4.1 Overnight temperature drop in December

Results showed that although Flat No. 3 — the most conservative case — had the highest average water temperatures, the overnight thermal energy losses from the solar tank were the least, as shown in Figure 6. Again, this is a proof of good management of hot water, since the users opted to partially fill

the tank with water during the winter months, to counter-balance the lower availability of solar energy. This action had also contributed towards lowering the overnight temperature drop, since there was less volume of water in the tank when compared to other flats. From this graph, one also concludes that it is important for the solar tank to reach a considerably high afternoon temperature in winter (say 45 or 50 °C), so that after loosing energy at night, the water temperature would still be warm enough in the morning, without resorting to back-up electric heating.

Fig. 6. Overnight temperature drop and average hot water temperatures in the solar tanks in December.