In Which Solar Goes Up Part 1

*****So about an hours worth of typing got deleted by blogger, again. Forgive the quality of this post, but I can't be bothered to do as nice a job as the first go round. Friggin cloud....******
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 After a lot of quote wrangling and some hemming and hawing, I decided to go ahead with getting a grid tied solar array installed. Despite the rather uncertain financial benefit, the solar heat gain and yuppie green-ness kinds made it worth pulling the trigger. Of course, the governor's big green energy push is going to make energy production in the state carbon neutral anyway, so I'm probably not saving any polar bears at the end of the day.

Load Ratio "Overclocking"

The system I settled on consists of 24 Silfab SLA-M 310 panels to convert sunlight into DC electricity, and 24 Enphase M215 microinverters to take the DC voltage from the panels and convert it to AC voltage compatible with utility supplied electricity. The total system is rated for 7.44 kw DC and 5.16 kw AC. The eagle eyed among you will notice the discrepancy between what the panels can theoretically produce in DC (310 w) and what the inverters can output in AC (215 w). The 44% difference between PV output and inverter output is known as the load ratio.
Historically, PV systems were sized with a 30% load ratio in a practice commonly know as "panel over-sizing" or "inverter overclocking." There are two characteristics of a PV system used to justify the use of undersized inverters:
  • photovoltaic cells rapidly drop in efficiency as surface temperature increases, like when they are in the sun
  • inverters, like motors, have a band in which they operate at peak efficiency and from 0-30%output, efficiency grows logarithmicly
These two factors are used by installers to explain why it is better to have the inverters running at peak output for as long in the day as possible, even if you lose some energy during the sunniest part of the day due to "clipping."
The math is supposed to work out that if the inverter reaches the peak efficiency zone earlier in the day, you can harvest more of the total day's sunlight. The claim is that over the lifespan of the system, you would end up making more energy than if you had an oversized or 1:1 sizes inverter; and you could save a buck since smaller inverters are significantly cheaper.
For some reason, when I heard this, I did a double take. Ignoring whether or not clipping loses a significant amount of energy, running the inverters at 100% output for as much of the day as possible seems like a terrible idea. Anyone can tell you that red-lining anything will be less efficient, generate more heat, and ultimately, result in premature death. Even with state of the art solid state MOSFETs and IGBTs, heat is the number one enemy to longevity of any electronics. Say it with me:
HEAT + ELECTRONICS = BAD
In fact, on the same page, an inverter manufacturer explains how running a system at maximum output for longer is better in section 5, then shows that the efficiency for their system reaches a maximum efficiency at 30% and at 100%, the inverter is generating double the heat output it would at 80% output. That seems like a huge problem for component longevity to me. 
Further more, we are talking about a ~10% difference in efficiency between 5% output and maximum efficiency at ~30%. I would rather lose half an hour in the morning than have to replace every module in 3 years due to the MOSFETs letting out the magic smoke.
Typical efficiency curve for an inverter.
I think there are two other factors leading to residential systems with higher and higher load ratios:
  • higher wattage PV panels are becoming cheaper every day
  • inventory and manufacture of undersized inverters remains robust due to their low cost and proven reliability
In commercial installations which use power optimizers and large string inverters, I think the oversized arrays make a lot of sense since the inverter can be actively cooled and stable long-term production has more value than maximum peak output at noon. They  also have the advantage of running HVDC input to the inverter, which boosts efficiency.
For residential, I am not certain I believe what they say about higher load ratios. That said, mine is 44% and, at the very least, I'll be getting as much as possible during the winter.

Array Layout and Sundries

The system will consist of two sub-arrays: 12 panels on the 2nd story roof facing south-east; 12 panels on the garage roof facing south-west. None of the panels will be visible from the street, so no worries about "ugly solar panel" complaints.
The original plan was to have the garage sub-array tie into the new sub-panel I had installed while the main house array would get a supply-side connection since the main breaker panel is so stupid-full.  
 
When the electrician arrived, we both learned that we could use one of the open 40 A quad breakers as a tie into the main panel from the external breaker box. The actual over-current protection sized breaker (20A) would be in the external breaker box. Sounded a little sketchy to both of us, but apparently it's kosher.
Additionally, the current NEC requires big ugly rapid shutoff disconnects for each sub-array, so I'll have a bunch of new electrical boxes on the exterior of the house.
My old PV system used the Envoy-R to talk to the micro-inverters and monitor production, but in this new system, I get a pair of Envoy-S units, one for each sub-array, that are hard wired into the appropriate breaker panels. The new units come with a transformers for measuring output current and, theoretically, consumption current as well if you spring for the extra kit.

Installation

To take advantage of the Washington sales tax exemption, we scheduled the installation for July 1st. Two guys came out at 9:00 and started installing the brackets for the iron ridge mounts. By the end of the day, they had the panels on the second story installed, but their supplier sent the wrong thickness spacers for the racking, so they had to end before finishing the install.
The electrician and installers came back on Wednesday and finished putting the panels up by lunch. The electrical didn't go quite as quick. The garage bit was as easy as I expected it would be, but the compound roof eaves on the house itself made for slow going. There was also another run of knob and tube that had to be cut for access that will need to be patched or replaced.

They'll be back on Monday to finish running the conduit and wiring the main panel connection.



Updates and final pictures after they are finished.








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