Typical hot water usage

From the data collected for Flat Nos. 3 and 6, one could calculate the average volume of hot water used per couple. Figure 7 shows the mean daily hot water usage for every month. It follows that the minimum conservative hot water consumption would be in the region of 20 litres/person/wash (Flat 3), while 30 litres/person/wash would be required for average users (Flat 6). It is noted that only showers were used and no baths. Typical energy savings, payback period and carbon dioxide mitigation

Figure 8 shows the equivalent energy savings from solar hot water usage for Flat Nos. 3 (500 kWhthermal/year) and 6 (1,414 kWhthermal/year). Although, the more careful users (Flat 3) saved less energy but they have saved more water than Flat 6. The important point here is not the amount of energy saved but that users were satisfied with the performance of their systems, without resorting to electric back-up heating.

If one considers the FUPF for each flat of Figure 5 above, and calculate the maximum energy savings that could be obtained for both cases, excluding the summer months, then one would obtain 1,500 and 1,850 kWhthermal/year for Flat Nos. 3 and 6, respectively. Hence, on average each 150-litre water-in­glass evacuated-tube solar system can potentially save 1,650 kWhthermal/annum.

Alternatively, one could have used an electric boiler as is usually the case for Malta, with an efficiency of 90%. Hence, the actual electricity savings would be 1,833 kWhelectric/annum. Therefore, it could easily be calculated that the payback period of these solar systems were around 4 years, based on a net cost — after Government grant — of €1,000 and an average electricity rate of €0.13/kWh.

Moreover, since every kWh saved would have generated 0.87 kg of carbon dioxide at the power station [7] , the potential savings for the ten solar heaters, would amount to 16 tonnes of carbon dioxide per year, excluding the summer months.