The Solar Bucket - Results

What is being measured A south facing 4.7 watt amorphous silicon panel is mounted on the end of a pole close to the roof on which a full size installation might be installed.  Both the panel and the roof are in shadow until around 09:00 GMT.  The energy captured by the panel is used to charge a lead acid battery, this is discharged overnight and readings of the stabilised voltage taken at the start and end of most days.  Follow the description link for more information. Whilst the project ran for approximately 18 months, meaningful data was only collected for an 11 month period starting in July 2008 and end ing May 2009.

 Where was it being measured The panel is located in Brighton on the South coast of England.  The latitude is approximately 50.8 degrees north.  The panel is located in a typical urban location.  During the early morning and late afternoon it is in the shadow of surrounding buildings.  Also during winter, some trees to the south slightly mask the panel.  In short, it is a real world location.  The results are specific to the location.  The yield of solar devices is a function of latitude (Sun-Earth geometry) and prevailing weather (cloud and precipitation) and both these factors need to be taken into account when analyzing the data.

Annual Yield The graph below shows the estimated anual yield broken down by month.  No data was available for Jun-09, the value in the graph is the mean of the values for Jul-08 and May-09.The results show both the seasonal variation in the yield, the yield in winter being approximately one third of the summer value and the variation in weather.  August 2008 was a drab month with the sky full of low cloud on most days and this is reflected in the yield.  It should be emphasised that some of the variance is due to different equipment configurations and possible batter degradation.

The estimated total yield over a 12 month period was 3.75 kwh.

Reality Check From discussions people selling roof mounted PV systems in the UK, there seems to be a consensus that 2.5 kw installation would yield around 2,100 kwh/year.  Simply scaling up the results from the Solar Bucket's 4.7 watt panel to a 2.5 kw installation gives an equivalent yield of around 2.0 kw.  Other than noting that the yields are of the same order of magnitude, I would not want to stretch the comparison to far.  The design and operation of the Solar Bucket was an evolutionary process, however, the results to allow some lessons to be learnt and some conclusions drawn.