What Size Generator Do I Need for My House?

Generator Size at a Glance

If you're just trying to establish a rough starting point:

These ranges are orientation only.
A 20 kW generator can be excessive for one house and insufficient for another.
The electrical loads determine the answer.

Generator Watts vs. Kilowatts

Generators are commonly rated in watts (W) or kilowatts (kW).
The conversion is simple:
1 kilowatt = 1,000 watts
So:

• 5,000 watts = 5 kW
• 10,000 watts = 10 kW
• 15,000 watts = 15 kW
• 20,000 watts = 20 kW
• 24,000 watts = 24 kW
  Portable generators are frequently marketed in watts, while permanent home standby generators are more commonly described in kilowatts.
  That does not change the underlying calculation.
  You're trying to determine how much electrical demand the generator needs to support.

The Three Numbers That Matter

Generator sizing becomes much easier once you separate three concepts.

Running watts

Running watts are the power an appliance uses while operating normally.
If several appliances run simultaneously, their running loads need to be added together.

Starting watts

Motors can require substantially more power for a short period while starting.
Equipment such as:

• Central air conditioners
• Well pumps
• Sump pumps
• Refrigerators
• Freezers
• Some HVAC equipment
  can therefore create a temporary demand greater than their normal running load.
  This is also called:
• Starting wattage
• Surge wattage
• Starting load

Simultaneous load

You don't necessarily need enough generator capacity for every electrical device in the house to operate at maximum demand simultaneously.
What matters is what could reasonably be running at the same time under the backup configuration you've chosen.
That is where load management can materially change generator size.

How to Calculate What Size Generator You Need

A useful homeowner-level calculation follows this process.

Step 1: Decide What You Want to Power

Start by dividing household loads into three groups.

Essential

Equipment that genuinely needs to remain available during an outage.
Examples:

• Refrigerator
• Freezer
• Well pump
• Sump pump
• Furnace blower
• Medical equipment
• Security system
• Basic lighting
• Internet equipment

Important

Things you strongly prefer to keep operating.
Examples:

• Central air conditioning
• Microwave
• Garage door
• Television
• Washing machine
• Selected outlets

Optional

Loads you could temporarily avoid during an outage.
Examples might include:

• Electric dryer
• EV charger
• Pool heater
• Hot tub
• Second HVAC system
• Other large discretionary loads
  This exercise often saves more money than obsessing over generator brands.
  A home that theoretically requires 35 kW to operate everything at once may need considerably less capacity if several optional heavy loads can be managed.

Step 2: Find the Running Wattage

Next, determine the electrical requirement of each load you intend to support.
Where possible, use information from the actual equipment rather than a generic internet chart.
Look for:

• Equipment nameplate
• Manufacturer documentation
• Owner's manual
• Electrical specifications
  If wattage is given directly, use it.
  If you have voltage and amperage, a basic single-phase estimate can be calculated as:
  Watts = Volts × Amps
  For example:
  120 volts × 10 amps = 1,200 watts
  or:
  1.2 kW
  Real electrical equipment can involve additional considerations, so this simple calculation should not replace a professional load calculation for the final installation.
  But it is useful for understanding the process.

Step 3: Account for Starting Loads

This is where basic generator calculations often go wrong.
Suppose your household's running loads total 12,000 watts.
That doesn't automatically mean a 12 kW generator is sufficient.
If your central air conditioner requires a significant additional surge when its compressor starts, the generator must be capable of handling that event without becoming overloaded or suffering unacceptable voltage drop.
The same issue applies to pumps and other motor-driven equipment.
When building your load list, record both:
Running watts
and:
Starting watts
where applicable.
Our Generator Size Calculator is designed to account for this distinction automatically.

Step 4: Work Out Simultaneous Demand

Now determine which appliances may operate together.
Suppose a house contains:

• Central AC
• Electric water heater
• Electric dryer
• Well pump
• Refrigerator
• Freezer
• Lighting
  Sizing for the theoretical maximum of every load running simultaneously can push the generator requirement considerably higher.
  But you may decide:
• EV charging is disabled during an outage.
• The dryer is optional.
• Water heating can be temporarily interrupted.
• One HVAC load receives priority.
• Other large loads can be automatically managed.
  The system then needs to support the managed peak, rather than an unrealistic everything-at-once scenario.

Step 5: Decide Whether to Use Load Management

Load management can be one of the most valuable parts of a residential standby system.
It allows the generator to prioritise electrical loads rather than requiring enough capacity for every heavy appliance to operate simultaneously.
For example:
Your air conditioner starts.
The system temporarily prevents another large load from operating.
Once generator capacity becomes available, that load is restored.
This can allow a smaller generator to provide broad household backup.
The trade-off is straightforward:
Larger generator: more loads can operate simultaneously.
Managed smaller generator: lower potential equipment/fuel cost, but some loads may need to wait.
Neither is universally better.
The right choice depends on how you want the house to operate during an outage.

Step 6: Allow Appropriate Headroom

You generally don't want a system designed around an unrealistic assumption that the generator will continuously operate at its absolute limit.
Current sizing sources use different recommendations for additional capacity, with observed guidance ranging from around 10–20% to 25–30% depending on methodology and application.
There is no reason to pretend those competing rules are one universal standard.
For preliminary sizing, some headroom is sensible.
For a permanent standby installation, the final capacity should be determined from the actual load calculation, equipment starting requirements and system design rather than simply adding an arbitrary percentage.

Whole-House vs. Essential-Load Generator Sizing

Before asking "what size generator will run my house?", decide what run my house actually means.
There are three useful configurations.

Essential-load backup

Only important circuits receive generator power.
This can dramatically reduce required capacity.
A system might support:

• Refrigerator/freezer
• Lights
• Furnace
• Well pump
• Sump pump
• Internet
• Selected outlets
  while leaving large discretionary loads unavailable.

Managed whole-home backup

The generator is integrated broadly with the home, but high-demand loads are controlled so they don't all operate simultaneously.
For many households, this is the sweet spot.

Full simultaneous whole-home backup

The system is sized to support the expected household load with minimal restrictions.
Large all-electric properties can require considerably more generator capacity under this approach.

What Size Generator Do I Need for a 2,000 Sq. Ft. House?

There isn't one correct generator size for every 2,000-square-foot house.
Square footage can provide a rough starting point, but it doesn't describe the home's electrical demand.
Consider two 2,000-square-foot homes.

House A

• Natural gas heat
• Natural gas water heater
• Gas range
• One central AC
• Municipal water

House B

• Electric heat
• Electric water heater
• Electric range
• Large central AC
• Well pump
• Pool pump
• EV charger
  House B can have dramatically greater electrical demand despite having exactly the same floor area.
  Current sizing guidance observed in our research gives different square-footage recommendations depending on the assumptions being used. That's precisely why we don't recommend choosing a generator from floor area alone.
  For a 2,000-square-foot home, calculate the actual loads first.

What Size Generator for a 1,500 Sq. Ft. House?

The same principle applies.
A relatively efficient 1,500-square-foot home using gas for major heating loads may be able to achieve broad backup with substantially less generator capacity than an all-electric property.
Instead of asking only:
How big is the house?
ask:
What are the largest electrical loads?
HVAC, water heating, pumps and electric heating usually matter far more than an extra few hundred square feet.

What Size Generator for a 2,500 Sq. Ft. House?

A 2,500-square-foot home often pushes homeowners toward larger residential standby units, but there is still no defensible universal kW recommendation based on floor area alone.
The system could potentially be designed around:

• Essential circuits
• Managed whole-home loads
• Full whole-home operation
  Each produces a different generator requirement.
  Use square footage as an initial input, not the final calculation.

What Size Generator for a 3,000 Sq. Ft. House?

Larger homes are more likely to contain multiple large loads, but again, the load matters more than the building footprint.
A 3,000-square-foot house may include:

• Multiple HVAC systems
• Electric water heating
• Well pumps
• Pool equipment
• Electric cooking
• Multiple refrigerators/freezers
• EV charging
  If several of those need to operate simultaneously, required generator capacity can increase quickly.
  A load-managed design can sometimes avoid moving into a substantially larger generator class.

What Size Generator for a 200-Amp Service?

A 200-amp electrical service does not mean you need a generator capable of supplying 200 amps continuously.
Your electrical service represents the capacity available to the home from the utility.
It does not mean the house constantly consumes that amount of power.
A 200-amp, 240-volt service theoretically represents:
200 × 240 = 48,000 watts
or approximately:
48 kW
But that does not mean every home with 200-amp service needs a 48 kW generator.
Most homes use far less than their maximum service capacity at any given moment.
Generator sizing should be based on the calculated backup load and system design.

What Size Generator Do I Need to Run Central Air?

Central air conditioning is one of the most important loads to account for when sizing a home generator.
The required generator capacity depends on the actual HVAC equipment, including:

• Unit size
• Voltage
• Running current
• Starting current
• Compressor characteristics
• Other loads running simultaneously
  Do not size an AC circuit solely from its tonnage.
  Two systems with the same nominal cooling capacity can have different electrical requirements.
  Check the actual equipment data.

Generator Size for a 3-Ton, 4-Ton or 5-Ton AC

Searchers often want a simple chart matching AC tonnage to generator size.
That's understandable, but it can be misleading.
A generator doesn't power the air conditioner in isolation.
It powers the AC plus whatever else is operating in the home.
The starting characteristics of the compressor also matter.
So instead of:
4-ton AC = X kW generator
the correct process is:

1. Identify the AC's actual electrical requirements.
2. Account for compressor starting demand.
3. Add the other simultaneous household loads.
4. Determine whether load management or a soft-start system changes the peak.
5. Size the generator for the complete system.

Can an AC Soft Starter Reduce Generator Size?

Potentially.
A soft-start device can reduce the starting demand imposed by some air-conditioning compressors.
That can make it easier for a generator to start the HVAC system without a large transient load.
This can be particularly relevant where air-conditioning startup is the main factor pushing the generator toward a larger capacity.
However, a soft starter does not reduce every other load in the house.
It should therefore be treated as part of the system design rather than a universal shortcut to buying a smaller generator.
Compatibility should be confirmed for the actual HVAC equipment.

What Size Generator Do I Need for a Well Pump?

Well pumps are another load where starting demand matters.
The correct generator capacity depends on factors such as:

• Pump horsepower
• Voltage
• Running current
• Starting demand
• Other household loads
  Use the actual pump specifications wherever possible.
  A generic "well pump wattage" table can be useful for preliminary planning, but it should not override the data for your installed pump.

What Size Generator to Run a Refrigerator?

A refrigerator is relatively modest compared with central HVAC, electric heat or a large well pump, but it still contains a compressor.
That means it has:

• Normal running demand
• Higher short-duration starting demand
  If you're sizing a portable generator for a few essential appliances, that starting requirement matters.
  For a 20+ kW standby system, the refrigerator is usually only a small part of the overall load calculation.
  Context matters.

Can a 5,000-Watt Generator Run a House?

It can run part of a house, and in some low-load situations it may cover many essentials.
A 5,000-watt generator might support a combination such as:

• Refrigerator
• Freezer
• Lighting
• Internet
• Television
• Furnace blower
• Selected outlets
  depending on the actual equipment and simultaneous demand.
  It is not reasonable to assume 5,000 watts will operate a typical modern house normally with central HVAC, electric water heating, electric cooking and other major loads.

Can a 6,500-Watt Generator Run a House?

A 6,500-watt generator can provide meaningful emergency power and may support a larger group of essential circuits.
Whether it can "run the house" depends entirely on what that phrase means.
If you mean:
keep important appliances and several circuits operating, potentially.
If you mean:
operate every household appliance exactly as normal, often not.
Starting loads still need to be considered.

Can a 10,000-Watt Generator Run a House?

A 10 kW generator can support substantial residential loads, particularly where heating, water heating and cooking use gas.
But an all-electric home with large HVAC loads can exceed that capacity quickly.
Again, the deciding factor is simultaneous electrical demand rather than house size.

Can a 12,000-Watt Generator Run a House?

A 12 kW generator can provide broad backup in some homes and essential-load backup in larger or more electrically demanding properties.
It may be sufficient where:

• Major heating loads use gas
• Heavy appliances are managed
• HVAC starting requirements are reasonable
• The homeowner does not expect every large appliance to run simultaneously
  Calculate the loads rather than assuming 12 kW is automatically a whole-house capacity.

Can a 13,000-Watt Generator Run a House?

Potentially.
A 13 kW generator can support substantial household backup, but its suitability depends on:

• HVAC
• Heating
• Water heating
• Pumps
• Cooking
• Other simultaneous loads
  Portable generators advertised around this capacity can also have different continuous and peak ratings, so compare the correct specification.

Will a 15 kW Generator Run My House?

For some homes, yes.
A 15 kW standby generator can be enough for broad backup where large loads are limited or managed.
For other homes, particularly those with multiple HVAC systems or substantial electric heating loads, it may not be sufficient for unrestricted whole-home operation.

Will a 20 kW Generator Run My House?

A 20 kW generator falls into a common residential standby range and can provide broad whole-home or managed whole-home backup for many properties.
It still isn't universally sufficient.
A large all-electric home can exceed 20 kW, especially if several heavy loads operate simultaneously.
Conversely, a home with gas appliances may require considerably less.

Is a 22 kW Generator Big Enough for My House?

A 22 kW generator is a common choice for residential standby systems.
It can support many homes effectively, particularly when paired with sensible load management.
But "22 kW" shouldn't be treated as the default answer to whole-house backup.
The correct question is whether your calculated load — including starting requirements — fits within the system's capabilities under the intended fuel and configuration.

22 kW vs. 24 kW vs. 26 kW Generator

When comparing nearby generator sizes, don't assume the largest is automatically the best value.
Look at:

• Actual rated output
• Fuel-specific output
• Your calculated load
• HVAC starting requirements
• Load-management capabilities
• Transfer-switch configuration
• Fuel consumption
• Equipment price
• Installed-price difference
  If moving from 22 kW to 26 kW adds little to the complete installed price and gives useful capacity, it may be sensible.
  If you don't need the extra capacity, there is little value in buying kW simply because it is available.

Portable Generator Sizing vs. Standby Generator Sizing

The basic electrical principles are the same, but the use cases differ.

Portable generator sizing

Portable-generator buyers often prioritise:

• Essential appliances
• Running watts
• Surge watts
• Manual load management
• Fuel availability
  The generator may connect to individual appliances or appropriately configured household circuits.

Standby generator sizing

A permanent standby system is more likely to involve:

• Automatic transfer
• Whole-home electrical integration
• Load management
• HVAC
• Fuel-system design
• Professional load calculation
• Automatic transfer switch configuration
  That is why a simple appliance-wattage worksheet can be useful for estimating portable-generator requirements but may not be enough to specify a permanent standby installation.

Does Fuel Type Affect Generator Output?

It can.
Some generators capable of operating on more than one fuel have different rated outputs depending on whether they are running on natural gas, propane or gasoline.
That means you should not look only at the largest number printed on a generator's marketing material.
When comparing a generator with your calculated requirement, check the rated output on the fuel you actually intend to use.
This is particularly important with permanently installed natural-gas standby systems.
A generator advertised under one headline capacity may have a different available output under different fuel conditions.

Do I Need to Size the Generator for Every Appliance?

Not necessarily.
This is one of the biggest opportunities to avoid unnecessary generator capacity.
Ask whether you genuinely need equipment such as the following during an outage:

• EV charger
• Electric dryer
• Pool heater
• Hot tub
• Second oven
• Multiple air conditioners simultaneously
• Other discretionary high-load equipment
  If not, exclude or manage those loads appropriately.
  You are designing a backup-power system, not necessarily recreating unlimited utility power.
  For many households, keeping the HVAC, refrigeration, water, lights, internet and normal household essentials operating is more important than being able to charge an EV while simultaneously heating a pool and running an electric dryer.

How Load Management Changes Generator Size

Consider a simplified example.
Suppose a home's expected normal backup load is:
15 kW
But three large appliances could add another:
12 kW
if they happened to operate simultaneously.
Sizing for the theoretical maximum could push the requirement toward:
27 kW+
Instead, a load-management system might temporarily control those large loads and keep peak generator demand within a lower limit.
That could potentially keep the project within a mainstream residential air-cooled generator class.
This is why the answer to:
"What size generator do I need?"
can sometimes be:
"It depends on how intelligently we design the loads."
Generator capacity and load-management strategy should be considered together.

Air-Cooled vs. Liquid-Cooled Generator Sizing

Generator size can eventually influence the type of equipment available.

Air-cooled generators

Air-cooled standby generators cover a large portion of the residential market and are commonly available into the mid-20-kW range.
For many homes, particularly with load management, that is enough.

Liquid-cooled generators

Once the required capacity moves substantially higher, liquid-cooled systems become more relevant.
They are commonly used for:

• Very large homes
• Large simultaneous electrical loads
• More demanding applications
• Higher generator capacities
  Moving from a mainstream air-cooled system into a larger liquid-cooled system can materially increase equipment, installation, fuel and maintenance costs.
  That makes accurate sizing commercially important.
  If intelligent load management can safely keep the house within a smaller generator class, the savings can be substantial.

Should I Oversize My Generator?

Some additional capacity can be useful.
Massive oversizing usually isn't.
Potential reasons for allowing additional capacity include:

• Future electrical loads
• Motor starting
• Avoiding continuous operation near maximum capacity
• Planned home additions
• Additional HVAC
• Future pumps or equipment
  But "buy the biggest generator you can afford" is poor sizing methodology.
  An unnecessarily large generator can mean:
• Higher equipment cost
• Potentially higher installation cost
• Greater fuel consumption
• Larger fuel-system requirements
• More expensive maintenance
  The objective is not maximum kW.
  It's enough capacity for the backup system you actually need, with appropriate margin.

Should I Size for Future Loads?

If you already know the property's electrical demand is going to increase, yes — account for it during the design.
Examples include plans to add:

• Second HVAC system
• Home extension
• Pool
• Hot tub
• Well equipment
• Workshop
• Additional refrigeration
• Electric appliances
  An EV charger deserves slightly different treatment.
  You may have an EV charger without wanting it backed up during an outage.
  Simply owning a large electrical load does not mean the generator must support it.
  Decide which future loads genuinely need backup power.

Why Square-Footage Generator Charts Can Be Misleading

Generator-size-by-house-size charts are popular because they're easy.
They can also create false precision.
A chart might imply:
2,000 sq. ft. = X kW 2,500 sq. ft. = Y kW 3,000 sq. ft. = Z kW
But square footage doesn't tell us:

• Fuel used for heating
• Number of HVAC systems
• HVAC starting demand
• Water source
• Water-heater type
• Cooking fuel
• Pool equipment
• EV loads
• Electrical service configuration
• Which circuits need backup
• Whether load management is used
  Square footage is useful context.
  It is not a substitute for electrical-load information.
  That's why our calculator uses the equipment you want to power as the important input rather than returning a confident-looking generator size from floor area alone.

Generator Sizing Example

Consider a hypothetical home where the homeowner wants to back up:

• Refrigerator
• Freezer
• Lights
• Internet
• Gas-furnace blower
• Well pump
• Central air conditioning
• Microwave
• Selected outlets
  The process would be:

1. Record normal running loads

Add the expected running wattage of the equipment likely to operate simultaneously.

2. Identify motor starting loads

Pay particular attention to the central AC and well pump.

3. Determine realistic simultaneity

Not every appliance will operate at maximum demand simultaneously.

4. Account for load management

Decide whether large loads need to be sequenced.

5. Select appropriate capacity

Choose a generator/system configuration capable of supporting the calculated running and starting requirements with appropriate design margin.
Notice what's missing from that calculation:
square footage alone.
That's intentional.

Why Professional Generator Sizing Still Matters

An online calculator is useful for:

• Establishing a likely capacity range
• Comparing generator classes
• Understanding which appliances drive demand
• Preparing for installer quotes
• Testing essential-load vs whole-home scenarios
  It should not pretend to replace the final engineering and installation decisions for a permanently connected standby system.
  A qualified installer can assess factors the calculator may not fully capture, including:
• Actual equipment nameplates
• HVAC starting characteristics
• Electrical-service configuration
• Transfer-switch design
• Load-management strategy
• Fuel-specific generator output
• Local electrical requirements
• Future electrical loads
  The calculator should get you into the right conversation.
  The final installation should verify the numbers.

Generator Size Calculator

If you've read this far, you don't need to calculate everything manually.
Use our Generator Size Calculator to build your household backup load.
The calculator will let you select the equipment you want to power and estimate:

• Running electrical load
• Significant starting loads
• Peak requirement
• Approximate generator capacity range
  You can then compare what happens when you switch between:
  Essentials only
  and:
  Broader whole-home backup
  That is much more useful than asking for a generator size from square footage alone.

Turn the calculator result into a specification an installer can check

A calculator result is a planning estimate, not permission to order equipment. Cummins makes the same distinction on its own sizing tool: the result is an initial estimate and actual requirements are best determined through an in-home assessment.[4]

Use our generator size calculator to prepare a load worksheet. Before requesting proposals, mark each load as required during an outage, optional, or acceptable to manage. Add model information for the largest motors and note loads you expect to add later. Leave unknown starting demand marked unknown; do not replace it with a guessed appliance average.

Ask the installer to return a written sizing decision that answers:

• Which loads can operate together, and which cannot?
• What starts the largest motor while the other essential loads are already running?
• Which loads will be shed or delayed, by what equipment, and in what priority?
• What is the selected generator's continuous rating on the fuel the property will actually use?
• Which assumptions still need an on-site measurement or equipment-manual check?

A useful proposal lets you trace the recommended capacity back to those answers. A larger advertised kW number is not a substitute for them. For an example of manufacturer document lookup, Generac provides model- and serial-specific specifications and manuals.[1]

If HVAC startup is the uncertain item, read the air-conditioner soft-start guide before assuming that a larger generator is the only option. Then use the brand comparison hub and installed-cost guide to compare suitable systems. The standby installation guide explains what happens after a design is agreed. Take the worksheet to a provider serving your area; ask for a site-verified design rather than a guarantee based on this calculator.

Sources

[1] https://www.generac.com/support/product-info-user-manuals [4] https://www.cummins.com/en-na/na/generators/home-standby