Cleaner Electrical Power usually refers to the consistency of the Alternating Current. The cycles of current in AC power is also known as Hertz (Hz). Most portable power equipment in the United States is designed to work on 120v 60 Hertz AC.
Most Conventiional Generators do not produce as consistent Frequency as you get from the power company. Most Inverter Generators have very Clean or consistent AC Frequency.
Why is Cleaner Power Important?
It is very important for most electrical equipment to have clean power to operate properly. Computers, Monitors, and motors all operate much better on power with nice clean even sine waves.
Surprisingly to me Electrical AC Motors are very dependant on clean power to run efficiently. If the power is not very consistant your motor will not run as well.
Inverter Generators normally produce very clean electrical power with smooth consistant sine waves.
This article is referring to the Clean electrical output of different types of Generators not the output of the energy source. However Propane or Natural gas generators will have cleaner exhaust output than Diesel or Gas generators.