How Do I Size a Whole-Home Generator for My McKinney House?

Whole-home generator sizing comes down to one goal. Your generator must handle everything you want to run at once. Here is the simple four-step method we use.

Step 1: List your must-run appliances. Start with the essentials you cannot go without:

• Refrigerator and freezer
• Heating and cooling (HVAC)
• Well pump or sump pump
• Lights and outlets
• Medical devices

Step 2: Add up the running watts. Write down the running watts for each item and total them.

Step 3: Add the highest starting watt. Find the one appliance with the biggest startup surge. Add only that single number, not every surge.

Step 4: Add a safety margin. Add 20% to 25% to your total. Then divide by 1,000 to get kilowatts.

You can find wattage on the appliance nameplate or in the manual. If it lists amps and volts, multiply them (Amps × Volts = Watts). The U.S. Department of Energy's appliance energy use guide can also help you estimate wattage for common household appliances.

Running watts and starting watts are not the same. Running watts keep an appliance going once it is on. Starting watts are the short surge it takes to turn it on.

This gap is big for anything with a motor. Your AC, sump pump, and fridge can pull two to three times their running watts at startup. That surge lasts only a second or two, but your generator must handle it.

Here is the key part. You only add the single highest starting watt to your total. Your motors do not all start at the same moment. Adding every surge together would push you toward a unit that is too large.

Ignoring startup surge causes the opposite problem. It leads to a generator that is too small. That unit will overload and shut down right when you need it.

Home size gives you a helpful starting range. Use these tiers to picture where your McKinney home may fall. Remember, these are estimates, not exact numbers.

• Small home or essentials only — about 10 kW to 15 kW. This covers lights, a fridge, a furnace, and medical devices.
• Most homes (2,000–3,000 sq ft) — about 15 kW to 22 kW. This runs most appliances, central AC, and pumps.
• Large or all-electric homes (3,000–5,000 sq ft) — about 22 kW to 32 kW. This handles electric ovens, water heaters, and dual HVAC.

Square footage is only a rough guide. Your appliances drive the real number more than your home's size. An all-electric home needs a bigger unit than one with gas heat.

A few key factors move your generator size up or down. Knowing them helps you plan before you call.

What raises your number:

• Central AC — This is the biggest driver here. McKinney summers make cooling a must-run load.
• Electric appliances — An electric heater, water heater, or dryer adds a lot of watts.
• Future loads — An EV charger, pool, or home addition means you should size with room to grow.

What lowers your number:

• Gas appliances — Gas heat and a gas water heater cut your electric load.
• Select circuits — You can cover key circuits instead of the whole home.
• Load management — This system spreads power use so a smaller unit can do more.

Home age matters too. Newer McKinney homes often carry higher electrical loads than older ones.

Not sure which loads to count? See our whole-home generator installation in McKinney for help.

A few common mistakes throw off generator sizing. Avoid these to get a unit that fits your home.

• Going too small — An undersized unit overloads and shuts down. It can also damage your electronics over time.
• Going too big — An oversized unit costs more upfront. It also wastes fuel when it runs at a light load.
• Sizing by square footage alone — Your appliance load matters more than your home's size. Count what you actually run.
• Adding all starting watts together — Only the single highest surge counts. Your motors do not all start at once.
• Skipping the safety margin — Leave 20% to 25% of room. Your power needs tend to grow, not shrink.

The math gives you a solid estimate. A professional load calculation gives you the exact fit. Here is what our McKinney team checks before we recommend a size.

• We run a load calculation on your real circuits, not a guess
• We check whether your panel can support the unit or needs an upgrade
• We confirm your AC, fridge, and essentials can run together safely
• We handle the permits and meet local code for a standby generator

This step protects your investment. The right size keeps your home powered without overloading. The wrong size can shut down or waste money for years.

Ready to size your generator the right way? Located at: 7300 State Hwy 121 Suite 300, McKinney, TX 75070. Call (469) 398-3229 for generator help in McKinney.