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What Size Generator Do I Need for My House
Match a standby generator to the loads you want running, with typical watts for common household equipment and a free written estimate.
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If you are asking What Size Generator Do I Need for My House, start with your loads, not your square footage. A standby generator has to carry everything you choose to run and still start your biggest motor on top of it. This What Size Generator Do I Need for My House guide walks through that math one step at a time.
You will see the difference between essential circuits and whole house coverage, typical watts for common household loads, and how a load calculation turns your list into a kW number. The wattages here are typical planning values. Your nameplates and your panel decide the real answer, which is why every whole house generator installation we quote starts with a look at your house.
Lectro's tip
Write the breaker size next to every big motor in your house. That list does half the sizing work before anyone shows up.
What sets the size
The loads you want running during an outage
The starting watts of your largest motor
Fuel type, since many units are rated lower on natural gas
Load management that shares capacity between big loads
Room for future loads such as an EV charger
What to have ready
A short list of must-run loads
The nameplate or breaker size on big motors
Your main breaker size: 100, 150 or 200 amps
Natural gas or propane, and where the meter or tank sits
Anything you plan to add in the next few years
Essential circuits or the whole house
Before any number, pick a goal. Generator size follows from what you want to keep running when DTE Energy power goes out.
Essential circuits
You keep the furnace blower, refrigerator, freezer, sump pump, well pump if you have one, some lights, internet and a few outlets. This needs a smaller generator and often a smaller transfer switch.
Whole house coverage
You want central air, the water heater, the range and everything else to work as if nothing happened. That takes a larger generator, a switch sized for your full service, or load management that shares capacity between big loads.
Which one fits you
Neither answer is wrong. The mistake is skipping the question and buying by the biggest kW on the sales sheet. Size is also the first thing that moves price, as our whole house generator cost guide explains.
Running watts and starting watts
Every motor load has two numbers. Running watts are what it draws once it is going. Starting watts, sometimes called surge, are the short spike when the motor kicks on.
A sump pump may run on under a kilowatt but need twice that for a second or two to start. Central air is the big one: its compressor can need two to three times its running watts at startup.
Your generator has to handle the largest start while everything else is already running. Heating loads such as a water heater or a range burner draw steady power with no real surge.
Typical watts for common household loads
This generator wattage chart shows typical ranges for equipment found in many southeast Michigan homes. Typical, so check your nameplate. The label on the motor or appliance, or the breaker that feeds it, gives the real figure for your unit.
Load | Typical running watts | Typical starting watts | Note |
|---|---|---|---|
Sump pump (1/3 to 1/2 hp) | About 800 to 1,050 | About 1,300 to 2,200 | Key load in basements |
Well pump (1/2 to 1 hp) | About 1,000 to 2,000 | About 2,000 to 4,500 | Well homes only |
Gas furnace blower | About 500 to 900 | About 1,300 to 2,400 | Gas furnaces still need power |
Central air (about 3 tons) | About 2,500 to 4,500 | About 6,000 to 9,000 or more | Soft-start kit cuts spike |
Refrigerator | About 300 to 800 | Up to about 2,200 | Count a second fridge |
Chest freezer | About 300 to 500 | Up to about 1,500 | Small, but it adds up |
Microwave | About 1,000 to 1,500 | Same as running | Short bursts |
Electric water heater | About 3,500 to 5,500 | Same as running | Heats in cycles, can wait |
Electric dryer | About 5,000 to 6,000 | Close to running | Easy to skip |
Electric range, full use | 8,000 or more | Same as running | Rarely all burners at once |
Lights, TV, internet, chargers | About 500 to 1,500 together | Same as running | LED keeps it small |
Don't add up every row. The chart shows which loads are cheap to carry and which use real capacity.
How to calculate what size generator you need
You can do a rough generator size calculation with a pencil. It shows which size range to expect.
Step by step
List every load you want running at the same time.
Add up their running watts.
Find the motor with the biggest starting watts, usually central air or a well pump.
Subtract that motor's running watts and add its starting watts. That is your peak.
Divide by 1,000 for kW and leave some headroom, so the unit isn't working at its limit.
Worked example: essential circuits
Say a home on a well wants the furnace blower (700 W), fridge (700 W), chest freezer (400 W), sump pump (900 W), well pump (1,200 W), plus lights and internet (1,000 W). Running total: about 4,900 watts.
The well pump has the biggest start, about 3,500 watts. So the peak is 4,900, minus 1,200, plus 3,500, or about 7,200 watts. That is roughly 7.2 kW at the moment the pump kicks on.
Adding central air
Now add a central air unit that runs on 3,500 watts and starts at 7,500. Running climbs to 8,400 watts. If the AC starts last, the peak is about 12,400 watts. Add the water heater and range and you move into the 20 kW range. That is where sizes like the 22 kW units in our recent installs come in.
These numbers are an illustration, not a size for your house.
Why a load calculation beats a rule of thumb
Square footage charts are popular, but they guess. A 2,000 square foot house with gas heat and a gas range needs far less than a 2,000 square foot house with electric heat, an electric range and two AC units. The size of a 2,500 square foot house depends on the same thing: what runs inside it.
A proper generator load calculation looks at your panel, the circuits you want covered, which motors start together and which loads can wait. The National Electrical Code sets the methods for calculating loads, and your electrician applies them to your service. The result is a generator that starts your AC without stalling, and isn't far bigger than you need.
What size generator for 100, 150 or 200 amp service
Your main breaker tells you the most your service can deliver, not what your house uses. So a 200 amp service doesn't call for a generator that matches 200 amps.
The service size matters more for the transfer switch. A whole house switch is usually rated to match your service, while a smaller switch covers selected circuits. We have installed both 200 amp and 100 amp automatic transfer switches. Our transfer switch installation page explains the choice.
If your panel is old, full or too small for what you want to add, sizing may include a talk about an electrical panel upgrade first.
Load management: covering more with a smaller unit
Load shedding means not running every big load at the same moment. You can do it by habit or automate it.
Load management modules sit on large loads such as central air, a water heater or a range. They hold those loads off when the generator is near its limit and let them back on when there is room. That is how a mid-size generator can cover a large house. The tradeoff is that some loads wait their turn.
What size Generac do I need?
The same load math applies to every brand. Generac, Honeywell and Kohler each sell home standby generators in a range of kW sizes, and each publishes ratings for natural gas and propane. Pick the size from your loads first, then compare models at that size.
For reference, our recent jobs include 22 kW Honeywell and 22 kW Generac units on natural gas, and a 35 kW Generac in Belleville. Those are examples, not a size for your home. See our Generac generator installation page for what that job includes.
Fuel, gas line and rating details to check
A generator's rating is not always the same on both fuels. On many models the kW rating is lower on natural gas than on propane, so read the spec sheet for the fuel you will use. Our natural gas vs propane guide covers the tradeoffs.
Size also drives fuel supply. A larger unit burns more gas at full load, which can mean a larger gas line or a meter change from your gas utility. On propane, tank size sets how long you can run before a refill. Your installer should check both before you settle on a size.
Common generator sizing mistakes
Buying the biggest unit you can afford. Extra capacity costs more and needs more fuel and pad space.
Ignoring starting watts. A unit that covers running watts but not the AC start will struggle or shut down.
Forgetting the second fridge or the chest freezer. Small loads add up.
Leaving out future loads. An EV charger, hot tub or addition changes the math. If an EV is coming, see our EV charger installation page.
Reading a propane rating as a natural gas rating. The numbers can differ.
How we size your generator
We start with a visit. We look at your panel and main breaker, ask what you need running, check the big motors and note where the gas meter or tank sits. Then we recommend a size, a transfer switch and any load management.
You get a free written estimate with each part listed before any work starts. Bring your list of must-run loads and we can talk it through. Request a free generator sizing estimate or call (734) 697-8477.
Generator guides
How size, fuel and the switch move the price
Whole house or selected circuits
What a Generac job includes
Fuel ratings and supply
The full process, start to finish
Written and itemized, in advance
Frequently Asked Questions
Many whole house setups land around 20 kW or more, but it depends on your loads. Central air, an electric range and an electric water heater push the size up. Load management can let a smaller unit cover the house by holding big loads until there is room.
A 200 amp service does not need a generator that matches 200 amps. The generator is sized to the loads you want running. The transfer switch is what usually matches your service, or it covers selected circuits.
Square footage alone can't answer it. A gas-heated home with a gas range needs much less than an all-electric home of the same size. List your must-run loads and size from those.
Central air needs extra capacity at startup, often two to three times its running watts. A soft-start kit or load management can lower what the generator must handle. The compressor nameplate gives the real numbers for your unit.
Use the same load math as any brand. Add your running watts, account for the largest motor start and check the rating for your fuel. Then compare Generac models at that size.
No. A unit larger than your loads costs more to buy and install and uses more fuel. Size it for the loads you actually plan to run, with sensible headroom.