What Size Generator Do I Need for My House?
The right generator size depends on what you want to keep running during a power outage. A generator for a refrigerator, a few lights, and a sump pump will be much smaller than one designed to power your air conditioning, water heater, kitchen appliances, and most of your home.
Generator size is usually measured in watts (W) or kilowatts (kW). One kilowatt equals 1,000 watts.
To estimate what size generator you need, make a list of the appliances and systems you want to use during an outage, find their running and starting wattage, and calculate how much power they may need at the same time.
How Do You Calculate What Size Generator You Need?
A basic generator load calculation starts with two numbers:
- Running watts: The power an appliance needs while it is operating.
- Starting watts: The extra power some appliances need for a few seconds when their motors or compressors turn on.
Starting wattage is especially important for equipment such as air conditioners, refrigerators, sump pumps, and well pumps. A generator may have enough power to keep these items running but still be too small to start them.
For a rough estimate:
- List everything you want the generator to power.
- Find the running wattage for each item.
- Add the running wattages together.
- Identify the appliance or system with the largest starting power requirement.
- Make sure the generator can handle both your total running load and that higher startup demand.
Manufacturer labels and owner’s manuals are usually the best places to find the actual electrical requirements for your equipment.
This gives you a starting point, but a professional load calculation is a better choice when sizing a permanently installed standby generator.
Start With What You Actually Need During an Outage
One of the easiest ways to narrow down generator size is to decide what you consider essential.
Ask yourself what you would want running if the power were out for several hours or several days.
For many homeowners, essential loads include:
- Refrigerator and freezer
- Lights
- Wi-Fi equipment
- Phone chargers
- Sump pump
- Well pump
- Furnace blower
- Medical equipment
- A few kitchen outlets
Other homeowners want much more coverage, including:
- Central air conditioning
- Electric water heater
- Washer and dryer
- Electric range
- Multiple refrigerators or freezers
- Home office equipment
- Garage doors
- Most or all household circuits
The more equipment you want available at the same time, the larger the generator will need to be.
How Much Generator Power Does a Refrigerator Need?
A refrigerator is one of the first appliances most homeowners want to keep running during an outage.
A typical home refrigerator may use a few hundred watts while running, but its compressor can require additional power when it starts. Actual wattage varies by refrigerator size, age, efficiency, and model.
If you also have a separate freezer, include it as its own load.
Do not estimate refrigerator needs based only on the running wattage listed online. Check the manufacturer’s information whenever possible, especially when you are trying to run several appliances from a smaller generator.
What Size Generator Do You Need for a Well Pump?
Homes with private wells need to account for the well pump when choosing a generator.
Well pumps can be a major factor in generator sizing because the pump motor may need a large burst of power when it starts.
The amount of power depends on factors such as:
- Pump horsepower
- Pump voltage
- Well depth
- Pump type
- Starting current
This is one area where guessing can lead to an undersized generator.
If your home depends on a well for water, include the pump in your load calculation and use the information on the pump or control panel rather than a generic wattage estimate.
Can a Generator Run Your Air Conditioner?
Yes, but central air conditioning can significantly increase the generator capacity your home needs.
An air conditioner has both a running load and a higher startup demand from the compressor. The size of the AC system, voltage, efficiency, and starting requirements all affect how much generator power it needs.
This is why two homes of the same size may need very different generators.
One homeowner may have a gas furnace and only want basic backup power. Another may have a large heat pump, electric water heater, well pump, and other high-demand equipment.
Their generator needs will not be the same.
If keeping your central HVAC system running during an outage is a priority, its electrical requirements should be one of the first things considered during generator sizing.
Don’t Forget the Sump Pump
If your home has a sump pump, keeping it powered during a storm may be more important than keeping the television on.
Power outages can happen during severe weather—the same time a sump pump may be working hardest.
Like a well pump, a sump pump uses an electric motor and may require additional power when it starts.
Include both its normal operating load and startup requirements when estimating your generator capacity.
What About an Electric Water Heater?
Electric water heaters can use a significant amount of power.
If you only want essential backup power, you may decide that heating water is not worth using a large portion of your generator’s available capacity.
For whole-home backup, however, the water heater may need to be included in the load calculation.
Gas water heaters may require much less electrical power, although some still use electricity for controls, ignition, fans, or other components.
Check the type of water heater you have before adding it to your generator estimate.
Essential-Circuit Generator vs. Whole-House Generator
You do not necessarily need a generator large enough to power every electrical device in your home.
There are two common approaches.
Essential-Circuit Backup
An essential-circuit setup focuses on the things you truly need during an outage.
That might include:
- Refrigerator
- Freezer
- Selected lights
- Furnace
- Sump pump
- Well pump
- Internet equipment
- A few outlets
This approach can reduce the amount of generator capacity you need.
It can make sense if your main goal is to keep your home safe and comfortable until utility power returns.
Whole-Home Backup
A whole-home generator is designed to support much more of the home’s normal electrical use.
Depending on the system and load management setup, that may include HVAC equipment, appliances, water heating, lighting, outlets, pumps, and other household circuits.
“Whole house” does not mean every home needs the same size generator. The correct capacity still depends on the home’s electrical loads and how many of them may operate at once.
Does the Size of Your House Determine Generator Size?
Not by itself.
Square footage can provide some context, but it does not tell you how much electricity your home needs.
A 2,000-square-foot home with natural gas heating, a gas water heater, and a few essential backup circuits may need far less generator capacity than another 2,000-square-foot home with:
- A large heat pump
- Electric water heater
- Well pump
- Electric cooking appliances
- Sump pump
- Multiple refrigerators
- Other high-demand equipment
Think about electrical load first and square footage second.
What Is the Difference Between Running and Starting Watts?
This is one of the most important parts of generator sizing.
Running watts are what an appliance uses once it is operating normally.
Starting watts are the higher amount of power certain equipment needs briefly when it turns on.
Equipment with motors and compressors often has the biggest difference between the two.
Examples include:
- Air conditioners
- Heat pumps
- Refrigerators
- Freezers
- Well pumps
- Sump pumps
If you only add up running watts, you could choose a generator that appears large enough but struggles when a major appliance tries to start.
That can lead to overloads or equipment that simply will not operate as expected.
How Much Extra Generator Capacity Should You Have?
You generally do not want to choose a generator whose maximum capacity exactly matches your estimated electrical demand.
Your home’s loads change throughout the day. Appliances cycle on and off, motors start, and several systems may operate at the same time.
Leaving some capacity available can help the generator handle these changes.
For a standby system, an electrician can calculate the home’s expected loads and determine whether load management can be used to control which high-demand appliances operate at the same time.
This can sometimes provide the backup coverage you want without simply choosing the largest generator available.
What Is Load Management?
Load management helps control how a generator supplies power to large electrical loads.
For example, your generator may be able to run your air conditioner and electric water heater, but not if both start at exactly the same time.
A properly designed system can temporarily delay one load while another is operating.
Once enough generator capacity becomes available, the system can allow the next appliance to run.
This can be useful in homes with several high-demand appliances because it helps make better use of the generator’s available power.
Portable Generator or Standby Generator?
Generator size also depends on the type of backup power you want.
Portable Generators
Portable generators are commonly used to power selected appliances and essential circuits during an outage.
They may work well if your main priorities are things like refrigeration, lighting, pumps, and a limited number of outlets.
Portable generators require more hands-on setup and must be operated according to the manufacturer’s safety instructions.
Standby Generators
Standby generators are permanently installed outside the home and are connected to the home’s electrical system through appropriate transfer equipment.
They can provide broader backup coverage and may automatically start when utility power goes out.
Standby systems are often chosen by homeowners who want to power major household systems or avoid setting up a portable generator during an outage.
Why a Professional Generator Load Calculation Matters
Online generator sizing calculators can be useful for getting a rough idea of what you may need, but they cannot always account for every part of your electrical system.
An electrician can look at the equipment that is actually installed in your home, including:
- Electrical panel
- HVAC system
- Water heater
- Well or sump pumps
- Major appliances
- Available fuel source
- Existing circuits
- Starting loads
- Desired backup coverage
That is especially important for a whole-home standby generator.
A proper load calculation can determine the right generator capacity based on your home’s electrical system and the appliances and circuits you want to back up. Generator installation and service should account for both normal power use and higher startup demands.
Could You Need an Electrical Panel Upgrade?
Sometimes installing a standby generator brings attention to an older electrical panel.
That does not mean every generator installation requires a panel upgrade.
However, an electrician may recommend additional electrical work if the existing panel is outdated, damaged, lacks capacity, or cannot safely support the planned generator setup.
This is another reason it helps to evaluate the generator and electrical system together instead of choosing a generator based only on an online calculator.
What Happens If Your Generator Is Too Small?
An undersized generator may work fine until several appliances need power at once.
Then you may experience:
- Generator overloads
- Breakers tripping
- Appliances failing to start
- Certain circuits losing power
- Generator shutdowns
A common mistake is adding only the running wattage of each appliance while ignoring the extra power required when motors start.
The goal is not simply to buy the biggest generator possible. It is to choose a system that can handle the loads you actually expect to use.
What Size Whole-House Generator Do You Need?
There is no single generator size that works for every house.
The correct whole-house generator size depends on:
- Heating and cooling equipment
- Fuel type
- Water heating
- Well and sump pumps
- Major appliances
- Electrical service
- Starting loads
- Number of circuits being backed up
- Whether load management is used
A home that relies heavily on natural gas may have a lower electrical backup load than a similar home with electric heating, electric water heating, and electric cooking.
That is why the best generator size is based on the home’s actual electrical demand rather than a general recommendation based on home size.
Frequently Asked Questions About Generator Sizing
What size generator do I need for my house?
Add up the running power needed by the appliances and systems you want to use during an outage, then account for the higher starting demand of equipment with motors or compressors. For a standby generator, have the home’s actual electrical loads calculated before choosing a size.
Is a 10,000-watt generator enough to run a house?
It depends on what you want to power. A 10,000-watt generator may handle many essential household loads, but homes with central air conditioning, electric water heating, well pumps, or several large electrical appliances may require more capacity.
Can I size a generator based on my home’s square footage?
Square footage alone is not enough. Generator sizing should be based mainly on the appliances, HVAC equipment, pumps, electrical systems, and circuits you want backed up.
Do I need to include starting watts when sizing a generator?
Yes. Equipment with motors or compressors can briefly require much more power when starting than while running. Ignoring that startup demand can result in an undersized generator.
Do I need a whole-house generator?
Not necessarily. If you only need refrigeration, lights, pumps, heat, and a few outlets during an outage, an essential-circuit setup may provide enough backup power. A whole-home system makes more sense when you want to maintain more of your home’s normal electrical use.
Should I use a generator sizing calculator?
A generator sizing calculator can provide a useful estimate, especially when comparing your essential loads. For a permanently installed standby generator, a professional load calculation based on the equipment in your home provides a more accurate answer.
