My modest solar setup
Original article: http://physics.ucsd.edu/do-the-math/2012/07/my-modest-solar-setup/


Breaker box detail. The placement and orientation of components closely corresponds to that in the diagram above.

More general wiring diagram for single off-grid system. Shunts placed at positions A, B, and C measure net battery current, solar input current, and load current, respectively (should add up). Red represents positive wires, and black (standing in for white) is for neutral, while green is for ground. If PV panels are attached to a dwelling, the ground bus must be separated from the neutral bus with a ground-fault protection device in between.

After the system stabilized and was happily powering my living room, I wrote an article for Physics Today on how to build and set up a small-scale off-grid PV system. It would be something of a waste for this post to rehash that work, so I strongly recommend you look at that article to fill in important gaps that I gloss over here, if you have not already (I’ll wait, in fact). It is there that you will find a more complete description of the roles that the various components play, how to size the system, and many other practical tips. In a sense, this post serves more as a detailed system composition and evolution and an update to the original article.

Growth Phase

As satisfying as it was to watch movies and entertain guests on the modest system, the house was begging for more. Anything that I could plug into an extension cord was fair game. So I took the plunge and bought seven more 130 W panels, upgraded to a 3500 W Outback VFX3524 inverter (24 volt), and also purchased additional communication and indoor display units for the Outback devices (now inverter and charge controller). The 24-volt inverter demanded that I put my two 12-volt batteries in series, so at this point I abandoned my dual system and consolidated into one. The 64 W panel took a break from the sun.

New Items:

The MPPT charge controller allowed complete freedom as to how the panels were configured. So in February 2008, I switched over to the single system, using two 130 W panels in series. After a mere four days, I added a third panel in series (the MPPT makes it that easy). Three months later, I had four panels running in series. At this point, I had an open-circuit voltage (maximum voltage that panels reach when no current is delivered) in the neighborhood of 80 volts. My circuit breakers were rated for 80 V, so I became shy about simply extending series combinations beyond this—even though during proper operation the voltage drop across the breaker is trivially small. The point of a breaker is to offer protection if something shorts out or goes wrong.

After some extensive tests of panels hooked up in parallel under various states of partial shading—for which I built my own IV curve tracer—I concluded that there was no penalty in configuring parallel/series combinations.

So by May 2009, I was up to six panels in two parallel chains of three panels. That’s about as much as I could conveniently accommodate on the carport roof, so I reined in my ambitions for the moment, even though two panels still sat inside waiting to be used.

At this stage, I was powering a refrigerator that averaged 75 W (50 W in winter, 100 W in summer); the entertainment system, and the living room lights. The extension cords were almost entirely concealed, but further expansion would have required unsightly runs.

The more sophisticated inverter can be configured to sense a low battery charge state—at a user-selectable voltage threshold—switching to utility power input to give the batteries a break. It can also use utility input to recharge the batteries, but I consider this to be cheating, and have disabled this service. I want my batteries to be 100% solar, for whatever reason. My inverter does not export energy back to the grid: it’s a one-way utility connection. But that limited utility connection saves the batteries from deep depletion during poor weather periods. And I don’t have to be vigilant about the battery state-of-charge with the ever-watchful smart inverter on duty.

The Next Big Move (to the present)

In late 2009, we stepped back into the housing market after a crash-hiatus. This meant I could configure my house any way I wanted, up to spousal approval. Okay. Eight panels this time. More power. More stuff connected. But extension cords running through the house was a non-starter. So I set about running standard household electrical cable through the house (attic/walls) to dedicated PV power outlets (colored gray) throughout the house in strategic locations. I put a breaker box next to the PV installation so the dedicated PV circuits would have over-current protection. At present, I have five outlets throughout the house running on PV, plus the direct-wired attic fan. The items powered by the PV system change a bit from time to time (no longer run TiVo; changed fridge; changed television). At present, we run:

History of electricity use in our household, showing the rise of the PV system. Many months of the year of late, our modest off-grid PV system provides more of our electricity than does utility. See the phantoms post for more on this plot.

The main configuration change in the new house—besides eight panels arranged as two parallel strings of four—is the requirement for a GFPD in the circuit. Because they sit atop our dwelling, the panel frames must be grounded, and a special breaker set between the ground and neutral buses that will also kill the power to/from the panels if current begins to flow to ground (e.g., if a short at the panel connects the positive terminal/lead to ground, thus potentially creating a fire-starting arc).

I also added a Lantronix UDS1100 terminal server to form an interface between the serial communication spoken by the Pentametric monitor and the ethernet protocol of the internets. As a result, I can query my system externally no matter where I am (once the home’s router is properly configured). I can also automate retrieval of the data logs twice a day (or more if I wanted) to guarantee that I do not lose any data (monitoring unit stores 1.3 days’-worth at 5 minute intervals). This way, I can disappear from internet access for days on end without losing knowledge of what my precious energy system is up to. My wife wants cat-cams to check up on our cats while we’re away. But I already have in place a way to check up on the PV system and on utility usage via TED—prompting my wife to question which I love more: our cats or my energy devices. I’m smart enough to change the subject.

Eight panels on the garage roof, plus our old 64 W friend now tasked with pumping water in my rain catchment system.

What’s Next?

As far as my blogging duties are concerned, I still owe you an efficiency analysis of my PV system. But for me personally, I’m pretty happy with my current PV setup. I have gained valuable experience through the process of setting up the various stages of the system. I have a system that can move with me wherever I go. The PV system has helped me develop a keener awareness of wasteful energy practices. I don’t have to worry about loss of refrigeration during power outages. The door is open to expansion if I need it: I can always throw more panels on the roof or add batteries for greater capacity. I can add circuits to my house to support more devices.

But mainly, having learned first-hand what it means to build, operate, and maintain a PV system has been hugely rewarding. I’m pretty content with the current setup, and have no burning drive to grow further. After all, we can’t expect growth forever. At some point, it’s nice to sit back and enjoy the steady-state.

Warning: Do Not Try this at Home (Apparently)

A few readers have informed me that the 2011 NEC standards on PV installations have taken the DIY out of solar installations. So doing what I did would now be against code, since I am not an authorized installer. Even John Wiles, who wrote much of the NEC code is not authorized to install a system, and another individual who trains installers to take the test is not himself eligible to take the test, and could not today install the 7 kW system that he previously installed at his home. So here I thought I was doing people a favor by providing information on how I did it myself. Turns out you can’t. Bummer.