What does generator size actually mean?
Generators are rated in watts. Two numbers matter: running watts, which is the steady load, and starting watts, the short surge that motors draw when they kick on. Well pumps, freezers, furnace blowers and shop tools often need much more power for a moment than they do once running.
The nameplate on each appliance or motor, or its manual, lists what it needs. A licensed electrician can read these and add them up properly, which is better than guessing.
If your generator is too small, the engine bogs down, voltage drops and sensitive motors can overheat. Giving yourself a margin avoids that, but oversizing wastes fuel because lightly loaded engines run poorly.
How do you decide what to power?
Start with a list of what you must run during an outage and what you would like to run. Essentials usually include the well pump, refrigerator, freezer, some lights, heat controls and medical equipment. Comforts like an electric range or clothes dryer can add large loads.
You do not have to power the whole house. Many people pick a handful of circuits and run those, rotating other loads by hand.
Think about time, too. Loads do not all run at once. A well pump runs a few minutes at a time, and a refrigerator cycles. A simple schedule of what runs when can shrink the size you need.
- Well pump and pressure tank
- Refrigerator and freezers
- Furnace blower or boiler controls
- Lights, phone and internet charging
- Medical equipment and barn essentials such as milkers or ventilation
What is a transfer switch and why does it matter?
A transfer switch connects the generator to your house wiring and keeps power from flowing back out onto the utility lines. Without one, you risk injuring or killing line workers and damaging your equipment.
Installation must be done by a licensed electrician, and your utility and county may have rules and permits. Never plug a generator into a wall outlet to power the house.
Portable or standby?
Portable units are cheaper and can move around the farm, but you must start them, fuel them and run cords. Standby units start on their own and run on a fixed fuel supply such as propane or natural gas, but they cost more to buy and install.
Think about who will be home during an outage. If an older relative is alone, an automatic system may be worth a close look.
Noise and fuel use matter on a long outage. Some quieter models run more efficiently at partial load, and a standby unit on a large propane tank can run for days if the tank is full. Ask a dealer for real run-time figures, not marketing claims.
How do you plan for fuel?
A generator is only as good as its fuel. Think about how many hours you want to run and how much fuel that takes, then store only what local rules and safe practice allow. Fuel stored for long periods can go bad.
Ask your fire department or state fire marshal about safe fuel storage. Rotating fuel through your vehicles or equipment keeps it fresh.
Keep fuel for your generator separate from fuel for farm equipment, so you always know what you have. Keep a written log of when you filled and what was used during each outage.
How do you test your estimate?
Before you buy, run a trial outage. Turn off the main breaker for a few hours and note what you missed and what you did not. Walk through what you would run and in what order.
Ask a licensed electrician or your electric provider for help with load calculations. Some providers offer guidance or have rules on generator hookups.
Photograph nameplates and keep them with your plan, so a dealer or electrician can help you without a long visit.
What mistakes do people make?
The most common is skipping the starting surge in the math, followed by overloading one circuit. Others forget fuel, run extension cords through doors, or place the generator too close to a window.
Read the manual and the safety labels before first use, and check CPSC and manufacturer notices for recalls.
Sizing is also a good time to ask whether some loads can be replaced by efficient or non-electric options, like a propane water heater or a wood stove, since smaller electric needs mean a smaller generator.
Key takeaways
- Add up running and starting watts from nameplates, with an electrician's help.
- Power essentials, not everything.
- Use a licensed electrician to install a transfer switch.
- Plan for fuel, and run a trial outage before you buy.
Sources and further reading
Last reviewed: November 12, 2026. Written by the Rural Farm Life Editorial Team. See our editorial policy.