Solar Water Heating

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Solar Water Heating systems must be correctly sized if they are to deliver tangible benefits.

This document outlines the simple process to assess your hot water requirements and select the correct size collector for your needs, taking into account geyser capacity, power demand, available sunlight and deliverable power.


The Q factor is the name given to the power generated by a solar water heating system in testing by the SABS. Usually issued in units of Mega Joules but can equally be expressed in Kilo Watt Hours.

Q Factor performance of different manufacturers products may vary by as much as 300%. Make sure you know the Q factor of the product you opt for and that it matches the demand of your size of geyser.

With the knowledge above of the power required by geysers of different volumes, it is easy to understand that the Q factor output of a product should be equal to or greater than the energy demand of the geyser to which it is attached.

Since the power output of solar water heating systems is dependant upon a variable input source, the sun, Sky Power systems have been designed to provide the required power output from winter sun conditions to ensure your dependence on electricity for water heating is reduced to a minimum.

Capacity (L)
Temp Rise
 kWh Energy
Sky Power
100404.59SP 4500
150406.89SP 6600
200409.18SP 9000
2504011.48SP 11000
3004013.77SP 13000

Table 1 Lists the energy required to heat the given volumes of water by 40 degrees. The last column lists the recommended Sky Power model for the application.

An effective solar water heating system must be able to provide a high proportion the power required from expected winter sun levels. If they can’t, you will still need electricity to provide the balance on the energy required electrically.

The Graph below attempts to forecast how hot the water will be at the end of the day from solar input. It is based on the measurements of the 200L system and should be typical of all of the Sky Power products is sized to our recommendations.

The insolation data used for the calculations below is for the Johannesburg area and represents the ten year average levels. This data has been collected from the NASA Surface Meteorology and Solar Energy web site.

Graph 1


Look up the size of your geyser and select the Sky Power model that matches that demand.

From the graph above which factors such variables as cold water temperature and the changing insolation levels it can be seen that each Sky Power system will provide hot water all year round. using as little electrical input as possible.

As an example a 200L geyser needs 9.18kWh per day – The SP9000 will deliver sufficient energy all year round. Typically requiring less than 1kWh of energy per day to be provided electrically in winter alone.

That’s less than 30 minutes of electrical top up a day given the average expected level of winter sun.

The Sky Power SP9000 product which consists of a 30 tube array in conjunction with a 200L tank has been given a Q factor of 9.23kWh. (33.227 MJ)

With reference to our comparisons page you will be able to see that this Sky Power product has the highest performance of any system tested to date and represents an efficiency level of 87%.

Sufficient insulation to both the geyser and attached pipe work is required to keep losses within tolerances.

The performance of various manufacturers systems differ wildly. This data is specific to Sky Power EVT products and should not be used to predict the performance of other products. Flat plate products in particular behave much differently due to there low efficiency levels particularly in winter.