The refrigeration unit of an electrically powered trailer refrigeration system includes an economized scroll compressor and an economizer circuit selectively operable to inject refrigeration vapor into an intermediate stage of the scroll compressor. An engine driven generator produces AC current to power the scroll compressor and other electric components of the system. During operation of the refrigeration system at high capacity demand, a controller will operate the refrigeration system in an economized mode if sufficient electric power is available from the generator and in the non-economized mode if insufficient power is available to sustain operation of the refrigeration system in the economized mode.