Longitudinal comparison of outputs from photovoltaic arrays

A longitudinal comparison of the performance of near identical solar arrays in Melbourne, Australia, from 2008 to present.

The concept of a national solar array began as a Sustainability Street programme in inner-suburban Melbourne (Australia). In late 2007 the community decided to make to make a group purchase of grid connected PV systems. This was prompted by the Australian Federal Government's offer of an $8,000 rebate, to anyone who installed a system greater than 1kW.

Installations occurred over 3 weeks in May/June 2008 but there were complications with co-ordinating energy distributors and meter changeovers so that not all systems were fully connected until mid-July 2008. 10 of the group of 13 agreed to collect data from their inverters in order to compare the performance of what were very similar systems:

Similar panels (175W Siemens rebranded as Solar World)
 Similar inverters (Sunnyboy)
 Close proximity – therefore minimising climatic variations
 Same orientation – all solar arrays face due north (except for one on a NW compound angle and later another with a NE orientation)

Six members of the group installed 1kW systems with 6 x 175W panels. One member opted for 7 x 175W panels, two members had 8 x 175W panels, and one had 10 x 175W panels. 2 more households (local with 12 Schott 170W panels and in Brunswick with 8 x BP 165W panels) have joined the group as further points of camparison.

Apart from the number of panels in each system, the main point of difference was the tilt angle of the panels (15 – 42 degrees). There was vigorous discussion about whether it was better to be optimised for summer (shallow angle) or winter (steep angle). Half the systems needed frames; the other half followed the roof pitch angle. A comparison of performance over 12 months would allow the group to see if tilt angle was a significant factor.

There are 20 different points of comparison. The data provides an insight into what actually happens when similar systems are compared. As expected, output was highest in late January and lowest in mid June. However, there is a noticeable departure from the expected bell-shaped day length curve. Real world factors that have impacted on the efficiency of the systems include:

Unexpected shading from trees, a chimney, a solar hot water heater
Bird poo on panels from birds sitting on a TV antenna above panels
 Layer of dust settling on panels blowing in from drought stricken areas
 Inverter burning out
 Extreme not weather
 Persistent smoke haze from bush fires

About the data

This collection of data is the basis for a longitudinal comparison (from July 2008) of the performance of identical - to similar, photovoltaic (solar) arrays in Melbourne, Australia. The initial research question was whether performance of these systems was significantly impacted by the tilt angle of the panels. Was it better to be optimised for summer (shallow angle) or winter (steep angle)? The comparison of performance over the first 12 months showed that tilt angle of the panels was a NOT significant factor.

The study has now been extended indefinitely to track the performance of the systems over time. The data is now serving as a diagnostic tool for identifying underperforming systems and provides real world data for ground-truthing theoretical solar output estimates used to determine Government policy.

At the end of each month, or the first day of each month, the numerical values displayed on the inverters have been recorded for two points of comparison: the monthly kWh output of each system (eTotal) and the monthly number of generating hours sensed by the inverters (hTotal). Data is listed for each system and systems are ranked by the tilt angle of the panels (lowest angel first).

Each system is described by the code (e.g. SP-6-37-N) = Owner’s initials - number of panels - angle of panels - orientation. Raw data has also been normalised to show kWh output per panel and per sqm of panel. Solar radiation tables have been used to plot % efficiency. Various other views of the data have been graphed to display the comparisons. These graphs are embedded in the spreadsheet. The transformation formulae are embedded in the column headers of the Open Office spreadsheet. Data is updated monthly.

Dr Simon Pockley, research leader and co-ordinator

Simon currently works for the Australian National Data Service.

Data and Resources

Additional Info

Field Value
Source http://nationalsolararray.net/
Author Simon Pockley
Maintainer Simon Pockley
Version June 2012
Last Updated October 10, 2013, 22:23 (UTC)
Created July 19, 2012, 21:17 (UTC)
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