What is an inverter generator?

Very simply, you have a gasoline engine that drives a (3 phase) alternator (AC generator) at variable RPM. That AC power is turned to DC power then back to 120 VAC @ 60 Hz power. The "good gensets" use a Sine Wave inverter... Have seen a cheaper version/knockoff a few years ago that used a MSW (modified sine/square wave) AC inverter (not good for induction motors and some AC power supplies).

The reason... A normal AC alternator needs to turn at a fixed RPM (typically 1,800 or 3,600 RPM for 60 Hz line frequency). That is how the standard genset works. And they have a voltage regulator of some sort to keep the 120/240 VAC output voltage stable across loads.

Works fine, however for gasoline and propane (ignition combustion engines), they are not very fuel efficient below ~50% load--Because the gasoline engine needs to run at a fixed 3,600 RPM (typical for 'smaller' gensets), the engines are just not that efficient--And suck fuel at 50% rate even down to near zero Watt loads.

Diesel fueled motors tend to be more efficient at lower loading because they are Otto cycle (compression ignition) engines... And below 50% loading, they use less and less fuel flow (gallons per hour of fuel). There are some other issues (nothing comes for free), but that would be another post--If you are interested.

The inverter-generator "breaks" the need to run the (typically) gasoline/propane engine at 3,600 RPM and instead allows the computer to run the motor at the most efficient speed (fuel burned vs Watts generated). So, at lower power output, the engine can be throttled back, and the inverter part keeps the voltage at 120 VAC and 60 Hz.

Think of a single speed bicycle vs a 10 speed bike (or the automatic transmission on a car)... One is fixed speed/gearing. The other matches the engine speed and loading to the road conditions and vehicle speed.

The upside, less fuel flow (at lower output power), quieter (slower running engine most of the time), can have longer life (slower average RPM), and vary stable 120 VAC @ 60 Hz output voltage. Also many inverter-generators support paralleling the AC output between two (identical models) of gensets for 2x more power (easy to "sync" two inverters, hard to "sync" two standard gensets).

Downsides... At higher output loads, can be less efficient (extra power conversions). More expensive. And most inverter-generators do not support high surge loads (inverter will shutdown, standard genset will supply more current and "sage" voltage & RPM/frequency for a few seconds). Also, when operating in "economy mode", if there is a hard surge (starting a heavy motor load), the inverter-generator may stall the gasoline engine or fail to start the load--And you have to turn off ECO mode.

If you are interested in genset--This Stage Lightning newsletter has a very complete discussion (without getting into the math):

http://www.screenlightandgrip.com/html/emailnewsletter_generators.html

Make sense?

-Bill