Solar Water Heating Q&A

Why not use solar electric power for water heating?

This is an excellent question. Understanding the answer provides some insight into why, 50 to 100 years from now, solar water heaters will still be the best way to heat water.

And why you should go ahead and install a solar water heater today.

The issue is conversion efficiency. A typical flat plate solar water heating collector transfers about 63 percent of the solar energy that strikes it directly into the water (or heat transfer fluid, in indirect circulation systems).

A typical photovoltaic (PV) solar electric cell converts only 15 percent or so of the energy striking it into electrical energy under ideal conditions: PV cells lose efficiency as operating temperatures rise. Additional inefficiency occurs when an inverter changes direct solar electric current (DC) into alternating current (AC). The end result is that a PV cell with a 15 percent rated peak efficiency only delivers about 10 percent of the energy striking its surface to a demand load.

So here is the problem: The solar electric PV panels would require more than six times the roof area of a flat plate solar thermal collector to meet the same (water heating) demand load.

Meeting the hot water needs of an average Florida family of four requires about 40 square feet of flat plate solar water heating collector surface area, so to do the same job with PV panels would take about 40 x 6 = 240 square feet of PV panel surface area.

Does a tankless water heater make more sense than a solar water heater?

We don’t think so. Every gallon of hot water you use still has to be heated. And the amount of energy required to heat a gallon of water to a certain temperature does not change just because it is being heated at a faster rate.

The tankless water heater only eliminates energy costs for maintaining the temperature of water that has been heated and is sitting in the storage tank awaiting use. So a tankless water heater saves about 15 percent of the heating energy needed by a conventionally insulated electric or gas water heater. Solar storage tanks have extra insulation, to keep water heated during the day hot overnight, so you get the same temperature maintenance savings with solar.

And tankless water heaters do have drawbacks. According to the U.S. Department of Energy:

“Sometimes … even the largest gas-fired model cannot supply enough hot water for simultaneous, multiple uses … taking a shower and running the dishwasher at the same time can stretch a (tankless) water heater to its limit.”

Most tankless water heaters use natural gas, which is currently about 32 percent cheaper[2] than electricity for water heating. But natural gas is a fossil fuel, so it is subject to the same climate change concerns and cost inflation pressures as powerplant-supplied electricity.

Solar energy is clean, inexhaustible and free, so it’s “greener” than gas and beats the economics of a gas tankless water heater hands down.

And one final point: Tankless water heaters can fall short on performance during periods of high demand; for example, if you are showering while the dishwasher or clothes washer are filling, or if more than one shower is in use at the same time.

Of course, you could overcome this problem by installing multiple tankless water heaters. On the other hand, tankless water heaters cost much more than conventional water heaters to install, so the total installation cost for more than one tankless water heater can quickly surpass the cost of a single solar water heating system.

Why not just install a solar water heater?

Solar water heaters provide all the benefits of tankless water heaters but eliminate up to 90 percent of your water heating costs.

Will my water be hot enough?

Yes. Solar heated water is often hotter than the thermostat setting on your water heater. In fact, for safety reasons our systems include mixing valves to make sure the hot water going into your house isn’t too hot. On the other hand, sometimes we may have extended periods of very cloudy and rainy weather. During these periods, a backup electric heating element in your water heater / storage tank will automatically heat water to the water heater’s thermostat setting.

Will I have hot water during cold weather?

Yes. Solar water heating collectors typically deliver excellent performance in Florida during cold weather because the sky is very clear during winter high pressure waves. The glass cover plate and insulation inside the collector prevent collected heat from escaping to the outside air.

Can you use my existing water heater as the solar storage tank?

Usually, no. Solar water heating systems are designed to heat and store 24 hours worth of hot water during the daylight hours, so the tank has to be large enough to store 24 hours’ worth of hot water. Most conventional electric water heaters in Florida homes have a capacity of about 52 gallons. Standard solar storage tank sizes are typically 80, 100 and 120 gallons, with 80 gallons being appropriate for most three- to four-person households. Also, solar storage tanks typically have better insulation than conventional electric water heaters, to minimize overnight heat loss