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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.

A portable generator can supply power to selected appliances and circuits in a home, but it's generally not designed to handle the entire household's electrical needs simultaneously. Portable generators have limited wattage capacity, so they are only designed to power essential devices and electronics temporarily until local power is restored. If your power needs outweigh the capabilities of a portable generator, consider installing a whole-home generator instead.

You can determine what size generator you need for a transfer switch by matching the switch to the largest outlet on your generator. Larger generators can power up to 10 circuits, while most 5,000-watt generators can provide power to six circuits. So, if your generator’s largest outlet is 30 amps, you’ll need a 30-amp transfer switch.

Here’s what you can run with different wattages and types of generators:

Portable generator:

• 2,000 watts can handle a large appliance like a freezer or refrigerator, plus a few smaller items like phones or computer chargers. 

• 4,000 watts will keep two large appliances operating and a few smaller devices.

• 8,000 watts should power most essential appliances and other devices, including a sump pump, well pump, or your dishwasher.

Partial to whole-house generator:

• 20,000 watts and above will make it seem like your home’s power is still on.