Is it an APU, a gen set or both? The Energy Act defines APUs, but does not specifically talk about generator sets, or gen sets. The Environmental Protection Agency considers them to be functionally identical, thus you see them lumped together.
Section 756, in the Energy Policy Act of 2005 defines an auxiliary power unit as “an integrated system (that) provides heat, air conditioning, engine warming, or electricity to components on a heavy-duty vehicle …”
An APU will have an engine-driven air conditioner compressor, while a gen set makes electricity and will power an electric, stand-alone air conditioner.
An APU can have a generator for electric power, but not always. Don’t assume that an APU will power the hotel loads. Many require the additional expense of an inverter to provide 120-volt power for microwaves, DVD players, etc.
There are more hybrid units than pure ones. In general, remember that all gen sets are technically APUs, but not all APUs are gen sets.
Both have gained popularity as fuel prices and anti-idling regulations have increased. But, for decades, cost-conscious drivers have been using idling alternatives.