This research aims to study the effectiveness of preventing heat loss in the hot water tanks are insulated biomass. By the banana fiber to produce insulation. Test conditions for thermal conductivity testing of insulation and the use of insulation made with hot water tank with a solar water heating system. And adjust the hot water temperature from 35 0C 40 0C 45 0C to 50 0C, and the amount of heat loss of the insulated hot water tank insulation, banana fiber, compared with black rubber.