Why choose solar thermal energy versus other alternatives. Two reasons: expense and efficiency. First, it is less costly to manufacture a solar thermal panel than a photovoltaic (PV) panel. Second, solar thermal panels are more efficient than PV panels. By efficiency, we mean that percentage of solar radiation that is converted to useful power. The sun delivers a fixed amount of power (Solar Constant) per square foot at the earth's surface. A solar panel converts this to useful energy.
The solar collector, either a PV or thermal panel, captures and converts that fixed amount of power into energy you can use. The solar constant at the edge of the earth's atmosphere is about 125 watts/hour per each square foot. At sea level that decreases to about 93 Watts per hour per square foot (see figure to right) . That is all you ever will have available, never any more. But of that 93 watts, not all can be captured, some will "escape". A PV panel will only harness about 15%, so that translates into about 14 Watts per square foot per hour. The energy harnessed will be in the form of a DC current (amps/hour). A solar thermal panel, in contrast, can harness about 66% of the available power, or 61 Watts per square foot per hour. This will be in the form of heat (more easily measured in BTUs (British Thermal Units).
When you examine this information, one fact becomes obvious; size of the panel is the key determinate of output power. The larger the panel in square feet, the more power it will generate. Since area is calculated by multiplying height times width (h x w), the output power is an exponential factor. A 4' x 4' panel produces 16 times the power of a 1' x 1' panel. That fact, combined with output efficiency makes a substantial difference as the size of a panel increases. A 5' x 5' solar thermal panel generates 5X the output power of 5' x 5' PV panel. Over the course of a day, the total amount of energy collected will vary based on location, season, and angle (tilt) that the panel is mounted.
Panel Dimensions 
PV panel output
@ 15% efficiency 
Thermal panel output
@ 66% efficiency 
1' x 1' 
14 Watts/hr 
61 Watts/hr 
2' x 2' 
56 Watts/hr 
244 Watts/hr 
3' x 3' 
126 Watts/hr 
549 Watts/hr 
4' x 4' 
224 Watts/hr 
976 Watts/hr 
5' x 5' 
350 Wattst/hr 
1525 Watts/hr 
6' x 6' 
504 Watts/hr 
2196 Wattst/hr 
7' x 7' 
686 Watts/hr 
2989 Watts/hr 
8' x 8' 
896 Watts/hr 
3904 Watts/hr 

The other issue with solar thermal versus PV panels is the issue of cost. It can be less expensive to build a solar thermal panel than a PV panel. A solar thermal requires an absorber, which can be just a coated aluminum foil and glazing, either plastic or glass. PV panels are built from more expensive poly silicon and rigid frame with glazing.
The most common solar thermal panel technology has been the flat plat solar collector. The cost of manufacturing these is comparable to PV panels because they use copper tubing that must be welded, an expensive and heavy steel or aluminum frame, and heavy solar glass. They also require expensive installation and heat exchangers because they require circulation of an antifreeze liquid (glycol) to prevent freezing. Zonbak panels eliminate all of these expensive materials in favor of lightweight aluminum foils, Teflon film, and mounting via an inexpensive but strong wire cable system. Instead of circulating and heating glycol, they just circulate and heat air. As a result they are inexpensive solar panels to manufacture and assemble. They can also be sold in 3' widths of lengths of 10', 20' or up to 30' long. The corresponding surface area will be 30 square feet, 60 square feet, and 90 square feet. At 66% efficiency, the 30' panel will produce an output of 21,000 BTUs/hr. In the mid Atlantic states, as an example, it would take approximately 40 BTUs/ft^{2} to heat a home, so a single 30' panel could heat approximately 500 ft^{2 } on a sunny day.

