Whole-Home Standby Generator
Sizing Calculator

How to Calculate Standby Generator Size for Your Home

Choosing the right whole-home standby generator is determined by the simultaneous electrical load of your home's appliances, rather than just the square footage of the house. A large home relying on natural gas appliances often requires significantly less electrical backup power than a smaller home equipped with multiple central air conditioning units, an electric water heater, and an EV charger.

1. Determine Your Base Running Load

The calculation begins with your base running load, which covers essential daily items. For the average Texas home, this baseline ranges from 1,200 to 3,500 watts, providing uninterrupted power for LED lighting, refrigerators, internet routers, televisions, and device chargers.

2. Factor in High-Draw Appliance Loads

Next, we account for heavy-duty appliances that draw significant constant wattage. If you intend to operate electric dryers (approx. 5,500W), electric water heaters (4,500W), or Level 2 EV chargers (7,200W) during an outage, these substantial loads must be calculated into the standby generator's continuous capacity.

3. Account for Compressor Starting Surge

Air conditioning systems are usually the largest determining factor in generator sizing. While a standard 3-ton AC unit might only draw 3,500 watts while running, it demands a massive, split-second surge of power (often up to 10,000 watts) to start the compressor. Sizing is heavily influenced by the Locked Rotor Amps (LRA) to guarantee the generator can handle this initial motor startup without tripping breakers.

4. Incorporate the 25% Safety Margin

A reliable generator should never run at 100% capacity. Pushing a generator to its absolute limit causes overheating and premature engine wear. We automatically factor in a 25% safety buffer so your unit operates comfortably between 50% and 75% capacity, maximizing fuel efficiency and overall longevity.

Frequently Asked Questions: Standby Backup Power

What size standby generator do I need for a 200 amp service?

While a 200-amp electrical panel can theoretically draw up to 48,000 watts, real-world residential demand is a fraction of that. Most homes equipped with a 200-amp service require a 22kW to 26kW standby generator for complete whole-house backup. A detailed load calculation determines your exact footprint, as the average home rarely draws more than 30% to 50% of its panel capacity simultaneously.

Can I run my air conditioner on a standby generator?

Absolutely. Standby generators are specifically engineered to handle heavy HVAC loads. The critical factor is the starting surge. A Generac standby unit intelligently manages the startup sequence of your central air conditioning, ensuring the compressor has enough initial wattage (which can be triple the running wattage) to start safely and keep your home cool during extended summer outages.

Do I need an Automatic Transfer Switch (ATS)?

Yes. By law (NEC Article 702), a permanent standby generator requires an Automatic Transfer Switch. The ATS safely disconnects your home from the utility grid the moment power drops, preventing dangerous "back-feeding" that could electrocute utility line workers. It also ensures your generator seamlessly turns on and off automatically without you lifting a finger.

How long can a stationary generator run continuously?

Premium standby generators, like Generac and Kohler, are designed to run for extended periods during prolonged grid failures. If connected to a natural gas line, they can theoretically run indefinitely as long as the fuel supply remains active. If running on a 500-gallon liquid propane tank, a 22kW generator operating at a 50% load will generally last 5 to 7 days before needing a refill. It is highly recommended to perform a brief oil check every 100 to 200 hours of continuous operation.