Estimations, uncertain as they may be,  suggest that there are about 1.5 million off-grid PV home systems providing lighting, radio and television are currently in operation, totalling 40MWp, and large rural electrification programmes comprising some thousands of SHSs are increasingly becoming a part of the rural market. Thus, SHS represent the most widespread PV application nowadays (when only the total number but not the total installed power is considered), and the trend is likely to continue.
Standardisation of equipment and its mass production represent efficient ways to obtain low prices and high technical quality. In consequence, SHS designers have to assume a single ‘standard’ value of energy consumption for a large number of different families. It must be noted that such standardisation is a requirement imposed by the technology itself in order to reduce cost
and guarantee quality, but it does not correspond well with the idea of needs at the individual level. PV history shows some interesting cases [78, 79]. For example, in reference  it is stated that ‘… an interesting aspect, clearly confirmed by the operating results of ENEL plants, is that the intake power by this type of user… is rarely the same on any given day, and it is linked to the particular lifestyle of the people involved, to periods of absence and to the number of occupants of the houses being supplied, and soon ’. Apart from PV, other types of rural electrification also provide examples. Figure 22.26 shows the distribution of the individual monthly electricity consumption measured during 4 years in the 63 dwellings of Iferd, a Moroccan village where a small diesel generator set provides 3 h of electricity per day (consumers are metered and pay for their energy use). The large observed spread leads one to question the real meaning of reliability parameters such as LLP.
It appears that ‘standard’ LLP values derived from sizing methodologies are scarcely representative of the realities in the field. The relationship between reliability and load, that is, the function LLP = LLP (L) for a given PV system, can be explored just by extending the previously described simulation procedure to a large number of cases. A certain baseline case has been, first, established by fixing the PV array power and the battery capacity values, CA and CS, for a given load, LBASE, and a given reliability, LLP = 0.1. Then, the load has been varied from 0.8LBASE to 1.2Lbase and the corresponding reliability has been calculated. Figure 22.27 shows the result. Roughly speaking, we can say that an approximately logarithmic relationship exists in such a way that LLP decreases one order of magnitude for each 30% of load reduction. This result, together with the observation that real L values are generally found within the range-50% to +100% of the mean, let us conclude that real individual LLP values can vary more than three orders of magnitude (for example, from 10-1 to 10-4) in the context of the same SHS project. This nullifies any attempt at finding a single representative LLP value. It is worth mentioning that the same is not true when centralised electricity generators are considered (PV or not), because the total energy consumed by all the families involved shows a much lower standard deviation than that corresponding to the individual consumptions (roughly, the standard deviation becomes reduced by a factor of N, the number of families), so that it is possible to find single L and LLP representative values for the whole population served.
Consumption in Iferd-distribution function
Figure 22.26 Distribution function of monthly electricity consumption in all the Iferd dwellings
However, even in extremely varying applications, such as SHS, PV sizing methods based on reliability can be of great help if large-scale programmes become a future reality. This will probably require the development of rigorous engineering: standardisation of different levels of service, technical quality controls and so on. For example, PV sizing methods based on LLP represent an interesting possibility of comparing different alternatives (different offers from various manufactures) on an objective basis, as the LLP value respectively associated with each alternative, for the same considered energy service .
It is worth considering the question: ‘How much electricity has to be provided to a rural house in a developing country to be socially and economically acceptable?’. Although this question is always at the origin of any PV rural electrification programme, its answer in terms of watt hour/day, is far from being clear. Energy consumption data, based on practical experience in developing countries, are scarce in the literature , which is paradoxical, considering that many thousands of SHS are currently operating in developing countries. Instead, there are a great number of consumption scenarios where, although starting from very different hypothesis concerning the number of appliances and the length of time they are in use, the SHSs finally selected have an installed power of about 40-50 Wp. This is because past in-field experience has shown PV designers that such systems are generally well accepted by the rural users, while the same is not always the case when small (20-30 Wp) PV modules are concerned. In this way, the SHS scenarios elaborated by PV designers must therefore be interpreted as explanation exercises, rather than as designs for systems starting from an evaluation of actual needs (see Chapter 23 for discussion of rural electrification programmes). So, we must conclude that energy scenarios for rural electrification purposes are still an open question, which need to be explored in depth.