Solar Power System Energy Yield Analysis
– Comparison of CEC Calculated Yield vs Actual Yield Data
Date: 21st February 2013
SUMMARY
Autonomous Energy has completed a technical study comparing the calculated electricity yield
estimations using the CEC approved methodology for solar power systems with actual yield data from
turnkey solar power systems that Autonomous Energy has installed in the real world. This report details
the methodology and results of this analysis.
The study includes the monthly analysis and comparison of the calculated and actual energy production
of the 10kW solar power system installed at Sydney’s Claremont College in Coogee. It also includes
comparison of the calculated and actual energy production of the 60kW solar power system at Pymble
Ladies’ College, the 20kW solar power system on the Hornsby Council Admin building and the 8kW
solar power system at St Philips Christian College in Newcastle.
The results show that the calculated electricity yields are accurate, though slightly on the conservative
side of the actual energy production and conversely that Autonomous Energy’s installation are
‘outperforming’ the modelled energy yields provided to the client prior to the project proceeding. These
results provide confidence to the financial ‘business case’ calculations that are derived from the
calculated estimation of yields prior to system construction, therefore the results of the financial analysis
will also be conservative.
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METHODOLOGY
Calculated Yield
The calculated yield methodology follows the CEC’s prescribed formula given below.
Figure 1: CEC's Prescribed Annual Yield Formula
Where:

Parray_STC is calculated from the number of modules in the solar array and the rated output of the
module from the manufacturer’s datasheet.

fman, ftemp and ηinv are taken from the module and inverter datasheets.

fdirt is taken to be 95% (i.e. 5% loss is attributed to sunlight being blocked by dirt on the surface
of the modules).

Htilt has been sourced from the Australian Solar Radiation Handbook. The Handbook software
produces average hourly incident energy data for each month of the year package taking tilt
and orientation of the PV modules into account. This is then compiled to give the total yearly
irradiation value for each site.

ηpv_inv and ηinv_sb are taken to be a total of 99% (i.e. 1% loss due to resistance in the cables) as
this is what our engineers aim for when designing our solar power systems.
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The Clean Energy Council (CEC) is Australia’s Government body that regulates renewable energy
technologies and installers across Australia.
The CEC prescribed methodology for calculating energy yields from solar power systems is the only
method that CEC accredited installers should be using to calculate energy yields as directed by the
CEC. This is the method that Autonomous Energy uses to calculate the energy yields for all our present
and future clients. There are many other companies that will use “rule of thumb” methods of yield
calculation which are not acceptable and will often overestimate annual energy yields.
Autonomous Energy calculated the annual yields for the 60kW solar power system at Pymble Ladies’
College, the 20kW solar power system on the Hornsby Council Admin building, the 9.99kW solar power
system at Sydney’s Claremont College in Coogee and the 8kW solar power system at St Philips
Christian College Newcastle using this method.
Actual Yield Data
The actual yield data has been compiled from the 20kW solar power system on 60kW solar power
system at Pymble Ladies’ College, the Hornsby Council Admin building, the 10kW solar power system
at Sydney’s Claremont College in Coogee and the 8kW solar power system at St Philips Christian
College in Newcastle.
The solar data monitoring system installed by Autonomous Energy gathers and logs live energy
production data from the solar power system and transmits this data to remote servers via the internet
where active performance monitoring and reporting is then conducted from Autonomous Energy head
office. The data is also displayed through an online portal for access and use by the client. The data in
this report was extracted from the online portal and compiled for this report.
Data from a number of years per site has been averaged to reduce the effects of year to year weather
variability. The data was also inspected for any abnormalities or missing data before use in this
analysis.
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RESULTS
Using the methodology above, we have compared the calculated and actual energy yields for a number
of solar PV installations.
Claremont College
1600
100
1400
Output (kW)
60
1000
800
40
600
20
400
Percentage Difference (%)
80
1200
Actual Annual Yield (kW)
Calculated Yield (kW)
% difference
0
200
0
-20
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Figure 2: Monthly Yield Comparison - Claremont College
The Monthly Yield Comparison graph above shows how similar the yield profiles are throughout the
year between the calculated model and the actual energy production figures. Over the whole year the
Calculated Yield is 5.9% less than the Actual Yield. Most of the variation between the Actual Yield
compared to the Calculated Yield can be attributed to the conservative nature of the Calculated Yield
formula. Some of the variability can be attributed to weather variations. The data used for the
Calculated Yield is based on 10 year averages. The Actual Yield is based on 2-3 years of data.
A similar analysis has been completed for the other sites. A summary of the results is as follows:
System Size (kW)
Actual Annual Yield (kW)
Calculated Annual
Yield (kW)
Difference in Yield
(Actual - Calculated) (kW)
% Difference
Claremont College
9.99
14968.1
14131.1
+ 836.9
+ 5.9
Hornsby Council Admin
20.21
30994.2
29293.0
+ 1701.2
+ 5.8
SPCC Newcastle
8.14
12604.0
11987.0
+ 617.0
+ 5.1
Pymble Ladies' College
60.18
90590.0
84477.8
+ 6112.2
+ 7.2
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CONCLUSION
The results clearly show that that in all cases assessed, the calculated energy yield of the solar
power systems is close to the actual energy produced by the solar power systems installed on
site. In each of the cases assessed, the calculated energy yield was conservative (by at least
5.1%) in comparison with actual yield. By extension, any financial calculation derived from the
calculated system yields would be a conservative estimate therefore actual saving would be
slightly higher than those presented in the business case. This allows for a high level of
confidence in payback periods and return on investment figures based on Autonomous
Energy’s calculated annual solar yields and therefore decision makers can have confidence in
making investment decisions based on Autonomous Energy’s calculated yield figures and
associated financial metrics provided as part of the ‘business case’ for solar PV projects.
Mark Gadd MEM (UNSW)
Managing Director
Ph: 02 9907 2466
Mob: 0405 368 759
Email: [email protected]
Matthew Linney
BE Photovoltaics and Solar Energy, honours (UNSW), MEngSci (Mechanical Engineering,UNSW)
CEC Accreditation Number: A2141300
Engineering Director
Phone: 02 9907 2466
Mobile: 0402 725 995
Email: [email protected]