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A 1,500 square foot home will likely need a 15kW to 20kW generator. When deciding on whole-house generator size, add up the total wattage of the devices that you’d like to continue running during a power outage. That way, you can ensure you purchase a generator that can power your necessary appliances or medical equipment during an electrical emergency.

To determine the right size generator for your house, you should calculate the total wattage of the appliances and devices you want to power during an outage. List all essential items, note their start-up wattages, and add them together.

Here are some typical wattage requirements for common appliances:

• Refrigerator: 600 watts

• Large dehumidifier: 700 watts

• Large window air conditioner: 1,400 watts

• Water heater: 3,000 to 4,500 watts

• Electric furnace: 5,000 to 25,000 watts

A 7,500-watt generator might be sufficient for essentials like a refrigerator, freezer, well pump, and lighting circuits. However, for high-demand items like a water heater or furnace, a whole-home generator is a better choice. A portable generator may be enough if you only need to power a few critical items.

While home square footage can offer a rough estimate, it is not always a reliable indicator. For reference, here are some typical generator sizes based on square footage:

• 1,000 sq ft: 6–9 kW

• 1,500 sq ft: 7–10 kW

• 2,000 sq ft: 10–14 kW

• 2,500 sq ft: 12–16 kW

• 3,000 sq ft: 16–20 kW+

For the most accurate sizing, consult a professional who can calculate your home's peak and average power consumption and recommend the optimal generator.

Natural gas generators are designed to run for extended periods of time, such as power outages during a natural disaster. Since there’s no limit to the supply of natural gas, they don’t need to stop for refills. However, generators do need maintenance, including pauses to check and add oil. Constantly running them will also cause a lot more wear and tear, leading to a shorter lifespan.

In most municipalities, you will need a permit to install a permanent home generator, including emergency backup, partial-home, and whole-home generators. A permit for a generator usually costs between $50 and $250, depending on where you live. Your electrician or certified generator installer will usually pull the permit for you—and might be required to do so by your building department—but you’ll be on the hook for the cost of the permit. You should ask your installer before agreeing to any work if they will pull the necessary permits for your project.