I'm not sure what size green house you have, but generally HID or "High Intensity Discharge" lights are what's used in growing greenhouses. As a good rule of thumb, metal halide lights "White colored", provide excellent lighting for "leafy vegetative growing". High Pressure Sodium "orange colored" provides more of a flowering type light. For the most part people usually use the high pressure sodium's if your only going to use one type, as it'll handle everything most plants need for the full term of it's growth. It really depends what your growing. These light's can be found at many Hydroponic stores and come in waterproof, sealed units.