Solar PV System Design Do’s-and-Don’ts
Guidelines for designing and installing your system
You can hardly fail to notice the surge in solar panels which are popping up on roofs all over the place. But how can you tell the good from the bad? The efficient from the inefficient? The thing is, if you don’t have any knowledge of how solar power works, then you run a high risk of falling foul of the many con artists out there.
It’s a sad fact of life, but even in Sydney people exist who would like to part you from your cash with no conscience about the job they perform. To safeguard yourself, the very first thing you should do when speaking to a solar power salesman is to ask to see the Clean Energy Council Solar Design accreditation of the designer for the system he is trying to sell you. Ideally, the person visiting your home should have some evidence of internal or external training in system design. If the system they are selling you is itself of a good quality, then it’s pretty likely that you’re dealing with a reputable company who will also install the system correctly.
We’ve covered choosing the right solar panel in our other guides, the Detailed and Brief guides to choosing a solar module. It should also be noted that it is necessary to take into account the voltage and current windows of the inverter when choosing the modules. Your professional installer can assist with this.
How far should the solar panels be from the edges of the roof?
No solar pv systems should extend beyond the edges of the roof. Wind will get underneath and weaken the fixings, leading to a potentially lethal situation. Depending on where you live, different laws exist as to how close solar panels are allowed to be from a roof edge. Of course, those in cyclone areas have the strictest laws, and are not allowed to be any closer than 1 metre from the edge or apex of a roof. If the panel position is proposed as being very close to an edge, further attachment points will be required. Most mounting system guides feature a diagram showing the allowed distances for the standard number of attachments.
There are two reasons solar panels need to have a decent roof clearance (height suspended above the roofing material). The first is so that the installer can reach underneath during installation to secure the wiring to the rails underneath. Not only will loose cables make the performance of your solar pv system less efficient and untidy, but it’s pretty dangerous as well. Roofs heat up immensely in strong sunlight, and electricity and heat is a dangerous combination. Ask the technician assessing your home if the rails are mounted straight onto the roof (not desired) or if there is a clearance (better).
The other reason panels need to be mounted with clearance from the roof material is heat. Despite what you may have heard, the hotter a solar panel gets, the less efficiently it works. Light is what powers them, not heat, and the cooler the solar panels stay, the more electricity they produce. Having a decent roof clearance allows air flow underneath and this helps to keep the panels cool. Some installations are still being performed with hot water mounting equipment, which is different because of weight of hot water systems helps to secure them. It almost goes without saying that this is not appropriate for Sydney.
Dissimilar metals inducing Galvanic corrosion
If a solar pv unit is placed on a tin or metal roof, then it is possible that a chemical reaction between the different properties of the metal parts of the solar unit and the roof will cause corrosion over time. This is known as dissimilar metal corrosion. If the solar panels are to be mounted on a metal roof, then a material such as rubber should be placed between the two as a preventative measure. Likewise, bolts and frames – or screws and saddles – need to be made of the same (or compatible) materials. Typically, the best partner for the largely aluminium components is stainless steel.
It is important to consider obstructions in the East-North-West direction from the proposed panel location, with most importance given to objects that will shade during the key 10am – 2pm window. Complete shading of one panel in a group can affect the output of the entire group, and in any case, the panel itself will have its output reduced or bypassed. As a result of poor training at some large companies, we’ve seen installations butted up against chimneys and flush to solar hot water tanks – this won’t occur if the house is professionally inspected prior to the installation day.
Industry Knowledge is so important
Of course, there are many other things to be taken into consideration when purchasing and designing the perfect system for you. These range from the obvious, such as the panels facing the correct direction – north – with no shade obstructing the passage of light, through to the less obvious, such as how design affects the solar rebates which you may be entitled to from the government. A good solar technician will be happy talk you through the basics, to show you photos of previous installations, and to answer any of your questions about positioning and about the equipment itself.
For more information on system design, we recommend the Clean Energy Council website, and the book Photovoltaic Systems Engineering, Third Edition by Roger A. Messenger and Jerry Ventre (We’ve read the 2003 and 2010 editions and found the books invaluable).
Collated by Greenforce Energy 2010