We developed an economic feasibility spreadsheet

to or greater than 175 Watts are ideal for a project of this magnitude. Last year alone the church consumed 102,000 kWh of electricity. To even begin to offset a fraction of the electricity used a solar system must be powerful enough to produce enough electricity to be viable. In conjunction with this, a minimum allowed solar cell efficiency was established at 10%. A higher efficiency also allows the solar cells to produce its maximum power output more than a lower efficiency counter-part in the same amount of sunlight. The size limitations arise from the desire to keep the panels as small as possible. Because space is a limited commodity the panels that occupy the space must be kept small to allow for more panels in the limited space. Smaller panels also provide a bonus when discussing shadow effects. Shadows that are cast on a given solar panel render that entire panel obsolete. A small shadow may render a large array useless. Having a larger number of smaller panels decreases the chance that a small shadow will create such detrimental effects as compared to the larger panel. In addition to size, weight was considered to ensure that the panel could be structurally supported by the rooftop without any unforeseen consequences. The weight limit was determined by questioning the building inspector on all possible points on the rooftop.
Model
Power
Price
$/Watt
L (in)
W (in)
H (in)
W/in^2
Weight
Efficiency
Kyocera’s KD200GT
200W
$870
$4.35
56.2
39
1.4
0.09
40.7
15.00%
Sun power Corp.’s SPR-315
315W
$1510
$4.80
61.39
41.18
1.81
0.12
53
19.30%
Sharp’s ND-V230A1
230
$1,150
$5.00
64.6
39.1
1.8
0.09
44.1
14.00%


Table 3: Potential Solar Panels
The table above displays three of the most promising photovolatics for utilization in no particular order.
5.2 Inverter Choice
Unlike the photovoltaic selection choice, inverters had a much smaller pool to choose from. Many inverters were found to either in the midrange, 3000W-7000W output, or in a commercial range, 100000W or above. Because the list of possible choices was so small, all types of inverters were added to the component matrix. This gave us the opportunity to contrast having one large inverter compared to several smaller inverters. Knowing that the maximum energy that could be utilized by filling the Church’s roof entirely with panels was approximately 25000W it became apparent that the much larger inverters were overkill. Inverters such as Satcon’s AE30, was capable of connecting up to 37500W to the grid. This meant that even if we were capable of covering the roof with panels we would only be utilizing 66% of the inverters capacity. On the other hand, smaller inverters such as Sunny Boy’s SB7000US would require three inverters to handle the electrical output of the solar array. While this may seem unfavorable, it actually carries many advantages. First, smaller inverters can be used to precisely match the power output of the photovoltaic array. This means that there is approximately 100% utilization of the inverters compared to only 66% when using Satcon’s AE30. In addition, one AE30 would cost $33,780.00 to handle 25000W of electricity, while three SB7000US would cost only $12,057.00, more than half the price. Another benefit to having smaller inverters is the ability to region the array. This means the array, in its entirety, can be broken into several smaller, independent parts. This method avoids a total shutdown of the array system if a single inverter fails. Finally, a smaller inverter increases maintainability when compared to a large commercial inverter. If a large inverter requires replacement it may take days or even weeks to swap out, not to mention the assistance of a professional. A smaller inverter is generally much simpler
to operate and maintain. The SB7000US is actually capable of being carried to the site and is simple enough to be installed by someone with elementary electrical knowledge. The table below shows a list of possible inverters.
Model
Efficiency
Power (W)
Current (A)
Cost
$/Watt
Sunny Boy’s SB7000US
95.00%
7000
34
$4,019.00
$0.57
Sunny Boy’s SB6000US
95.00%
6000
29
$3,725.00
$0.62
Fronius IG 4500-LV
94.40%
4500
21.6
$2,899.99
$0.64

Table 4: Potential Inverters Once it became apparent that smaller inverters were more beneficial in this situation than their larger, commercial counter-parts, it was only a matter of reducing cost to arrive at the chosen inverter. Above is a table which represents the top three chosen inverters for Wesley United Methodist Church.
6. Economic Context
We developed an economic feasibility spreadsheet in Microsoft Excel as described in the Methodology section to aid us in feasibility calculations for various scenarios. In the Methodology section we explained how the spreadsheet worked. An accurate economic model depends not only on accurate formulas, but on various parameters that can be estimated with a high degree of certainty, backed by historical figures or measurements. This section explains our justification for the parameters that we entered into the spreadsheet. A screenshot of the spreadsheet can be seen below.



Figure 12: A screenshot of the economics spreadsheet. In the first section of the spreadsheet, “System Size and Cost” we are required to determine the cost per Watt of the solar panels and the cost of the inverter and other equipment. This data comes directly from our list of solar panels and our inverters and is explained in depth in the section titled “Possible Solar Panels and Placements”.
The second section, “Installation and Fees”, describes an estimated cost for installation on a per Watt basis, an estimation of electrical inspection costs, and several other fees. The installation cost per Watt is the largest factor in determining system cost, so it must be estimated with high precision. We used the average cost for a solar system, installation and component costs combined, of $8.03 provided by the MTC36 and subtracted out the component cost to leave us with our installation cost alone.
36 Massachusetts Technology Collaborative. Information on installers and costs. Retrieved 1/15, 2009, from http://www.masstech.org/SOLAR/CS%20Installer-Cost-Location%20Data%20for%20Website%20as%20of%2001-31-09.xls
Unfortunately, without getting an actual bid from contractors, this is the most accurate estimate we cost estimate we can get. We also came in correspondence with some installers, who confirmed the rate of $8.00 per watt and gave us a further cost breakdown. A summary of these correspondences can be found in “Appendix X: Correspondence”. Section three titled “System Life and Maintenance” provides an estimation of the system life expectancy, the yearly degradation factor, and the yearly maintenance cost. To use a safe estimate of system life expectancy, we chose twenty-five years, as that is the typical warranty on solar panels. It could be the case that the system continued to work after twenty-five years; however we would rather use a conservative estimate and analyze the feasibility over the next twenty-five years. The yearly degradation factor is the percentage that the electricity production is decreased each year, due to several factors such as: packaging material disintegration, adhesional degradation, interconnect loss of integrity, moisture intrusion, and semiconductor device degradation.
Unfortunately, an effort to collect data regarding photovoltaic degradation has not been well coordinated. There are, however, two studies that look at degradation on single and multicrystalline photovoltaics. The Sandia study, which was on multicrystalline photovoltaics, reported 0.5% degradation per year. The National Renewable Energy Laboratory reported 0.7% degradation per year on a study they did looking at single and multicrystalline photovoltaic. The graph below shows what the overall efficiency of a solar panel would be over the course of twenty five years, assuming it started at 13% efficiency37.