Rheem is the most trusted manufacturer in the industry. The Solaraide tank series features 2" of rigid urethane insulation giving the tank an unbelievable insulating factor of R-17. Also provided is a backup element with 97% thermal efficiency to maximize energy savings on days without sun. These tanks have shown to lose only 1 degree per hour in standby loss. The diameter of the 80 gallon model is 24.5". Other tanks are available with smaller diameters if the water heater closet or attic access door is less than 24".