The invention relates to a power unit, in particular for a hybrid vehicle, having a two-cylinder reciprocating piston engine which comprises two pistons guided in two cylinders in a tandem arrangement and two counter-rotating crankshafts connected to the pistons by connecting rods, and having an alternator that can rotate in the opposite direction to the first crankshaft and in the same direction as the second crankshaft. The invention is characterized in that the alternator is in driving engagement with at least the first crankshaft via a traction mechanism and comprises a counterbalance, wherein the first crankshaft is connected via a timing chain or a timing belt to a balancing shaft which carries a further balancing mass. The invention also relates to a vehicle, in particular a hybrid vehicle, having such a power unit.