Generator Size Calculator

This generator size calculator totals the watts your appliances need so you can pick a generator that will actually run them. Tick the items you want to power at the same time, adjust quantities and wattages, and the tool adds up running watts and the biggest starting surge a motor will pull when it kicks on.

It then recommends a running rating with 20% headroom, which you can change, and a minimum starting (surge) rating. The appliance list uses typical values from widely published generator wattage charts. Real appliances vary, so replace any number with the one on the nameplate or in the manual whenever you can.

How to Use the Generator Size Calculator

1
Tick the appliances you will run together
Check each item you want powered at the same time during an outage. Unchecked items are ignored. A starter set of essentials is ticked by default.
2
Edit quantities and watts
Change the quantity, running watts or starting watts for any row. Use the nameplate values if you have them. For items with no motor, starting watts equal running watts.
3
Add custom items
Enter a name, running watts and optional starting watts, then press Add item. Custom rows can be removed with the x button.
4
Read the recommendation
Check total running watts, peak starting surge and the recommended generator size. Adjust the headroom percentage if you want more or less margin.

How the generator sizing works

Every appliance has a running wattage, the power it draws while operating. Appliances with electric motors or compressors, such as refrigerators, sump pumps, well pumps and furnace blowers, also draw a brief surge when they start, often two to three times their running watts for a second or two.

The calculator adds up the running watts of everything you tick. Because motors rarely start at exactly the same moment, it then adds only the single largest extra starting load (starting watts minus running watts) to find the peak surge. Finally it adds headroom to the running total so the generator is not working at 100% of its rating.

Running total = sum of (running watts x quantity); Peak surge = running total + largest (starting watts - running watts); Recommended running rating = running total x (1 + headroom %), rounded up to the next 100 W; Recommended starting rating = peak surge, rounded up to the next 100 W

Worked example: essentials during a winter outage

Using the default ticked items: a refrigerator (700 W running, 2,200 W starting), a 1/2 HP sump pump (1,050 W, 2,150 W), a 1/2 HP furnace fan (875 W, 2,350 W), ten 10 W LED bulbs, a 150 W TV, a 1,000 W microwave and a 20 W modem and router.

Running total: 700 + 1,050 + 875 + 100 + 150 + 1,000 + 20 = 3,895 W
Extra starting loads: refrigerator 1,500 W, furnace fan 1,475 W, sump pump 1,100 W. The largest is the refrigerator at 1,500 W
Peak surge: 3,895 + 1,500 = 5,395 W, rounded up to 5,400 starting watts
With 20% headroom: 3,895 x 1.2 = 4,674 W, rounded up to 4,700 running watts
Look for a generator rated at least about 4,700 running watts and 5,400 starting watts

Typical appliance wattage chart

These are the default values in the calculator, taken from commonly published generator wattage charts. Your own appliances may be higher or lower, so check the label, especially for well pumps, which vary widely by horsepower.

Appliance	Running watts	Starting watts	Extra at start
Refrigerator	700	2,200	1,500
Chest or upright freezer	500	1,500	1,000
Sump pump (1/2 HP)	1,050	2,150	1,100
Well pump (1/2 HP)	1,000	2,100	1,100
Furnace fan (1/2 HP)	875	2,350	1,475
Window AC (10,000 BTU)	1,200	1,800	600
Space heater	1,500	1,500	0
Microwave	1,000	1,000	0
Washing machine	1,150	2,250	1,100
Electric clothes dryer	5,400	6,750	1,350
Electric water heater	4,000	4,000	0

Tips and common mistakes

Size for what runs at the same time, not everything you own. You can stagger loads, for example running the microwave only while the space heater is off.
Do not forget the starting surge. A generator that covers your running watts can still stall or trip when a refrigerator or well pump starts.
Microwave labels often list cooking power. The power drawn from the outlet is usually higher, so use the input watts from the label if shown.
Heating elements are the big loads. Space heaters, electric dryers and electric water heaters use far more power than lights and electronics.
Fuel use rises with load. Headroom helps, but a generator far larger than you need burns more fuel than necessary for light loads.

Generator safety and connecting to your home

Portable generators produce carbon monoxide, a colorless, odorless gas that can kill within minutes. Run a generator only outdoors, well away from doors, windows and vents, and never in a garage, basement or enclosed porch, even with the door open. Keep battery-powered CO alarms working inside the house.

Never plug a generator into a wall outlet to backfeed the house. It can energize utility lines and injure line workers. To power hardwired loads such as a furnace or well pump, have a licensed electrician install a transfer switch or interlock kit and follow local code and the generator manual.

Features

Checklist of 16 common household appliances with typical running and starting watts
Edit quantity, running watts and starting watts for every item
Add your own custom appliances
Total running watts and peak starting surge from the largest motor load
Recommended generator rating with adjustable headroom (default 20%)

Frequently Asked Questions

What size generator do I need to run a refrigerator?

A typical refrigerator runs at about 700 watts but needs roughly 2,200 watts for a moment when the compressor starts. On its own, that means a generator with at least 2,200 starting watts. In practice you will run other things too, so add them in the calculator. The essentials example on this page, with a refrigerator, sump pump, furnace fan, lights, TV, microwave and router, comes to about 4,700 running and 5,400 starting watts.

What is the difference between running watts and starting watts?

Running watts, also called rated watts, are the power an appliance draws continuously. Starting watts, or surge watts, are the brief spike some appliances need to get a motor or compressor turning, often two to three times the running figure. A generator lists both. Its running rating must cover everything on at once, and its starting rating must cover that load plus the biggest motor start.

Why does the calculator add only the largest starting surge?

Motors rarely start at exactly the same instant, and the surge lasts only a second or two. The common sizing method is to add up all running watts and then add the single biggest extra starting load. If two large motors could start together, such as a well pump and a sump pump, add the second one's extra starting watts to the peak surge yourself and look for a generator with that starting rating.

How much headroom should I leave when sizing a generator?

A margin of about 20% above your total running watts is a common rule of thumb. It keeps the generator from running flat out, leaves room for a load you forgot, and accounts for appliances drawing more than their label. The calculator uses 20% by default. You can set anything from 0 to 100%, and the result is rounded up to the next 100 watts.

Can I plug my generator into a wall outlet to power the house?

No. Backfeeding through an outlet with a double-male cord can send power back onto utility lines, endanger line workers and neighbors, damage equipment and start fires. To run hardwired circuits like a furnace, well pump or lights, have a licensed electrician install a transfer switch or an approved interlock kit. Otherwise, run heavy-duty extension cords from the generator directly to the appliances.

Is a 5,000 watt generator enough to run a house?

It can run the essentials in many homes: a refrigerator, freezer, sump pump, furnace fan, lights and electronics. It usually cannot also run central air conditioning, an electric range, an electric dryer or an electric water heater, each of which can need 4,000 watts or more on its own. Tick exactly what you plan to run in the calculator to see whether 5,000 watts covers it.

Last reviewed September 21, 2026