Watts and kilowatts
Generators are rated in kilowatts (kW). One kW is 1,000 watts. A 20 kW generator can supply about 20,000 watts at once.
Backup power
What Size Generator Do You Need for a Whole House? A Guide Before the Next 24 Hour Outage
Buy a generator that's too small and it stalls when the AC starts. Buy one that's too big and you've given up yard space for power you'll never use. This guide walks you through sizing in plain steps, with real numbers for common household items and examples from homes like those in Marlboro and nearby towns.
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- Local to Monmouth County
The short answer
Add up the running watts of everything you want on at once, add the starting surge of your largest motor, and leave some headroom. Essentials-only homes often need a smaller standby unit, while mid-size homes with central air often land around 18 to 22 kW. A load calculation by an electrician confirms the exact size.
The basics
Watts, Kilowatts and Starting Power in Plain Words
Three ideas make generator sizing easy to follow.
Running watts
The power an appliance uses once it's on. A fridge might only need a few hundred watts while running.
Starting watts
Motors need a big burst to start. AC units, well pumps and sump pumps can briefly draw two or three times their running power.
What do you need to run?
Typical Power Use of Common Household Items
These rough numbers help you see where the power goes.
| Item | Typical running watts | Starting surge? |
|---|---|---|
| Refrigerator | About 100 to 800 | Yes, small |
| Sump pump (1/3 to 1/2 HP) | About 800 to 1,050 | Yes, large |
| Furnace blower | About 500 to 800 | Yes, moderate |
| Well pump (1/2 to 1 HP) | About 1,000 to 2,000 | Yes, large |
| Lights and outlets (a few rooms) | About 500 to 1,500 | No |
| Wi-Fi, TV, computer | About 200 to 600 | No |
| Microwave | About 1,000 to 1,500 | No |
| Central air (about 3 ton) | About 3,000 to 4,000 | Yes, very large |
| Electric dryer | About 4,000 to 5,500 | Small |
Homes with natural gas heat, cooking and hot water need far less generator than all-electric homes.
Step by step
A Simple Way to Size a Whole-House Generator
You can get a good rough idea in about ten minutes.
List what must stay on
Start with must-haves: heat, fridge, sump or well pump, some lights, internet, medical equipment.
Add the comfort items
Then decide on nice-to-haves: central air, cooking, laundry, more rooms of lights.
Add up running watts
Total the running watts of everything you'd use at the same time.
Add the biggest starting surge
Add the extra starting power of the single largest motor, usually the AC or well pump.
Leave some headroom
Don't plan to run a generator at 100 percent. A cushion keeps it from struggling.
This gets you in the right range. A licensed electrician confirms it with a proper load calculation, the same method we describe in our 100 vs 200 amp guide.
Real-world examples
What Size Fits Which Home?
Typical ranges we see in Monmouth County homes.
| Home and goal | Usual range |
|---|---|
| Essentials only: heat, fridge, sump pump, lights, Wi-Fi | Smaller standby unit, often around 10 to 14 kW |
| Most of a mid-size home, plus one AC unit | Mid-size unit, often around 18 to 22 kW |
| Whole large home with two AC units | Larger air-cooled unit with load management, or bigger |
Load management changes the math
Smart load management modules pause a big load, like a second AC unit, for a few seconds while another one starts. That lets a smaller generator cover a bigger home. Brands like Generac and Kohler offer these.
Don't forget
Fuel Type Affects How Much Power You Get
Two details that change the final size.
Natural gas vs propane
Many air-cooled standby generators produce somewhat less power on natural gas than on propane. Always check the generator's natural gas rating, since most Marlboro homes use natural gas.
Your gas meter also has to supply the generator at full load, along with your furnace and other gas appliances.
Portable instead?
A portable generator can run essentials through a transfer switch or interlock. Most portables can't start a central AC unit, but they keep heat, fridge and sump pump going.
Ready to plan a standby unit? See whole house generator installation.
Keep reading
More Guides From Our Electricians
Plain-English answers to the questions homeowners ask us most.
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Questions Readers Ask
What size generator do I need to run my whole house?
It depends on your appliances. Many mid-size homes with one AC unit land around 18 to 22 kW. A load calculation gives the real answer.
Will a 22 kW generator run central air?
Often yes, especially with load management. It depends on the size of your AC and what else is running.
What's the difference between running and starting watts?
Running watts keep an appliance going. Starting watts are the short burst a motor needs to start.
Does natural gas reduce generator output?
Many air-cooled generators produce somewhat less power on natural gas than on propane. Check the natural gas rating.
Can a portable generator power my house?
It can run essentials through a transfer switch or interlock, but usually not central air.
Want Help Sizing Your Generator?
A licensed electrician can do a load calculation and recommend the right size, with same-day site visits.
(732) 419-5195