Buyer beware!

This seems to be the season of the Salesmen, and several local companies have knocked on my door trying to persuade me to buy insulation and photovoltaic and/or solar thermal panels.  The only problem was that, as I hope my information website ( shows, I have considerable knowledge on the siting of the variety of solar panels and on the variety and action of insulation materials available – all I will say on the latter here, is that dry stuff settles by about 20%.  The knowledge of the salesmen suggested an admirable keenness to keep their businesses afloat by taking on the new technology, rather than by having in-depth knowledge of the subject.

Before you agree to anything, do question such Salespersons in depth and preferably consult the notes on my, or another website.  The solar equipment is very expensive to install and only brings a good return on your money if it is sited to take advantage of the highest light intensities available.  My visitors told me that the panels were so good now, that they responded to any light and that they could therefore be put anywhere on the roof.  They were also unaware that: the slope was important; as was the proximity of a neighbours end wall to my, only approximately, south facing roof.  The answer is that: yes, you will probably be able to generate a trickle of electricity, sometimes even on a north facing slope, but you will only get a good return on your investment if the panels are sited on a south facing slope where the light intensity and heat is greatest at all times of the year.  For maximum returns it is even worthwhile mounting the panels on a tracking unit – a practice that is becoming common across the rest of the world.  This means that not only will the units tilt, tracking the sun i.e. the brightest part of the sky, from east to south to west; but the sun’s rays will be kept at right-angles to the surface of the panels even in the winter when the sun never rises very high.  If roof mounted, then the panels will only reach their rated production level if they are on a south facing roof that is free of shadows from surrounding roofs or trees; and in this country they should be angled at 45° – 60° from the horizontal.  0° – 20° is only good enough for the tropics where the sun really does pass overhead. 

The same thoughts apply to Solar Thermal, where the idea is to use the heat of the sun, whether behind clouds or directly, to heat your water for household use and space heating.  Always bear in mind that you require most heat and electricity in the winter when the sun’s arc passes roughly from SE through S to SW; therefore, N, E and W slopes will be unable to meet your demands even in the best winter.  A final tip is to site the panels where they can be reached with a brush or soft scrapper to remove snow and also algae and dust.  The rain cleans them fairly well, but like your windows, they will require some attention occasionally if they are to perform at their best.  The more electricity that you can supply to the grid, the faster the return on your investment will be.

Local companies may very well be capable of doing a good job.  However, if you want a high rate of interest on your investment, you will need to tell them exactly what you want and how you want it done!

 Alison Tottenham BSc (Agric), MSc (Environmental Sciences)

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