This is a guest post from ReVision Energy, Maine's leading installer of solar hot water and solar electric systems.  Evergreen frequently partners with ReVision on efficiency/solar projects, and you can take it from us: these guys are the best.  Contact ReVision for a free solar site assessment and learn more about renewable energy.    

The dizzying drop in cost of photovoltaic (PV) panels is great news that’s making headlines, as well as shifting the way architects, green builders, and even solar installers think about venerable solar thermal technology.

With high-efficiency electric heating equipment available, and PV at record-low prices, doesn’t it make more sense to install an electric water heating system and then use PV to generate electricity to offset that electric load? Isn’t that “solar hot water?”

Well… not so fast. While electric water heating backed up by PV works great for certain cases, the full story is a bit more nuanced. For homes occupied year-round with moderate (60GPD+) hot water consumption, solar thermal remains a clear winner for domestic hot water loads

Solar Thermal: A Use it or Lose It Solution

For solar thermal to make sense you should occupy your home during the summertime, so as to take advantage of free hot water during peak production months. Unlike photovoltaic systems, which are able to bank excess summertime production for the winter by selling power to the grid, solar hot water systems are not good at providing storage for more than a few days at a time.

When properly sized, a solar hot water system will provide around 76% of year-round hot water consumption: 100% of the hot water a household consumes from May to September (and not too much excess), and ~50% of hot water use in the off-season.

Solar Thermal vs. Electric Tank + PV

Solar thermal technology is much more efficient at converting sunshine to heat than photovoltaic panels are at converting sunshine to electricity (~60% vs. ~19%, respectively). This means that more hot water is available, at a lower cost, than a comparable electric-powered system. The differences are minor for a house with small hot water loads (<30gal/day) but as a household's hot water needs increase, solar thermal outpaces an all-electric system pretty quickly.  For an active family, solar thermal is the least expensive way to meet domestic hot water loads. 


Solar Hot Water vs. Heat Pump Water Heaters

The introduction of heat pump water heaters makes the the solar hot water vs. electric water heating debate more complicated. These high-efficiency electric water heaters offer 2.5x more heating per kWh than conventional water heaters because they use ambient air temperature to heat water.  As they remove heat from the air, they also leave the basement cooler and drier.

While the heat pump and PV solution costs slightly less to install than solar hot water and is light years beyond a clunky old tankless coil, solar hot water is still the better choice for a busy family who lives there year-round (especially with the new state rebates!).

Here's why:

  • If you are already investing in a PV project, adding solar thermal opens up additional state rebate money you would not other tap into. At $1,000 or $1,500 (ME and NH, respectively), the state rebates for solar hot water are more generous than for heat pump water heaters.
  • Heat pumps are noisier typically than a solar hot water tank. This matters most in new, efficient buildings where mechanical rooms may be close to living quarters.
  • Warranties on heat pumps and solar hot water equipment are similar (10 years), though we feel that the long-term maintenance costs of solar hot water will be less than service costs of heat pump systems.
  • Solar hot water collectors will take up less space to produce a household's hot water than a comparable PV array offsetting an electric device. This is important for households that want to make the deepest cut into their electric bill as possible and have limited south-facing roofspace.

Solar Thermal Stays Strong

The dramatically shifting landscape of solar electricity makes for exciting times. It's never been better to invest in a photovoltaic system, particularly while both Maine and New Hampshire have generous state rebates.  That being said, solar hot water systems still make great financial sense in many cases. Seasoned solar designers will look at the overall picture of a home rather than assuming that "one size fits all."