One of the most important aspects of any solar renewable installation is collector siting. Although it sounds incredibly obvious, you'd be surprised how many people choose an awful location for their solar collector array. A mistake like that can cost you thousands of dollars over the years, so make sure you know how the sun and Earth relate to each other, how the seasons affect sunlight distribution and how best to site your collectors for optimum efficiency.

Let's start with the basics: the sun moves across the sky in a different path every single day of the year. It rises in the east, reaches a peak point at the middle of the day and sets in the west. The exact position depends on the time of year and your location. It's all relative. For example, the sun at its peak may be directly overhead, at its highest point in the northern sky or at its highest in the southern sky, depending on where you live – but it will be at its highest point.

In general, solar collectors must:

• be positioned to face within 30 degrees of (true) south
• be in direct sunlight from 9:00 a.m. to 3:00 p.m., as this is when the most sunlight will hit them and they will gather between 80% and 90% of the day's solar radiation
• not be in any shade at all during that time
• be tilted at an angle to maximize their performance, which will depend on your location
• directly face the sun at noon, when the most powerful sunlight is available

The first thing to do is find true south. To do this, you locate true north and look the other way. True north is traditionally located by planting a stick in the ground and waiting until the stick's shadow is at its shortest – that's when the shadow points true north. It's a lot quicker and easier to consult an isogonic map like the one on this page and adjust the compass reading by either adding or subtracting the number of degrees on the closest line to your location!

The solar window

As you know, the sun travels a higher or lower path through the sky depending on the season. That path is higher in summer than in winter. In the US, the sun travels its highest path on June 21, the summer solstice; its lowest path occurs on December 21, the winter solstice.

Now, imagine those two paths across the sky. Imagine they draw two lines which determine the outline of a curved window, through which all the solar energy available passes before hitting your collectors. The June path draws a line that defines the top of the solar window's frame for your location; the December path defines the bottom of the frame; at both Equinoxes, the sun passes right through the middle of the curved frame. The first diagram here shows the top and bottom lines.

Now that you've imagined the top and bottom of the window frame, you need to consider the sides. As mentioned above, the sun is