Another important aspect that affects building performance is orientation. An effective passive solar heating design assumes that the building is orientated to receive as much solar radiation as possible in winter, when heating is required, whilst rejecting as much as possible in summer when it is not.

To derive the most effective orientation, the Weather Tool calculates the amount of solar radiation incident on a 1m² vertical surface for each 5° of orientation angle. Three values are stored for each angle, the average daily radiation taken over the whole year, over the coldest 3 months and over the warmest 3 months. These three values can then be plotted on a polar graph where the radius of any point from the centre represents the incident radiation value.

Figure 1 - Chart showing optimum orientation angles based on solar radiation received in the coldest 3 months (blue), the warmest 3 months (red) and over the entire year (green).

The most favourable orientations occur where the amount of incident radiation in winter is greater than that incident in summer, where the blue line extends out beyond the red line.

However, it is also desirable to provide as much protection from the maximum summer radiation as possible. Hence the optimum orientation provides maximum winter collection with maximum summer protection which, assuming the building is roughly orthogonal, involves a trade off between the two based on the relative amounts of heating and cooling stress in the climate. Thus, in the graph above, the compromise angle is not exactly at the point of maximum winter collection, but slightly to the east in order to 'turn away' slightly from the hot afternoon sun in summer.

This calculation is available in the Weather Tool by clicking the Best Orientation... button in the Solar Position panel. You should try a few different climates, especially Kuala Lumpur in Malaysia as its solar radiation distribution is markedly asymmetrical.