Sundial

One of the most overlooked uses of the sun is in keeping time. This is one case where the effeciency is 100%. Ancient people sure didn't overlook it. Not only can you tell time, but you can also tell the time of the year depending on the dial you use. I personally only have one dial, a portable one from Pennsic War. Unfortunately it only works in Pennsylvania and similar latitudes, but it looks great and it's portable.