I wouldn't use a batch heater somewhere I thought it might freeze. No one (that I know of) makes commercial batch heaters. They're a DIY project. A full up solar water heater system has thermostatically controlled valves and recirc pumps. I guarantee you that even up north, they will reduce your water heater bill and not dowse you with cold water. For christ's sake, they've been making and selling them since the turn of the LAST century. You think they might have also thought about some of these issues or are you just the smartest human being there is?When it is December 22nd, the shortest day of the year and very cold, when the water goes outside to be warmed by said solar panel, how much heat is absorbed during the night? So when you wake up the next day your tank is nothing but freezing cold water? Or is there a electric/gas water heater inside your house just for those emergencies?