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are needed for the 112-cable system. This also reduces labour requirements. Again assuming eight person-hours of installation time per combiner box, only 40 hours of combiner box installation labour are needed.


The total cost of a parallel system is thus far lower than the cost for a series system. With the same global cost assumptions ($0.30/ft for #10 AWG, $1,000 for a 24-pole combiner box, and $65/hr for electrical installation labour) the total cost of the solution (omitting the cost of the distributed electronics) is only $15,320, or $0.015/watt-peak (Wp).


Thus, we can see that a parallel system design yields a bill of materials cost savings, driven by wire, combiner box, and labour reduction, of $417,320. On a dollar/watt basis, this is a savings of $0.42/Wp (given the reference system size of 1MW).


Conclusion We have shown the economic savings from a parallel system design, primarily driven by longer cable runs, which require fewer combiner boxes and less wire. We will now conclude with a few final comments.


First, note that the cost savings cited above are largely scale-independent. As systems scale, the electrical bill of materials (wire content, combiner


box requirement) will largely scale with the number of PV modules. Therefore, while the above calculations were done on a 1MW reference design, they would apply proportionally to any commercial and utility-scale systems, from 30kW to multiple megawatts3.


The savings outlined also do not require any cost reduction from the inverter. Identical models were specified for each example, and cost savings are comprised entirely of wire and combiner boxes.


Finally, we should keep in mind that a parallel architecture brings a host of other benefits in addition to cost savings. Since each current source in a parallel architecture runs directly into the inverter, it is completely independent from its neighbours.


As a result, the system requires no balancing, and is more robust during failures or other adverse conditions. PV modules can be added or removed, without any modification to the other modules or the inverter. Different PV types can be combined on a single cable run, and fed into a single inverter.


Also, a parallel system such as that enabled by the vBoost will improve inverter performance. This is primarily because the voltage sent to the inverter is carefully controlled at the inverter’s peak efficiency point, which leads to less heat generation in the inverter.


Excluded components from analysis


The design & engineering of a solar plant can be infinitely complex. The paper here has explicitly focused on the electrical system installed cost. We have also focused only on first-order cost drivers. While we could add dozens of other factors to increase the accuracy of the analysis, it would obscure the major point. Here, we want to acknowledge the other second-order effects that we have excluded, and outline the reason behind excluding them.


Field installation labour: The labour to install the vBoost is small. The units are mounted using two screws, and all electrical connections are MC3/MC4 connectors (done by hand). This is also not purely incremental labour: it replaces the field connections of strings, which also use MC3/MC4 connectors. Plus, the added simplicity of the parallel wiring (no strings to plan around) will offset much of the additional hardware mounting costs. These calculations will be the focus


of future white papers, but the current field testing shows the total parallel install time to be on par with a series installation.


Conduit: Because parallel wiring reduces the number of strings and combiner boxes, this also leads to a significant reduction in the conduit required for the system. Conduit requires significant labour, special hardware, and also results in the de-rating of the cabling (requiring the designer to upgrade the cable used). This is a strong benefit of parallel design, but is omitted here for simplicity.


Design time: Designing a parallel system is much simpler than designing a series system. The designer no longer must balance strings; instead they just lay out all of the units, and connect them together until they reach the current limit of the wire. However, since this simplification is difficult to quantify and monetize, we are omitting it from this analysis.


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www.solar-pv-management.com Issue V 2010


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