How to know if wall or roof faces true south

How close to true south is the wall or roof on which you would like to mount your solar panels? There is a very simple way to find out. All you need is a watch, a pencil, a sunny day, and a protractor, which can be made by pasting the accompanying diagram on a piece of cardboard. Now follow these steps:

* Take the protractor and place it up against a window in the wall in question.

* Find out from a local weather station the times of sunrise and sunset in your region. Solar noon occurs exactly midway between these two times.

* At solar noon, hold a pencil vertically at the center of the flat side of the protractor. The fall of the pencil's shadow will indicate the number of degrees off true south your collectors would be. Thirty degrees off true south will still provide 90 percent solar efficiency. At 45 degrees, efficiency is only 72 percent.

Another way of finding true south is to go outdoors at solar noon and stand at the end of the shadow cast by a telephone pole (the pole must stand straight up and not lean to one side). Now turn directly toward the pole and you will be facing true south.