?
Usually not. A 3 ton central AC runs on about 3,250 W and can need around 5,500 W to start, so it needs at least a 9,500 W portable or a standby unit once the rest of the house is added, unless a soft start kit is fitted.
Why add headroom?
Generators run cooler, quieter and longer below full load, and appliance labels vary. Sizing so the running load is no more than about 80 percent of the rating, 25 percent headroom, leaves room for a fridge and a pump that start together.
How do I connect a generator to my house?
Only through a transfer switch or a panel interlock fitted by a licensed electrician. Plugging a generator into an outlet can backfeed the utility lines and kill line workers, and it can damage the generator when power returns.
Related Calculators and Reference
- Watts to Amps Calculator: current for any load and voltage.
- Electricity Cost Calculator: what each appliance costs to run.
- Solar Panel Calculator: panels to cover your yearly use.
- Voltage Drop Calculator: cable size for a long generator cord.
- BTU Calculator: air conditioner size by room.
- Champion Power Equipment generator wattage chart: published running and starting ranges.