We recently finished a 4 panel Stiebel Eltron domestic solar hot water system in Sunderland, Massachusetts. The system is a little larger than usual to compensate for a somewhat shady site. The Stiebel tank is 110 gallons and, using a upper heat exchanger, is backed up by a Buderus propane boiler. Stiebel Eltron makes fantastic tanks--with 33% more insulation than every other tank on the market. We decided to place the panels towards the upper part of the roof to maximize solar gain. In the closeup picture of the controller, you might notice that the collector temperature is at 148--the tank is therefore probably 140!