Conviene far bene l'amore (1976)

In the near future, in a world without energy resources that has returned to a preindustrial age—where horses and bicycles are the only means of transport and gas is the sole source of lighting—an eccentric scientist, Enrico Nobili, devises a device that generates electricity from sexual intercourse, testing it in a ward at the old Policlinico in Rome. The handsome young Daniele and the busty Francesca are inexplicably admitted to the same room after suffering accidents caused by Nobili himself. Once the two become attracted to each other, the experiment succeeds, and after decades the old light bulbs begin to glow. To repeat the experiment and confirm his theories, an amusing attempt follows to reunite the two involuntary guinea pigs, since the young assistant Piera lacks sufficient sensuality to generate an appreciable amount of energy. The invention is later stolen, but the researcher has the satisfaction of seeing the world restored to the splendor of the twentieth century, despite a new form of exploitation that discourages love by depriving people of the last pleasure left to them—namely, sex.