Vacuum tube solar collectors have minimal heat losses, therefore, their efficiency is relatively insensitive to ambient temperatures, wind conditions and insolation levels. Inlet temperature increases from 85°C (185°F) up to 132°C (270°F) reduce average efficiency by only 13%. Thus, it is simpler to predict performance of vacuum tube collectors than conventional flat plate collectors. A simple manual procedure is presented to predict the performance of vacuum tube collector systems. It is based on the results of many hourly computer simulations for various applications and of correlative test data developed at GE/Valley Forge.