A heavy-duty hybrid vehicle power system includes a main power unit located in an engine compartment; an electric propulsion motor; a DC bus that receives DC power from the main power unit; a vehicle accessory traditionally powered directly from an internal combustion engine of a conventional heavy-duty vehicle and requiring more than 1 kilowatt of power; an electric accessory motor, which is electrically coupled with the DC bus and mechanically coupled with the vehicle accessory, configured to supply power to the vehicle accessory, and the electric accessory motor and vehicle accessory carried by the heavy-duty vehicle either inside or outside of the engine compartment; and at least one of an ultracapacitor pack and a fly wheel coupled with the DC bus and configured to store power when the main power unit is on and supply power in place of the main power unit when the main power unit is off.