Power outages hit without warning. One moment, your home feels safe and normal. The next, you are in the dark, hoping your fridge stays cold and your air conditioning returns soon.
A whole-home generator fixes that, but only if you choose the right size. The wrong size creates overload, waste, and breakdowns. This guide gives you clear size ranges, wattage calculations, and simple examples you can trust.
Why Whole-Home Generator Size Matters
A whole-home generator must match your home’s load. An overloaded generator trips breakers and wears itself out fast.
An oversized generator burns fuel you don’t need and costs more than it should. A properly sized unit keeps your home steady, safe, and efficient during any power outage.
How to Determine What Size Whole Home Generator You Need
You can size your home generator with a simple step-by-step process. It keeps the math clear and helps you avoid guessing.
Step 1 — List the Essential Appliances You Want Powered
Make your list first. Think about the things that keep your home steady during a power outage:
- HVAC
- Refrigerator and freezer
- Water heater (especially electric)
- Well pump or sump pump
- Lighting
- Home office gear
- Medical devices
Write down the wattages or check appliance labels. This list becomes the base of your generator sizing.
Step 2 — Add Running Watts
Running watts are the steady power your appliances need once they’re already running.
A fridge sits around 600W.
A central air conditioning system may sit around 2,000W.
Add the running wattage for everything on your list.
Step 3 — Add Surge (Starting) Watts
Surge watts are different. They’re the highest starting power required when a motor kicks on.
- Refrigerator: 600W running / 1,200W surge
- AC: 2,000W running / 4,000W surge
Surge watt handling matters because it keeps your generator from overloading when appliances start at the same time.
Step 4 — Add a 20% Safety Margin
Once you add running watts and surge watts, give yourself a little room. A 20% buffer keeps your home steady during a simultaneous load spike. It also helps if you add appliances later.
Whole-Home Generator Size Chart (By Home Size & Power Needs)
Here’s a simple reference that helps you see how generator capacity lines up with different homes.
Under 1,500 sq ft
- 7–10 kW
- Essential appliances only
- Basic heating or cooling, depending on the system
1,500–3,000 sq ft
- 16–20 kW
- Most whole-home needs
- Strong enough for HVAC, refrigeration, and lighting
Over 3,000 sq ft
- 22–25 kW+
- Multi-HVAC homes
- Higher running wattage and heavier appliance loads
Note: Square footage isn’t the only driver. HVAC load often controls the size of the home standby generator.
Example Wattage Calculation for a Typical Home
Let’s look at a 2,000 sq ft home.
- Refrigerator: 800 running / 1,200 surge
- 3-ton AC: 3,000 running / 5,000 surge
- Water heater: 4,500 running
- Lights + outlets: 1,500 running
Total running watts: about 9,800
Estimated surge watts: around 12,000
Recommended generator: 17–20 kW whole-home generator
This size covers your daily loads and protects your home from overload.
Best Fuel Types for Whole-Home Generators
Your fuel choice affects performance, cost, and convenience. Each type of generator brings its own strengths.
Natural Gas
Natural gas works well if your home has a gas line. It burns clean, needs less daily care, and stays steady during long outages.
Propane
Propane works well in rural areas. It stores well, and you can keep a tank on your property. You’ll need to refill the tank during extended outages.
Diesel
Diesel provides heavy-duty power and handles large homes. It needs more maintenance, but it stays strong under high loads.
Additional Tips for Choosing the Right Generator
A few simple choices make your generator sizing more accurate.
Tip 1 — Consider Your HVAC System
Your HVAC system often uses the most running wattage and the highest starting wattage. Multi-zone systems may require 20–25 kW+.
Tip 2 — Plan for Future Add-Ons
Think about what you want later:
- Remodeling
- EV chargers
- More HVAC zones
- Additional appliances
A small bump in generator capacity saves you from needing a bigger unit later.
Tip 3 — Always Get a Professional Load Calculation
A licensed electrician performs a proper load calculation. It verifies the running wattage, protects against overload, and ensures the transfer switches match your generator size.
Frequently Asked Questions
What size generator do I need to run my whole house?
Most homes need 16–24 kW, depending on HVAC load and total running watts.
What size generator do I need for central air?
A 3-ton AC often needs 4,000–6,000 surge watts. Most homes with AC need at least 17–20 kW.
Can a 10 kW generator run a whole house?
It can run a small home with light loads, but it can’t support central air conditioning or full whole-home use.
How do I calculate my generator size?
Add your running watts, add the highest starting wattage, then add a 20% buffer.
What is the best fuel type for whole-home generators?
Natural gas if you have a gas line. Propane or diesel based on your location and usage.
Should I oversize “just in case”?
A small buffer is good. Too much buffer wastes fuel and increases cost.
Schedule Your Whole-Home Generator Assessment Today
A whole-home generator keeps your home steady during any power outage, but the size has to be right. This guide provides clear steps, numbers, and examples. You can use it to avoid overload and confidently choose the right home generator.
Get a custom load calculation and personalized sizing recommendation from our certified electricians. Comfort Experts™ installs and repairs all major house generators in Ft. Worth and nearby areas.
