Solar thermal energy: What you need to know

There are two key methods for harnessing the power of the sun: either by generating electricity directly using solar photovoltaic (PV) panels or generating heat through solar thermal technologies. While the two types of solar energy are similar, they differ in their costs, benefits, and applications.

Solar thermal encapsulates any technology that takes sunlight and converts it into heat. That heat can then be used for three primary purposes: to be converted into electricity, to heat water for use in your home or business, or to heat spaces within your house. Each of these options requires distinct technologies, but all of them harness the power of the sun to offset some portion of your energy needs.

Solar thermal is different from solar photovoltaics in that solar thermal technologies use the heat from the sun to produce energy, while solar photovoltaics take advantage of the "photovoltaic effect" of some semiconductors like silicon to produce a flow of electricity right from the sun's rays.

Using solar thermal technology to generate electricity is most popular for large, utility-scale solar projects. In this process, mirrors focus the heat from the sun onto a collector, where a liquid is converted into steam to spin a turbine. These types of power plants are typically called concentrating solar power (CSP), as they use mirrors to "concentrate" sunlight onto a set collector (typically a central tower or pipes in front of each of the rows of panels.)

CSP plants are large and powerful - they're typically built to be a minimum of 100 megawatts, which is more than 10,000 times larger than the average residential installation. According to the National Renewable Energy Laboratory and the Solar Energy Industries Association, there are currently 11 CSP projects up and running in the US.

Instead of relying upon natural gas or electricity to power your hot water heater, solar thermal hot water heaters allow you to convert the heat from the sun into hot water for your home or business. These systems require a solar collector (sometimes referred to as "solar thermal panels"), which transfers solar energy to water, as well as a storage tank, which then collects and saves the solar-heated water for later use. To learn more about how these technologies function, check out our solar hot water explainer.

There are two ways to heat your home using solar thermal technology: active solar heating and passive solar heating. Active solar heating is a way to apply the technology of solar thermal power plants to your home. Solar thermal collectors, which look similar to solar PV panels, sit on your roof and transfer gathered heat to your house through either a heat exchanger or via piping that runs hot water through your house.

The second method of heating your home with solar thermal technology, passive solar heating, does not include or require any mechanical processes. Instead, it uses the design of your home–for instance, by installing large south-facing windows–to collect and store heat from the sun to then radiate it into the ambient areas of your home over the course of the day.

Both active and solar heating are great options for houses with high heating loads in cold, sunny areas. These technologies can significantly reduce the amount of electricity or fuel you use for heating your home or business. Passive solar heating is especially cost-effective for new construction because it allows builders to incorporate passive solar heating design into the initial construction plans for your home or business.

With so many different types of solar thermal technologies, and can be tough to perform a like-to-like comparison with a solar PV system. However, breaking the question out into comparing solar PV systems to CSP technology, solar heating, or solar hot water individually allows for one-off comparisons.

For instance, on a large scale, CSP plants are more efficient than standard solar farms. However, CSP facilities require large amounts of land and are only well suited for certain geographical areas of the country or world.

At a household level, passive solar heating is a great way to design your house to reduce your overall electric demand over the lifetime of your house and is a perfect system to pair with solar PV since it will make the impact of each solar panel that much greater. Active solar heating and solar thermal hot water should be evaluated in comparison to solar PV on a case-by-case basis. Which technology makes the most financial sense for your house may be a mix of technologies determined by the specifics of your geography, electricity load, and overall heating or hot water needs.

While solar thermal technologies allow you to offset your home heating energy needs with the heat of the sun, installing solar PV panels on your property allows you to offset most–or all!–of your home energy needs with solar. In fact, our most recent Intel report found that the average customer on EnergySage received solar quotes that would offset 94 percent of their electricity need. To see how much you can save on your energy usage by going solar, check out EnergySage's free-to-use Solar Calculator. Ready to offset your home energy use with solar? Register for EnergySage's Marketplace to receive personalized quotes from pre-screened solar installers today.