Ask ten people how to size a generator and nine will say “add up the watts.” They are right, but incomplete. The tenth will say “and remember the surge,” which is the part that explains why a generator that looked big enough on paper trips its breaker the moment the refrigerator compressor tries to start. Sizing is not difficult, but it follows a method, and skipping a step is the most common reason people buy the wrong machine.
This guide walks through the method: building a load list, understanding running and starting watts, adding a margin and checking the answer against the machine’s ratings. It also covers what to do when the numbers show you need more than you can afford, because the right response is usually to run less, not to buy more.
Read this if you are about to buy a generator and have not written down what you want it to run. You can skip it if an electrician has already produced a load calculation for your panel.
Why sizing matters
An undersized generator trips, overheats or damages the equipment it is feeding. An oversized one costs more to buy and burns more fuel at light loads, which is wasteful and, for some engines, bad for them. The goal is a machine with enough capacity for the loads that matter and a sensible margin, not the biggest on the shelf.
Step 1: write the load list
List everything you want to run during an outage, grouped by priority.
- refrigerator, freezer, furnace blower or heating controls, well pump or sump pump if you have one, some lights, phone charging, internet equipment.
- microwave, coffee maker, television, a few extra lights, a window air conditioner in summer.
- washer, dryer, electric range, water heater, central air conditioner.
Be realistic. Large heating and cooling loads and electric ranges and dryers use far more than everything else combined, which is why many households choose to leave them off.
Step 2: find running watts and starting watts
Every appliance has two numbers.
- (or continuous watts) is the power it uses once it is running.
- (or surge watts) is the brief extra power needed to get a motor or compressor going. It can be two to three times the running figure, and sometimes more.
The label or manual may list watts directly. If it lists volts and amps, multiply them to estimate watts. If you have an electrician or a clamp meter, you can measure. Appliances with motors, such as refrigerators, freezers, pumps, furnace fans and air conditioners, are the ones with significant surge. Lights, electronics and heating elements, such as toasters, have almost no surge.
Step 3: add running loads, then add the largest surge
The method most dealers recommend is simple.
- of everything you plan to run at the same time.
- among those items.
- , meaning the starting watts minus its running watts, to the running total.
You do not add every surge, because motors rarely start at the same instant, and you can start them one at a time. That gives you the minimum output the generator must be able to deliver at peak.
An example, with illustrative numbers
The figures below are illustrative. Check your own appliances.
- Refrigerator. Running watts: 200. Starting watts: 600.
- Freezer. Running watts: 150. Starting watts: 450.
- Furnace blower. Running watts: 500. Starting watts: 1,000.
- Lights and phones. Running watts: 300. Starting watts: 300.
- Wi-Fi and modem. Running watts: 40. Starting watts: 40.
- Sump pump. Running watts: 800. Starting watts: 1,600.
Running total: 200 + 150 + 500 + 300 + 40 + 800 = 1,990 watts. The largest surge extra is the sump pump, 1,600 minus 800, which is 800 watts. Peak requirement is therefore about 2,790 watts. Add a margin of about 20 percent for safety and you are near 3,350 watts, which points to a generator in the 3,500 to 4,000 watt range. Many people choose that class for exactly this kind of essential load.
Step 4: compare with the generator’s ratings
Generators list a running (rated) output and a starting (maximum) output. Use the rated figure for sustained loads and the maximum for the surge. Do not run a generator constantly at its absolute maximum. A common guideline is to keep the sustained load at about 80 percent of the rated output or less, which also saves fuel and wear.
Altitude and temperature reduce engine output. At higher elevations, a generator may deliver noticeably less than its sea-level rating. If you live in the mountains, ask the dealer how the unit is rated.
Voltage, outlets and the plug problem
Check that the generator has the outlets you need. A 120-volt outlet is standard. Some appliances, such as well pumps, central air conditioners and electric dryers, need 240 volts. If you plan to connect to the house through an inlet and transfer switch, the generator must match the inlet type and amperage. We do not give wiring instructions. A licensed electrician decides the right connection, and our guide to transfer switches and safe hookup explains what to ask.
Sensitive electronics and inverter power
Modern electronics, such as computers and some furnace controls, prefer clean power. Inverter generators produce a smoother output than many conventional machines. If sensitive equipment is on your priority list, ask about total harmonic distortion in the specifications or choose an inverter model. Our overview of portable, inverter and standby generators compares them.
If the numbers are too big
When the total comes out larger than your budget allows, reduce the load before you increase the machine.
- Run the refrigerator and freezer, then the furnace blower, instead of all at once.
- Water heaters, ranges, dryers and central air conditioners are expensive to power. Gas versions of these appliances reduce electrical demand.
- LED lights and modern refrigerators draw less.
- Phones, lights and a router can run from a battery, leaving the generator for the heavy items. See our guide to portable power stations.
- For some motors, a soft-start device reduces the starting surge. Ask an electrician whether it suits your equipment.
Fuel use and run time
Larger generators burn more fuel. Manufacturers publish run times at a quarter, half and full load. A machine that runs for ten hours at half load on a tank will not run for ten hours at full load. Plan the fuel you will need for a multi-day outage, and read our guide to fuel storage and maintenance.
Checklist before you buy
- Written load list with running and starting watts.
- Peak requirement and a margin of roughly 20 percent.
- Required voltage and outlet types.
- Fuel type and expected run time.
- Noise limits in your neighborhood.
- Warranty and service network.
- A safe place outdoors to run it, away from openings.
- A professional plan for connecting it to the house.
The short version
Sizing is arithmetic with one twist. Add the running watts of what you want on, then add the largest starting surge. Add a margin, check the machine’s rated and maximum output and reduce loads rather than buying a bigger machine than you need. Never run a generator indoors, and use a licensed electrician for any connection to the house.
Match the size to the job
- Fridge, freezer, furnace fan and lights only: a modest inverter or small portable is often enough.
- Add a sump pump or well pump: plan for the larger starting surge.
- Want central air or an electric range: that points to a standby unit or a much larger machine.
- Only phones, lights and a router: a power station may be all you need.
