Solar System Description

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I thought that as I contemplate my project design that I'd use this blog post to provide a brief description of my solar PV system that I will be monitoring.

System Tradeoffs

As I mentioned in the earlier post, I wanted a system that generates 5 MWHrs per year at the lowest possible cost.  I was willing to sacrifice some performance and features but not quality and reliability.  I had some physical constraints that I had to accommodate - the usable roof area, the roof pitch, roof orientation, and possible shadowing.  My house unfortunately was not designed with any consideration for solar.  I have a kind of chopped up rooftop (multiple planes) that's common in my neighborhood.  I have usable surfaces that face directly south and west.   My roof pitch is 40 degrees, so for Oregon it's probably best to have a south facing installation.  I had space for 18 panels (nominally 3'x5') in a somewhat convoluted configuration to accommodate the roof geometry, a skylight, and vent pipes.

List of tradeoffs

1)  Module type - I chose to use an older generation module in order get lower pricing.  I am using American made Solar World Protect Mono Black 285W panels.  This monocrystalline panel has an efficiency of 17% and has a 30 year linear performance guarantee (maintains 86% of output capacity).  I could have used newer 300W+ panels which have higher energy densities and higher efficiency at a higher cost.  A solar survey indicated that I would need 16 panels to meet my energy requirement using only south facing panels..

2)  Inverter type - I chose to use a string inverter rather than more efficient micro inverters, again for cost reasons.  Using micro inverters requires that you have an inverter per panel.  This gains a lot of efficiency because you have power point tracking per panel and panels are essentially independent so you are more resistant to performance degradation due to shadowing or defective panels.  Another difference with micro inverters is that you have AC power distribution rather than DC from the panel.  I am using an SMA Sunny Boy 4000TL-US inverter.  This is a 4200W grid-tie transformerless string inverter.  It can accommodate 4 panel strings but only has 2 independent MPP (Maximum Power Point) trackers.

3)  Connectivity - The inverter model that I selected included a optional Webconnect module, but it was only available in wired Ethernet although newer models are available with WiFi capability.

4)  Orientation - Because I had 2 MPP trackers available I could have configured a west facing and a south facing string to try to optimize the array performance during seasonal variations.  Since the design simulation indicated that I would have some annualized power margin with only a south facing array, I decided to do that to ease the installation effort and cost.

Final Configuration

As the picture above shows, I installed 18 south facing panels configured in 2 strings of 9 panels each.  That gives me a spec capacity of 5130 W (this is under "optimum" conditions: 1000 W/m^2, 25C).  We used a 4200 W inverter because in reality that power level is only exceeded for short periods of time and the inverter just "clips" during those intervals.  Each string with modules at MPP is around 279V @ 9.2A.

I used my Tello drone to take the picture.