Bulbs are considered to be very inefficient sources of light. Bulbs give light and heat. As we use them for a long time, especially in winter, a large part of the heat produced by bulbs lowers the power consumption of the heating system. In this paper the problem of the real efficiency of a bulb is solved when both the lighting and heating effects of illumination of the interior of buildings are taken into account. It is analysed from the viewpoints of the energy consumed and the price of that energy. The real efficiency of bulbs and the savings obtained by replacing them with fluorescent lamps are calculated for various locations. (Contains 4 tables.)