In an engine, Parasitic Power Loss specifies the power not devoted
to the engine's main function (such as moving the vehicle). This power
usually goes to accessory functions, such as air conditioning, headlights,
radio and perhaps some engine overhead functions. It is also caused by
antipollution devices installed on vehicles.
Modifications can be done to vehicles to reduce this loss, such as
turning off of decreasing antipollution devices, and the removal of
accessories. The legality of this depends on the regulations
for the area where you live. As usual, do this at your own risk.
This is usually expressed on kilowatts.
Source: NASA Power and Propulsion Office. http://space-power.grc.nasa.gov/ppo/projects/heva/edit_veh.html