When it comes to wattage bulb for a salt lamp, it depends on the lamp's size. That is: . Small light lamps of up to10 pounds need a 15 watt light bulb . Medium salt lamps of 11-20 pounds use 25 Watt light bulb .large slat lamps with over 21 pounds need 40 Watt light bulb If you live in a high humid climate, you should go for a light bulb with higher wattage