In order to translate the technical results of efficiency into an easy-to-understand term, which would not alarm the users but give a proactive impression of their system, a new term had to be defined as:

FUPF = n-~nmonth, for nrnonh >0 Equation (2)

Tjmonth

Where, FUPF = Future Utilisation Potential Factor, which gives the ratio of hot water reserved for future use to current hot water consumption, for a specific period.

Птах = maximum attainable efficiency for the specific solar technology under local

operating conditions (found to be 81.7% for these systems under local climatic condition), and

nmonth = mean efficiency for a specific period (a month for this case).

A comparison was made between the FUPF values of Flats 3 and 6, which had complete data sets of at least 1 year each. Figure 5 showed that even though both owners were satisfied with the performance of their systems, it was apparent that Flat No. 6 had virtually no extra hot water storage during most months, while Flat No. 3 had up to almost twice the amount of hot water used, in reserve. This is a direct result of proper management of hot water usage, which was facilitated by the electronic control unit and the interest of the users to follow the daily weather forecast. The FUPF values for summer have little significance, since Flat No. 3 had fully covered the evacuated-tubes, in order to avoid overheating, while Flat No. 6 did not cover the tubes fully. As a result, one could notice that for July, the FUPF for Flat No. 6 was higher than Flat No. 3. This also translated into alarmingly high temperatures in the solar tank, which could be a potential hazard for skin burns and for a reduction in the lifetime of plastic hot water pipes of the apartment..