What Size Generator Do I Need For My House?

Sep 23, 2025 | Electrical

whole home generator

Whole House Generator Sizing Guide

When the power goes out, life stops — your food spoils, your Wi-Fi drops, and your HVAC system shuts down. For some households, a power outage can even be dangerous. That’s why more homeowners are asking:

  • What size generator do I need for my house?

  • How much does it cost to install a whole-house generator?

  • Are whole-house generators worth it?

This guide covers everything: generator sizing charts, cost breakdowns, brand comparisons, how generators work, installation steps, pros and cons, and even FAQs like “how long can a generator run continuously?” 

By the end, you’ll know exactly how a generator works, what size generator your home needs and whether a whole-house generator is the right investment. Carini Home Services is a licensed electrical contractor in San Diego, providing reliable electrical services at an affordable rate.

 

How Does a Generator Work for a House?

A generator is a backup power system that provides electricity to your home when the grid fails. When asking, “what size generator do I need?” it’s important to think about the type of generator you want.

Standby generators (whole-house):

  • Permanently installed outside your home.

  • Fueled by natural gas or propane for continuous supply.

  • Connected to your electrical panel via an automatic transfer switch (ATS).

  • Automatically start within seconds of a power outage and shut off once power returns.

Portable generators:

  • Smaller, gasoline or propane-powered.

  • Require manual setup and refueling.

  • Plug into appliances or a limited number of circuits.

 

What Size Generator Do I Need for My House?

The right size depends on your wattage needs and your home’s square footage.

How to Determine Generator Size

  • List Essential Appliances – fridge, HVAC, lights, Wi-Fi, medical devices.

  • Check Running & Starting Watts – appliances have two watt ratings:
    Running watts: continuous power needed.
    Starting (surge) watts: short spike required to start motors (AC, fridge, pump).
  • Add Them Up – total running watts + highest starting watts.
  • Add a Safety Buffer – increase total by 20–25% to avoid overloading.

Generator Sizing Chart

Home Size (sq. ft.) Typical Wattage Needed Recommended Generator Size
1,200 sq. ft. 5,000–7,500 watts 7–10 kW
1,500 sq. ft. 6,000–9,000 watts 10–12 kW
2,000 sq. ft. 12,000–18,000 watts 18–24 kW
2,500–3,000 sq. ft. 15,000–20,000 watts 20–25 kW
3,500+ sq. ft. 20,000–25,000+ watts 25–30 kW+

👉 For precision, use the Honda Wattage Calculator. Running an HVAC system increases your power needs.

 

What Size Generator Do I Need to Run My House with AC?

Air conditioners are power-hungry appliances:

  • Window unit: 500–1,500 watts

  • Central AC (3.5-ton): 3,500+ watts running / 7,000+ surge watts

Example: If your central AC requires 3,500 running watts and 7,000 surge watts, plus your fridge (800W) and lights (500W), you’ll need at least an 8,500–10,000W generator to handle startup surges.

👉 For whole-house AC, you’ll need a 15–20 kW standby generator.

 

Portable vs. Whole-House Generators

Feature Portable Generator Whole-House (Standby) Generator
Cost $500–$2,500 $7,000–$15,000 installed
Power Capacity 3–10 kW 12–30+ kW
Fuel Type Gasoline, propane Natural gas, propane
Setup Manual, cords, refueling Automatic with ATS
Best For Short outages, small budgets Long outages, full-home backup

 

How Much Does a Generator Cost for a House in 2025?

Typical Costs:

  • Portable: $500–$2,500

  • Standby unit only: $2,000–$7,500

  • Installed whole-house: $7,000–$15,000

Additional Costs:

  • Installation labor: $3,000–$5,000

  • Transfer switch: $500–$2,500

  • Fuel connections: $500–$2,000

  • Permits/inspections: $100–$500

  • Annual maintenance: $300–$600

 

How Long Can a Generator Run Continuously?

  • Portable generators (gasoline): 6–18 hours per tank

  • Portable propane generators: up to 150–200 hours with large tanks

  • Standby natural gas generators: days to weeks (as long as utility gas runs)

  • Manufacturer limits: oil checks every 72 hours, major service ~500 hours

👉 In real-world terms, a standby generator can power a home for weeks on end with proper maintenance.

 

What Type of Generator Is Most Efficient?

  • Diesel generators: Most fuel-efficient (138,700 BTUs per gallon). Best for high power loads, but noisier and higher emissions.

  • Natural gas: Convenient, unlimited fuel supply, great for standby units.

  • Propane: Cleaner burning, stored on-site, slightly less efficient.

  • Gasoline: Widely available but least efficient and shortest runtime.

 

The Quietest Home Generators

If noise matters, look for inverter generators and models rated under 60 dB.

Top Quiet Picks (2025):

  • Honda EU2200i (48–57 dB) – gold standard portable

  • Westinghouse iGen4500 – quiet + efficient dual-fuel

  • Champion 4650W Inverter – budget-friendly quiet power

  • Kohler standby units – sound-dampening enclosures

 

Are Whole-House Generators Worth It?

Pros:

  • Automatic operation = peace of mind

  • Runs HVAC, sump pumps, and appliances = prevents home damage

  • Adds 3–5% resale value

  • Critical for medical equipment & work-from-home households

Cons:

  • High upfront cost ($7K–$15K)

  • Ongoing maintenance ($300–$600/yr)

  • Fuel consumption can be significant

  • Requires permits + professional installation

👉 Worth it if: you live in outage-prone areas, rely on medical/work systems, or want to protect home value.

 

Best Generator Brands in 2025

  • Generac – most popular standby brand in the U.S.

  • Cummins – premium standby, quiet, remote monitoring

  • Honda – leader in quiet portable inverter models

  • Kohler – durable, premium standby units

  • Champion, Briggs & Stratton, Westinghouse – strong budget-friendly options

 

How to Install a Whole-House Generator

This is not a DIY project — requires electrician + plumber.

Steps:

  • Site selection – at least 5 ft. from openings, near panel & gas line.

  • Concrete pad – stable, level base.
  • Fuel connection – licensed plumber connects natural gas/propane.
  • Electrical work – transfer switch wired to main panel.
  • Permits & inspections – code compliance required.
  • Startup test – installer simulates outage for confirmation.

 

FAQ: Quick Answers

  • What size generator runs a whole house?
    Typically 15kW–25kW, depending on home size and appliances.

  • How much is a generator for a 2000 sq ft house?
    $7,000–$15,000+ installed for a 20–24 kW standby unit.

  • How long can a generator run?
    Portable: 6–18 hrs; Standby: days/weeks with natural gas.

  • What’s the quietest home generator?
    Honda EU2200i (48–57 dB) or Westinghouse iGen4500.

  • Do I need a permit?
    Yes — standby generators require electrical + plumbing permits.

  • Are generators tax deductible?
    Sometimes, if used for medical equipment or home office (check local codes).

 

Conclusion

The size and type of generator you need depends on your home size, appliance load, and budget. For short outages, a portable generator may suffice. But for full-home peace of mind, a standby generator (15–25 kW) is the best long-term solution. If you have any questions call us to learn more or visit our Electrical Services page to see our services.

Other Resources:
How To Prepare Your Home for a Blackout
Should I Repair or Replace my Electrical Panel?
Whole House Surge Protector: Are They Worth It in 2025?
Why Are My Recessed Lights Turning On and Off By Themselves?
EV Charger Installation Guide