> Even then, the systems are of such different breed that you > could have a problem comparing them at all, for instance, > photovoltaics do not allow for easy

> Even then, the systems are of such different breed that you > could have a problem comparing them at all, for instance, > photovoltaics do not allow for easy storage and thermal > collectors give you the option of storing the energy in > the form of heat for a while before using it for > conversion. And then there's the difference between energy efficiency, energy return on energy invested, cost efficiency, and energy return on money invested which are all totally different measures. The energy efficiency of solar PV is really rather ridiculously high all things considered, I think it's rarely acknowledged that the ~10% conversion efficiencies common for PV are actually extremely high compared to plants with their more typical ~1% for even organisms like algae. Really kind of puts things in perspective. I think solar thermal is less energy efficient generally for producing electricity, but is obviously more efficient for generating heat. But the cost efficiency is less clear. I've seen studies that indicate solar thermal can be on par cost-wise even though the efficiency is lower, because it's an array of mirrors instead of semiconductors. Not really sure what the result of that analysis ended up being, but probably varies by site.