Choosing the wrong generator size leads to either wasted money or a unit that can’t power your essentials when you need it most. Generator sizing is about matching the unit’s output to your home’s critical electrical loads. Here’s how to determine the right size.
Understanding Generator Capacity: Generators are rated in watts (or kilowatts). A small portable generator might produce 3,000–5,000 watts, while a whole‑home standby unit produces 20,000–26,000+ watts. To know what you need, you must calculate the starting and running watts of the appliances you want to power. Running watts are the continuous draw; starting watts are the higher surge required by motors (AC units, well pumps, refrigerators) when they first turn on.
Step 1: Decide What You Need to Power. Do you want to run just essentials—refrigerator, some lights, a furnace—or your entire home including central AC? Essentials can often be handled by a 5,000–10,000‑watt portable generator. Whole‑home backup typically requires 20,000+ watts. For the different types of units, see our types of home generators.
Step 2: Calculate the Load. List every appliance you want to power and its running watts. Then add the highest starting watt requirement among motor‑driven appliances. The total is your minimum generator capacity. A simplified method: for whole‑home, use your home’s square footage and major appliances to estimate—your electrician can perform a precise load calculation.
Step 3: Consult a Professional. The most accurate sizing comes from a licensed installer who will perform a detailed load calculation. They’ll account for your specific appliances, HVAC system, and any plans for future additions. For help finding a pro, see how to choose a licensed generator installer. For cost implications of different sizes, see generator installation cost. For the fuel requirements of different sizes, see generator fuel options. And for the installation process, see what to expect during installation.
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