PV has gotten cheap and is easy to install, providing opportunity to defect hot water heating from the electrical grid or from propane or heating oil. Electric resistance water heaters can be powered by direct current, straight from a PV array, with no batteries, no inverter, and no grid-tie. The PV is connected to the bottom element, with the top element still connected to the electric grid as a backup.
At one time, solar thermal was a tenth of the cost of PV. At about 50 cents per watt, solar thermal equipment is still cheaper than PV, but often not by enough to offset its higher installation and storage costs. A PV-based domestic hot water (DHW) system using a large electric water heater for storage, and a resistance element as the “heat exchanger” can be cheaper for homes with minimal hot water needs, and very much a DIY project. You can even use the water heater you already have. I use the 50-gallon unit that has been here since 2007.
Another benefit of using PV for DHW is that the distance from the array to storage isn’t nearly as critical as it is with solar thermal. That’s because electrical line loss can be lower than thermal loss and the cost of wire is so much less than plumbing plus insulation. With my PV array, the distance was 240 feet, but it cost just over $250 for the #8 cables to keep line loss under 2%.
For homes with more hot water needs, solar thermal may still be a less expensive option. A solar thermal system will collect more energy per collector area than a PV array. For modest hot water needs, such as for one or two people in a single-family home, using PV for hot water makes sense. Using PV for DHW also presents an opportunity for backup power, since the PV electricity could in a pinch be redirected to DC loads or to a battery-inverter system.
Pete Gruendeman • La Crosse, Wisconsin