Does It Make Sense to Use Photovoltaics to Heat Water?

solar thermal vs photovoltaic
CC BY 2.0. Lloyd Alter (left image)

Lloyd Alter (left image)/CC BY 2.0

Three years ago I suggested that it was almost criminal that people were putting photovoltaic panels on their roofs before they put in solar hot water heaters. I calculated that a watt of energy from a PV panel cost ten times as much as from a solar thermal panel.

Times have changed, and PV panels cost a third of what they did then, while water heaters have got a lot more efficient. But as Martin Holladay points out in Green Building Advisor, there is a lot more going on in this comparison than just the price of equipment, particularly in more northern states and Canada.

  • The main point is that thermal systems generate a lot of energy in the summer, far more than you need. But you can't really store it and put it away for the winter very easily. With a grid-tied PV system, you can sell it. This makes a watt of electricity a LOT more valuable than a watt of heat.
  • Solar thermal systems have lots of piping, valves and pumps. I have learned from the sad experience of my interconnected hydronic heating and domestic hot water system that you don't want your basement looking like the set for Das Boot, that simplicity matters a lot.

Martin goes so far as to say that " unless you’re building a laundromat or college dorm, solar thermal is dead." That may be a bit extreme, but he has certainly made me rethink the conventional wisdom.