We have 300,000 BTU of Evacuated Solar Thermal Tubes for Solar Heat generation on our roof. The heat is piped into our Heat Micro-Grid and is used for heating and cooling our 840o square foot high bay facility.
These tubes are highly efficient, and even in below zero weather our largest problem is keeping them cool with-in operating parameters. (we have to pump ground water into the system to do this)
Even on a cold day we can get 200 F degree heat