Saving Power at Home & Work

Solar PV Panels

solarpanelSolar panels use the photovoltaic effect to convert sun light into direct electric current. Models that are currently available are quite expensive and do not produce large amounts of power for a given surface area. On average, about 750-1000W of solar power per square meter reaches the earth’s surface, depending on your location. Current solar cell efficiencies are approximately 10-15%. Thus solar panels currently available produce no more than about 150W per square meter of panel.

Typically one or more solar panels are arranged in an array to produce enough power to meet normal requirements. Their low power rating typically makes them uneconomical for applications that are power hungry such as any form of heating.

Because solar panels only supply power when the sun is out, they are normally used to charge batteries to store the energy collected.   When ever power is needed, it is supplied from those batteries, via an inverter to the building appliances (please see the Battery Power page for more information on inverters and batteries used as an alternative source of electrical power).

Solar panels only produce their rated power under ideal conditions. They need to be pointed directly at the sun for maximum power. And even then, they only produce 100% of their maximum power close to noon on a bright sunny day. Earlier in the morning, or later in the evenings, they produce less. Also, some panels perform poorly at higher temperatures, which can be a problem in Africa.

Solar panels usually produce a voltage of about 17.5 V, or even higher (some as high as 110V), which may damage your batteries and/or inverter if not controlled properly. A solar charge controller is thus required to control the voltage supplied by the panel to the batteries/inverter.

Recent advances in solar panel technology, most notably “thin film” technology promise to substantially increase the performance while reducing the cost of solar panels.  These types of panels perform very well in the hotter African climate as well.


Because you are getting power from the sun on a daily basis, the energy produced by the solar panels needs to be sufficient to at least replace your daily energy consumption.  So you need to know what that daily consumption will be.  A good way to calculate your daily energy consumption is to use the PowerProphet electricity consumption modelling tool.  Alternatively you can take measurements, although that is sometimes difficult if you are going to be only providing power to selected appliances and not everything in the house or business.

You should take into account the inverter/system efficiency rating when calculating overall daily consumption (i.e. divide the expected daily energy consumption through by the inverter/system efficiency to get the overall energy that needs to be supplied by the panels each day).

Once you have a good idea what your daily energy required is likely to be, then you can calculate the number of panels needed by simply multiplying the power rating of a single panel by five hours (which is a typical estimate of the amount of time you will get usable power from the sun each day). This will give you the expected energy (in Watt-hours) produced by a single panel each day under good conditions.  If you divide your required daily energy estimate through by the energy produced by a single panel per day, you will get the number of panels required.  This is the minimum number of panels which will replace energy consumed each day.

An example of this would be as follows:

If you estimated that your daily energy consumption will be 5kWh, and your inverter/system efficiency is 90%, then the minimum amount of energy required from the panels each day will 5 / 0.9 = 5.667 kWh

If your panel rating is 200W, then the amount of energy typically produced by that panel each day will be 200 x 5 = 1000 Wh or 1kWh.

Given that panel power performance steadily degrades over time (a typical degradation rating is that the panel should still produce 80% of the rated power in 25 years), then it is probably wise to use that reduced power rating, so in the above example 200 x 5 x 0.8 = 800 Wh.

Then to calculate the number of panels required, you divide the daily energy requirement by the energy produced per panel per day :

Daily energy required = 5.667 kWh = 5667 Wh

Therefore number of panels needed = 5667 / 800 = 6.9

So in this example 7 panels would be the minimum requirement.

But be aware that sometimes manufacturers are optimistic about their solar panel power ratings (especially the cheaper versions).  Some panels also suffer significant power loss at higher temperatures. Lastly, remember that all panels suffer a steady degradation in power performance over time (again the cheaper panels often degrade much faster).  So you may want to add a few more panels to provide a suitable safety margin

Grid Tied Systems

One popular method of gaining benefit from power generated by solar PV panels, is to use a special grid tie inverter which feeds the power generated directly onto your AC network.  These systems can be very useful in situations where you use most of your power during the day, especially if the peak demand is around midday.  They also enable the export of excess power generated out onto the grid.  But unfortunately the regulatory environment in South Africa is still somewhat unclear in this regard, and perhaps even unfriendly towards this practice, although it is done in many cases.

Storing the Sun’s Energy in Batteries

If you are using the solar panels to charge batteries, please consult the battery power section to decide on the number of batteries and inverters you may require in a battery powered system.

Financial Viability

Solar panels have become far more affordable in the last few years, while  Eskom power has become considerably more expensive. Payback times for  PV investments are starting to make sense in certain cases. Especially if one accepts that while solar PV panels may be an expensive upfront investment,  the self sufficiency they offer may justify the capital outlay over time. Happily, provided good quality units are used, they have a very long life expectancy and their maintenance requirements are very low once correctly installed.


Solar panels should be aligned so that they point directly at the sun for best results. Obviously the earth’s rotation means that they need to be pointed to the “average” position of the sun in the sky, if they don’t have the capability of tracking the sun which is usually too costly to consider. This is done essentially by tilting the panel surface upwards from horizontal to at least the same angle as the angle of latitude of its position on earth, whilst facing true north in the Southern Hemisphere.

Equally important is its micro location with respect to horizons and shadows during the day/year. Obviously if it’s stuck behind the chimney, or under a tree, where it only gets direct sun during part of the day, the panel will probably not generate the energy you expect each day. The north face of the roof is obviously a good position.

Because some panels operate at a lowish voltage of 17.5V, the current produced by the panel array will be relatively high for any given power carried by the cables between the array and the regulator/battery. Ensure that appropriately thick cables are used to reduce voltage drop, particularly for long lengths.

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