How does a generator work?
Published October 3, 2026.
Built by Jeremy Panasuk.
The engine only turns a shaft. The electricity is induced in the windings. Sizing starts after that, when a motor asks for a short extra current the shaft has to supply.
What the machine is actually doing
EIA says most electricity generation uses generators based on Faraday: moving a magnet inside a coil induces a current in the wire. A basic electromagnetic generator has insulated coils forming a stationary cylinder, the stator, around an electromagnetic shaft, the rotor. Turning the rotor makes current flow in the coils. Electromagnetic generators driven by a mechanical prime mover account for nearly all U.S. electricity generation.
The DOE Fundamentals Handbook states Faraday's law the same way: a voltage is induced in a conductor when that conductor is moved through a magnetic field, or when the magnetic field moves past the conductor. If nothing is moving relative to the field, no voltage is induced. That is why a generator that is not spinning does not make usable power, and why a heavier electrical load asks the engine for more torque to keep the rotor turning at speed.
Grid plants usually use a turbine as that prime mover. A portable generator uses a small engine on the same frame for the same job: spin the rotor. This page does not claim a Honda or any other portable unit is a steam turbine. The electrical half is the EIA/DOE description. The fuel half is a separate engine.
Worked example: one refrigerator
Induction explains where the watts come from. It does not tell you the size. Honda's wattage guide lists an Energy Star refrigerator or freezer at 1,200 starting watts and 132–192 running watts. This example uses the top of that running range, 192 W, so the case is not sized on the low end of Honda's band.
The home calculator in assets/app.js does this, and only this:
- Running total = 192 W.
- The single largest starting wattage is 1,200 W, on that same fridge. Extra surge = 1,200 − 192 = 1,008 W.
- Peak = 192 + 1,008 = 1,200 W.
- With the page's 20% headroom, 1,200 × 1.20 = 1,440 W. Rounded up to the next 500 W, the recommendation is 1,500 W.
- Gasoline use at 50% of that recommendation is 1,500 / 7,000 = 0.214 gallons per hour. A 4 gallon tank in the calculator lasts 18.7 hours at that rate. Propane volume in the same calculator is 1.37 times the gasoline gallons per hour, so the same 4 gallons last 13.6 hours.
Replay it on the generator size calculator by adding a custom row of 192 running watts and 1,200 starting watts, headroom 20, 8 hours a day, 4 gallon tank. The fuel rate uses the 1,500 W recommendation, not the 192 W the compressor actually draws while running. That is what the script does.
A common mistake
Adding the starting watts on top of the running watts a second time (192 + 1,200 + 1,200) treats the motor start as if it were a second appliance. The calculator adds the extra once: the difference between that motor's starting watts and its own running watts. Honda already put both numbers on one row.
Limits, and when to call a pro
This is a planning sketch of one published chart row through one formula. It is not a model of your fridge's nameplate, a locked-rotor amp reading, or an inverter's overload timer. Permanent connection to house wiring needs a transfer switch installed by a licensed electrician, and a permit where local code requires one. Placement is a separate rule: run the set outdoors. See how to run a generator safely.
The coincidence method for several motors at once is on methodology, not on this page.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].