Determining the orientation and tilt angle of your solar power generation system is one of the most important considerations in designing your solar power system. As we have mentioned before, in the southern hemisphere, due North is the optimum orientation for panels. But not everyone has a perfectly oriented roof. When your two best options are East or West, which should you choose?
During the course of a day, the sun passes through the sky in a particular arc that varies throughout the year due to the earth’s orbit (see top image). The arc is always symmetrical from East to West, measuring out from the centre point, which would be midday. This means that from sunrise to midday (not counting during daylight savings time) and from midday to sunset, there is an equal number of hours of sunlight on one particular location. (You can see how this applies to your particular location using applications like SunCalc, which shows you exactly how the sun moves through the sky at different times of day.)
So, if your home or roof does not have a Northern aspect, but does have two roofs that face due East or due West, you may be asking yourself which side would be a better location for the most power generation. Making the right decision will impact on how much money you can save or possibly even make under the solar power incentive schemes available throughout Australia.
The first thing to consider is shading, of course, which can potentially have a big impact on the productivity of your system. Is either side of the roof shaded by nearby objects? If one side is shaded, choose the other side.
If you’ve determined that the shading effects are either non-existent or basically the same on both sides, then the next thing to consider is the angle of your roof–if possible, you don’t want to have to put in any mounting brackets or solar trackers. The two sides of your roof may be tilted at different angles: you will want to choose the side which is closer to being horizontal, as this will result in the most insolation (incident sunlight) throughout the day.
So, you have determined that shading is not an issue, and that both sides of your roof are symmetrically angled. What do you do now? Although Australia is well-known for its unpredictable climate, you will need to consider when you tend to have cloudy weather more often–in the morning or in the evening. This will depend on your location and climate, but for many locations (including Sydney), mornings have a greater tendency for overcast skies. Shading, even from clouds, can reduce the output of a system by up to 90%: any time it is not sunny, energy production will drop. So, in essence, the answer is that you should try to put your panels on the ‘sunnier’ side of the roof in terms of weather: if you have cloudy mornings more often, the West roof, and if you have cloudy afternoons more often, the East roof.
That being said, if you are thinking strategically about power consumption and pricing, you will want to keep in mind that in most states the price of electricity is higher around 5pm. For example, in NSW the peak electricity price at 5pm is $0.36, which is roughly twice the shoulder rate just prior to that time. If your panels are west-facing, you will be producing optimally right about this peak electricity time, potentially providing you significant savings on your bills.
For more information on west facing solar panels see our article Solar panel orientation: have we been facing the wrong way?
Written by James Martin
© 2010 Solar Choice Pty Ltd
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Previous related Solar Choice Blog Entries: Orientation and tilt angle for your solar power system : Solar Power Incentive Schemes in Australia : How to install solar panels on your roof : Solar Trackers