Choosing the right Generac generator size depends on your home's square footage, the appliances you want to run at once, and whether you need whole-house or partial backup power. A unit that's too small trips breakers and shuts down under load. One that's too large adds cost without real benefit.
This guide walks through wattage calculations, a sizing chart by home size, whole house vs portable options, and tips for confirming your final choice with a local Generac dealer, so you pick the right kW rating the first time.
Quick Answer: Generac Generator Sizing at a Glance
Most homeowners need between 14kW and 22kW for whole-house backup power. Smaller homes running only essentials, like a fridge, well pump, and a few lights, can get by with a 7kW to 11kW unit. Larger homes with central AC, electric water heaters, and multiple HVAC zones often need 24kW to 26kW or a commercial-grade standby model.
Here's a simple starting point based on home size:
- Under 1,500 sq ft: 7kW to 11kW
- 1,500 to 3,000 sq ft: 14kW to 18kW
- 3,000 to 5,000 sq ft: 20kW to 22kW
- 5,000+ sq ft: 24kW to 26kW or larger
These numbers assume a mix of standard appliances, not every high-draw item running at full capacity at once. A home with gas heat and gas water heating can often run on a smaller unit than one with electric heat and an electric water heater, even at the same square footage.
Think of this chart as a starting point, not a final answer. Your actual load depends on which appliances you plan to run at the same time, along with local climate demands and any future additions like an EV charger or pool equipment. A proper load calculation confirms the exact size your home needs.
Why Generator Size Matters for Your Home
An undersized generator trips breakers, shuts down under load, and shortens its own lifespan from constant overwork. An oversized unit runs fine but burns more fuel than needed and adds thousands to your upfront cost without any real benefit in return.
Right-sizing matters for a few practical reasons:
- It keeps your HVAC system, well pump, and everyday appliances running without tripping or overload
- It prevents nuisance shutdowns during storms, exactly when you need reliable power most
- It avoids paying for standby capacity you'll never actually use
- It supports future additions, like an EV charger, hot tub, or a home addition down the road
- It reduces strain on the engine, which extends the generator's overall service life
A properly sized Generac unit balances upfront cost against real-world power needs. This isn't just about covering today's appliances, it's about planning for how your home's electrical demand might grow over the next 10 to 15 years. Getting this balance right from the start saves money on both ends, avoiding an oversized purchase now and an undersized headache later.
How to Calculate the Wattage You Need
Every appliance draws a certain number of watts. Add up the running watts of everything you want powered at once, then add extra for starting watts on motors like AC compressors and well pumps, since these draw more power the moment they switch on.
Step 1: List your essential appliances
Common household loads include:
Appliance
Average Running Watts
Starting Watts
Refrigerator
600 to 800
1,200 to 1,800
Central AC (3-ton)
3,500 to 4,000
5,500 to 7,000
Well pump
1,000 to 2,000
2,000 to 3,000
Furnace fan
600 to 1,000
1,200 to 2,200
Electric water heater
3,000 to 4,500
N/A
Sump pump
800 to 1,050
1,300 to 2,200
Lights and outlets
400 to 600
N/A
Step 2: Add it all up
Add the running watts of everything you'd realistically use at once. Then add the single highest starting watt figure, since not every motor starts at the exact same second.
Step 3: Add a safety buffer
Add 20 to 25 percent to your total. This buffer covers voltage dips, future appliance additions, and everyday fluctuations in demand.
Once you have this final number, you can match it directly against the size chart below to find the closest Generac model for your home.
Generac Generator Size Chart by Home Type
Use this chart as a practical shortcut once you know your rough square footage and appliance load.
Home Size
Typical Load
Recommended Generac kW
Small home or condo (under 1,500 sq ft)
Fridge, lights, a few small appliances
7kW to 11kW
Mid-size home (1,500 to 3,000 sq ft)
Central AC, furnace, fridge, well pump
14kW to 18kW
Large home (3,000 to 5,000 sq ft)
Two HVAC zones, electric water heater, multiple large appliances
20kW to 22kW
Estate or large custom home (5,000+ sq ft)
Multiple HVAC systems, pool equipment, EV charger
24kW to 26kW+
If your home has gas heat and gas water heating, your electrical load drops significantly, and a smaller generator may cover the whole house. Homes with electric heat, electric water heaters, and central AC need the largest units on this chart.
Whole House vs Portable Generac Sizing
Not every home needs full backup coverage. Understanding the difference helps you avoid overspending or under-preparing.
Whole house standby generators, connect directly to your home's electrical panel and natural gas or propane line. They start automatically during an outage, power your entire home, and typically range from 14kW to 26kW.
Portable Generac generators run on gasoline and power select circuits through extension cords or a manual transfer switch. These typically range from 3,000 to 10,000 watts and work well for short outages or partial coverage.
Feature
Whole House Standby
Portable
Startup
Automatic
Manual
Fuel
Natural gas or propane
Gasoline
Coverage
Entire home
Select circuits
Typical size
14kW to 26kW
3,000W to 10,000W
Best for
Frequent or extended outages
Occasional short outages
If your area sees multi-day outages during storm season, a whole house unit is almost always the better long-term choice.
Factors That Change Your Ideal Generator Size
Two homes of the same square footage can need completely different generator sizes. Local conditions and household habits shift the math.
Key factors include:
- Climate zone: homes in hot or cold climates run HVAC systems harder, increasing load
- Well water systems: well pumps add a large, sudden starting load
- Electric vs gas appliances: electric water heaters and stoves draw far more power than gas versions
- Future plans: adding a pool, hot tub, or EV charger later means sizing up now
- Number of HVAC zones: multi-zone homes need more running watts than single-zone homes
- Local utility reliability: areas with frequent long outages benefit from extra buffer capacity
A local Generac dealer familiar with your region's climate and typical outage patterns can factor these details into your recommendation more accurately than a generic online calculator.
Average Cost by Generator Size
Cost scales closely with kW output, plus installation complexity like panel upgrades, concrete pads, and gas line work. A larger unit costs more upfront, but it also often means more labor and materials during installation.
Generator Size
Average Unit Cost
Average Installed Cost
7kW to 11kW
$2,000 to $4,000
$5,000 to $8,000
14kW to 18kW
$4,000 to $6,500
$8,000 to $12,000
20kW to 22kW
$6,500 to $9,000
$12,000 to $16,000
24kW to 26kW+
$9,000 to $14,000+
$16,000 to $22,000+
Installation cost varies based on a few key factors:
- Distance between the generator and your gas line or electrical panel
- Local permitting fees, which differ by city and county
- Whether a transfer switch upgrade is needed for your existing panel
- Site prep work, like pouring a concrete pad or clearing space
- Trenching costs if the gas line needs to be extended
Homes with an older electrical panel or a gas line that needs upsizing typically fall on the higher end of these ranges. Getting a quote from a local installer gives you the most accurate number, since these site-specific factors can shift the final price by several thousand dollars either way.
How a Local Generac Dealer Can Help You Choose the Right Size
Online calculators give a rough starting estimate, but a certified local installer performs a full load calculation based on your actual electrical panel, appliance list, and home layout. This kind of on-site review catches details a generic tool simply can't account for.
Search for an "authorized Generac dealer near me" to find installers certified to size, install, and service these systems correctly in your area.
A qualified local dealer typically will:
- Perform an on-site load assessment specific to your home
- Review your electrical panel capacity and condition
- Check your existing gas line size and pressure
- Recommend the correct transfer switch type for your setup
- Handle local permitting and inspection requirements
- Advise on the best placement for noise, clearance, and code compliance
Since permitting rules and gas line codes vary by city and county, working with a local Generac dealer helps you avoid delays and ensures the system meets your area's building codes. It also gives you a single point of contact for future maintenance, warranty claims, and repairs, rather than piecing together support from multiple sources after installation.
Common Mistakes When Sizing a Generator
Avoid these frequent errors when choosing a Generac unit:
1. Sizing only for today's needs, not accounting for future appliances or an addition
2. Ignoring starting watts, motors like AC units and well pumps need extra surge capacity
3. Skipping a professional load calculation, online estimates can miss appliance-specific details
4. Choosing based on price alone, an undersized unit costs more in repairs and shortened lifespan
5. Forgetting gas line capacity, a generator needs enough gas supply to run at full output
6. Not planning for panel upgrades, older homes sometimes need panel work before installation
Taking time upfront to size correctly saves money, reduces repair calls, and extends the generator's working life.
Final Thoughts
Sizing a Generac generator comes down to matching real power needs with the right kW output, not guessing based on home size alone. Start with a wattage calculation, check it against a size chart, then confirm with a local authorized dealer who can account for your electrical panel, gas line, and climate. A properly sized system keeps your home running safely through outages for many years, while an undersized or oversized unit leads to unnecessary cost or frequent problems. When in doubt, a professional load assessment from a certified Generac installer near you is the most reliable way to land on the right size.
FAQs
What Size Generac Generator Do I Need For A 2,000 Sq Ft House?
Most 2,000 sq ft homes need a 14kW to 18kW Generac generator, depending on whether the home uses electric or gas heat and water heating.
Can A 22kw Generac Generator Run A Whole House?
Yes, a 22kW unit can power most homes up to around 5,000 sq ft, including central AC, electric water heater, and standard appliances.
How Many Watts Does A House Need To Run Everything?
An average home needs between 5,000 and 15,000 running watts, though homes with central AC, electric heat, or well pumps can exceed 20,000 watts during peak load.
What Size Generator Do I Need To Run A Well Pump?
Well pumps typically need 3,000 to 5,000 watts of starting capacity, since the motor draws a large surge when it switches on.
Is A 7kw Generator Enough For A House?
A 7kW generator covers essential circuits like a refrigerator, some lights, and a few small appliances, but it won't run central AC or a whole electrical panel.
What's The Difference Between Running Watts And Starting Watts?
Running watts are the steady power an appliance uses once it's on. Starting watts are the short burst of extra power needed to start a motor, often two to three times higher.
How Much Does A 22kw Generac Generator Cost To Install?
Installed cost for a 22kW Generac generator typically runs between $12,000 and $16,000, depending on gas line work, electrical panel upgrades, and local permitting fees.
Do I Need A Permit To Install A Generac Generator?
Most cities and counties require a permit for standby generator installation, since it involves gas line and electrical panel work. A local authorized dealer usually handles this process.
Can I Use A Portable Generator Instead Of A Whole House Unit?
Yes, portable generators work well for short outages or partial power needs, but they require manual setup and won't power an entire home like a standby unit does.
How Do I Find A Certified Generac Dealer Near Me?
Search "Generac dealer near me" or use Generac's official dealer locator tool to find authorized installers in your area who can perform a proper load assessment and installation.