What Guarantee can be given for the Electrical Power Output from a Fixed Rooftop Solar Installation?

Abstract

Commercial and residential rooftops commonly use solar photovoltaic systems to reduce their electricity bills and to supply excess electricity to the grid. However, the fluctuations in irradiation, sometimes drastic, limits them to be regarded not as supplying capacity, but only energy to the electricity grid. A knowledge of these daily and monthly achievable targets will help supply authorities to consider solar energy as adding a partial capacity to the grid. This paper quantises the daily and monthly solar electricity production from fixed solar installations. Analysis is based on actual data from a 4.16 kWp rooftop solar system in Mount Lavinia, Sri Lanka [1]. Results of the present study show that the average energy produced during a given month can be estimated somewhat accurately, but there may be large daily variations even within a given week. Since accurate statistics from a real site are not freely available, no payment is made for capacity provided even for the large solar producers, as supply authorities have to plan generation schedules without a knowledge of the likelihood of such variations. Results of the study show that while 90% of the maximum solar energy production can be guaranteed for any month on only 17% of the days, 70% of the possible energy can be supplied on 60% of the days. The days on which less than 10% of the possible energy can be supplied is less than 2%. These results can be made use of to offer a partial capacity payment to rooftop solar producers. Further, knowledge of month-based statistics would be able to provide better guarantees on availability of solar energy.