The 41 or 21 watt 5,500°K T5 HO replacement LED lamps are an excellent choice for the propagation and veg cycles of your plants life. The iSunlight® T5 lamps are an intelligent LED that is specifically designed to work with your existing T5 HO fluorescent fixture. The iSunlight® 4 ft lamp produces over 4,900 lumens with only 41 watts of consumed power and the 2 ft lamp produces over 2,400 lumens with only 21 watts of consumed power. These are environmentally friendly and mercury-free lamps. Not compatible with HF Agrobrite T5 fixture.