Correlating Photovoltaic Clear-Sky and Actual Output to Sky Cover Measurements

As distributed energy resources are added to building systems, and variable priced electricity tariffs are introduced, facility owners may profit from understanding the expected performance of their building systems. Hourly, forecast energy generation data from photovoltaic sources allows owners to manage energy storage and consumption for their optimal benefit. This paper documents an empirical method which uses publicly–available sky cover observations to estimate the output of a photovoltaic (PV) array. Two functions are developed which together relate PV array output to sky cover observations reported by the National Oceanic and Atmospheric Administration’s National Weather Service (NWS) for a nearby airport. Results are evaluated by comparison of estimated PV array output with actual output. Results show this approach can produce useful estimates of hourly and daily PV system output which may be applied to forecast weather data in order to manage PV array outputs. Although some large deviations are observed between forecast and actual PV output, the method presented uses data that would be available to a building owner with insufficient on-site instrumentation beyond a meter.