A low-wattage metal-halide discharge lamp has a tube of the double ended type that forms a bulb or envelope, a pair of electrodes, e.g., an anode and a cathode, which penetrate into an arc chamber inside the envelope, and a suitable amount of mercury plus one or more metal halide salts. The electrodes are each formed of a refractory metal, i.e., tungsten wire, extending through the respective necks into the arc chamber. Heat transfer along the tube wall and through the necks is controlled by designing the bulb wall so that the cross sectional areas at the necks and at quarter-chamber planes halfway between the necks and a midplane have a respective quarter chamber loading factor and a neck loading factor each within a desired range. Lamps of this design achieve high efficacy at relatively low power, i.e., below 40 watts.