With advances in photovoltaic efficiency and the development of heat-pump water heaters, photovoltaic systems are now quite competitive for heating water.
Here is my effort to do an apples-to-apples comparison of the installed cost of a solar thermal water heater, versus the installed cost of a heat-exchanger electric water heater and a solar electric installation large enough to support it. A lot of the specifics of the comparison depends on your assumptions, so I’ll start out with the assumption that you need 40 gallons of hot water a day, and the water has to be raised 60F degrees. So that’s 19,920 BTU per day, or 5838 Watt-hours per day.
I’ll do the electric first. Over the course of the year, where I live averages 3980 Watt-hours of insolation energy per square meter. Commercially available panels (see
Grid tie Solar Power Systems - Grid tie Solar Panel Systems) have an efficiency of 14.4%. Assuming a heat pump efficiency of 200%, to get 5838 Watt-hours of heating you need 2919 Watt-hours of electricity. At 14.4% efficiency, to get 2919 Wh of electricity you need 20,271 Wh of insolation. At 3980 Wh/sm you need just about exactly five square meters of panels. Those commercial panels are 1.6 square meters so you would need three of them. These panels are called 250 watt panels so this would be a 750 watt system. A complete 750 watt grid tied system kit costs about $1200 (See
1200W 900W 750W 600W 450W 300W Complete Kit Grid Tie 160W Solar Panel System | eBay, I'm not recommending it, it's just the first one I found on Ebay) . You’d also need a heat pump water heater, a GE Geospring 50 gallon is about $900. So $2100 for the pieces. I’m going to assume that the installation cost is going to be roughly the same for either system since the pieces are fundamentally similar. I’m also leaving out subsidies and credits because they seem to be similar as well.
Now for the thermal system. Interestingly, evacuated tube panels have almost completely replaced flat panels in the marketplace, I wasn’t even able to price a flat panel kit any more. Evacuated tubes have come way down in price and offer superior cold weather efficiency. A thermal system has to be sized to provide 100% of the energy needed on the lowest-output day of the year. Here, December is the lowest insolation month of the year, averaging 1810 watt-hours/square meter per day. The efficiency of thermal collectors depends on the temperature. The average daytime temperature in December here is 47F, a commercially available collector gives an efficiency of 27% at 47F (see
Solar Water Heaters with Vacuum Tubes ) Efficency is lower in January when the temperature is lower, but January is sunnier, so December is the lowest producing month. To produce 5838 Watt-hours per day with 1810 watt-hours/square meter of insolation and 27% efficiency you need 11.9 square meters of panels. Those commercially available panels come in arrays of 18 tubes, each one is 2.35 square meters, so you’d need 90 tubes. They don’t come that big off the shelf, but you could get a 72 tube system for $4675 and an additional 18 tubes for $500. So $5175 for the pieces.
So $2100 for the PV and $5172 for the thermal, it’s not looking good for thermal. Now thermal systems are often sized to provide less than 100% of the hot water needs, the costs are much better that way. Often they are sized so that they provide 100% on the best day of the year instead of the worst day, that way 100% of the energy they provide is utilized. The best month for solar here is August, if you sized your system to provide 20,000 BTU on a typical August day it would only have to be 37% of the size. So instead of 90 tubes you could have 33 tubes. A 30-tube system is $2450, 36 tubes is about $125 more. Such a system would provide an average of 14,500 BTU per day year-round, or 72.5% of your annual heating need. It’s still not cost competitive, but it’s much closer, within a few hundred dollars. However, you would need a backup heating source to provide that other 27.5% of your hot water.
The advantages of PV swing even further if you look at two other factors. The first is economy of scale. The bigger your system, the less it costs per installed watt. That 750 watt system is $1.60 per installed watt. However, if you’re already installing a PV system the marginal cost of three additional panels is the cost of the panels. Those 250 watt panels cost $210 each at quantity 25, so that’s $0.84 per installed watt. The second factor is space on the rooftop. If there is limited space, PV makes even more sense. The 90-tube thermal system occupies 128 square feet. Even the modified 36-tube system occupies 47 square feet. The three-panel 750 PV system occupies 50 square feet. So if you only have limited space PV is a better use of that space.
It varies by state, but here the electric company allows reverse metering, so excess electricity produced by the PV panels is returned to the grid and I get a credit on my electric bill. Excess hot water produced by a hot water solar panel is just lost.
Where the jury is still out is on the efficiency claims of the heat pump. Interestingly, even without the efficiency of the heat pump PV is still more competitive than thermal. What’s telling is I’m starting to see gadgets that allow you to connect your PV panels directly to a resistance-heated water heater, bypassing the inverter and the grid-tie when the panels are producing and hot water is needed. See
Solar Hybrid Hot Water Solution - No Plumbing Mods Required - At TechLuck
