Lamps are first compared with other electrical apparatus as regards the possibility and the method of economic choice. Choice of type (carbon, metallic vacuum and gas-filled) is first considered, and then choice of rating for a given type, the latter being treated both graphically and algebraically. Simple formulæ are developed for the most economical rating, first with a fixed lamp and then with a fixed candle-power. These are applied to a number of different cases, and various courses of action are suggested for the manufacturer and the lamp user.