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Suppliers tend to claim various performances and efficiencies for their products. When suppliers claim superior performance and make outrageous claims about the potential costs savings of their systems, the credibility of the solar industry is jeopardised.

At SunScan we see no need to inflate the performance figures of our systems. Our calculations are based on facts obtained from our SABS test results as well as radiation figures from NASA's 22 year radiation averages study. Below are details of our efficiency and payback analysis.


Example: 200L Kwikot Retro Fit System

SABS test results : Q-factor = 27.9 MJ or 7.745 Kwh

According to NASA's 22 year radiation tables, the average annual radiation for Cape Town (taking cloudy days into account) is equal to 6.42 KWhr / sqm / day.

Our SunScan system will deliver 11.15 KWhr of energy on an average day in the Western Cape.

11.15 KWhr / day = 4070 KWhr / annum

Current electricity prices (including VAT) for high-use Eskom clients is R1.29 / KWhr

Annual savings = R5,250.00

20 year cummulative savings = R286,385.00


At an average system cost of approximately R16,000 (after Eskom rebate)

Payback period = 3 years

Technical analysis



SABS test submitted solar systems by "plugging" it into a sophisticated performance monitoring station.

Each system is tested with varying amounts of radiation (measured in MJ / sqm) and differing input temperatures of water. The water is then pumped from the geyser via the monitoring station which measures the temperature as well as the volume of water. From this, a measure of the amount of energy that has been stored in the geyser can be obtained. This energy value is given in MJ and must not be confused with the amount of sunlight that the system was exposed to which was measured in MJ / sqm.

Each system is exposed to varying amounts of sunlight (irradiation) on different days (eg: 10, 16, 20, 25 MJ / sqm) from which the results are plotted.

The cold water supply temeprature (Tc) to the geyser is also varied and graphs are plotted for various irradiation amounts as well as different cold water supply temepratures and air temepratures (Ta). A lne is plotted for each (Ta - Tc). For instance: the red line above shows performance of a system fro temperature diff of (Ta - Tc = 10deg) between the air temperature and the cold water supply temperature.


From the above graph, a specific set of conditions can be chosen to be able to compare different supplier's efficiencies.

The values that ESKOM decided on are 16 MJ / sqm irradiation and (Ta - Tc = 10deg), represented y the red line on the above graph. This provides us with the Q-factor for this system and is a measure of the energy delivered by a solar system on an average day of the year.

From the above graph the Q-factor for this system is measured at 27.9 MJ.


SABS uses MJ to measure energy.

Consumers pay for electricity in KWhr.

To convert from MJ to KWhr proceed as follows: 1Kwh = 3.6 MJ

Therefore a Q-factor of 27.9 MJ equates to 27.9 MJ / 3.6 = 7.745 KWhr

Eskom Rebate

Eskom uses the Q-factor in KWhr to calculate the rebate they offer. They offer R925.00 per KWhr with the new rebate. Therefore the rebate on the above system equates to 7.75 X 925 = R7,164.00

Annual Energy Production

NASA provides us with the 22 year average Solar radiation values. This takes into account cloudy / rainy weather. So it is an actual value of the energy that a system is exposed to. The table below is for Cape Town.

Cape Town receives an annual average of 6.42 KWhr / sqm / day. This equates to 6.42 X 3.6 = 23.1 MJ which is higher than the SABS standard day of 16 MJ

To calculate how much energy this system will accummualte, scale up the SABS value of 16 MJ to 23.1 MJ (23.1 / 16 = 1.44)

Multiply the Q-Factor by 1.44

27.8 MJ X 1.44 = 40.13 MJ

40.13 MJ / 3.6 = 11.15 KWhr

Using NASA's 22 year average daily average daily radiation values it can be seen that on an average day, this system will delivere 11.15 KWhr of energy.

Annual Cost Savings

11.15 KWhr X 365 = 4070 KWhr / annum

4070 KWhr @ R1.29 / KWhr = R5,250 savings / annum