Whole-Home Standby Generator
Sizing Calculator

Professional sizing tool for permanently installed standby systems. Adjust your home's parameters to find the ideal Generac solution.

Home Profile

Square Footage 1,500 – 2,500 sq ft
< 1,5001,500–2,5002,500–3,5003,500+

Electrical Panel Size

Fuel Source

HVAC & Heating

Central AC Unit Size 3 Tons
2 Tons3 Tons4 Tons5 Tons

Number of AC Units

Heat Type

Appliances & Heavy Loads

Recommended Size

22 kW

Generac 22kW Guardian Series

Running Load

—

Transfer Switch

200A ATS

Panel Upgrade Required: A 100A panel cannot safety support this load during a generator transition. A 200A service upgrade is recommended.
Natural Gas: Ideal for continuous power in Spring/The Woodlands area with no refueling required.
Propane: Systems derate ~5-10% on LP. This calculation includes a safety margin for liquid propane.
Gasoline: Portable/Residential fuel. Requires frequent refueling; storage safety is critical.
Diesel: High efficiency for large standby systems. Excellent for remote areas without gas lines.
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Disclaimer: This is an estimate based on average loads. Per NEC 220.82, a physical load calculation by a Licensed Master Electrician is required for all legal permit applications.

How to Calculate Stationary Generator Size for a Home

Choosing the right generator isn't about the size of your home; it's about the simultaneous load of your appliances. A large house with gas appliances may require less power than a smaller house with an electric water heater and multiple AC units.

1. Calculate Base Running Load

We start with the basic lighting and outlet load. For a typical home in Texas, this ranges from 1,200 watts to 3,500 watts depending on square footage. This covers your LED lights, phone chargers, and televisions.

2. Add Large Appliance Loads

Next, we identify the heavy hitters. Electric dryers (5,500W), electric water heaters (4,500W), and EV chargers (7,200W) are constant loads that must be accounted for if you intend to run them during an outage.

3. Account for Compressor Surge

Air conditioners are the most critical factor. An AC unit might use 3,500 watts while running, but it requires up to 300% more power (the "surge" or "starting" load) for a split second to start the compressor. We use the Locked Rotor Amps (LRA) from your AC nameplate to ensure the generator can handle this initial kick.

4. The 25% Safety Margin

To prevent the generator from running at 100% capacity (which causes overheating and premature failure), we add a 25% buffer. A generator should ideally run at 50-75% of its rated capacity for maximum longevity.