I have made repeated references in past posts to the modest off-grid photovoltaic (PV) system I built to cover a large share of our—again modest—electricity usage. By popular demand, I’ll take you on a tour of the system: it’s history, its composition, and adaptation to my house.
In 2007, I acquired a single, second-hand solar panel—intent on doing something useful with it. Confronted with a variety of options, and eager to explore multiple paths, I purchased a second panel and proceeded to set up a dual system: two stand-alone off-grid PV systems mounted side by side. It was really cool. I was able to power my television console and living room lights off of the two systems, while experimenting with different components and learning to live (part of) my life on natural power. I wrote a comprehensive article about how to size and design such a system, which may be worth reading first. Since that initial success, I have incrementally expanded my system so that I now get more than half of my electrical power from eight panels sitting in the sun. This is their story.
I have enough to say about my solar setup (and PV systems in general) that I must break this topic into multiple posts. In this, the first, I will describe the components, functions, and evolution of the system. In a future post, I will present system performance data and an assessment of efficiency of the various components. Perhaps even later I can explore the impacts of panel orientation, tracking, horizon obstructions, and geographic location.
I was teaching a class at UCSD on Energy and the Environment (for the second time), and was about to get to the part where I described solar power and photovoltaics when I happened upon an Earth Day demonstration on campus. Some guy had a truck decked out in solar panels, and was thumping out youth-magnet music on large speakers. I chatted with him for a bit about practical issues of PV panels and systems, which led to his giving me a sweet deal on a spare panel he had. I figured I owed it to myself and to my students to transcend the theoretical and learn more about the practical side of an apparently important player in our energy future.
To develop an idea of how to put a PV system together, I turned to the Solar Living Sourcebook (12th edition, in my case). This book—a mixture of tutorial and catalog—allowed me puzzle out the components I would need, conveniently interspersed with parts selection and prices. The book also offered an appendix on the National Electrical Code (NEC) suggested practices for photovoltaic installations, compiled by J. Wiles and also available here.
Even then, I was surprised at how difficult it was to find a definitive, comprehensive wiring diagram for an off-grid system. Picking up tidbits from a variety of sources (including helpful diagrams from Outback, and referring to the NEC standards), I was able to cobble together a code-compliant schematic, and proceeded to buy parts and build up my system.
Because the endeavor had a large educational focus for me, I was willing to: purchase a second panel and build two independent systems; try out multiple charge controllers; and spend money on monitoring/data collection. So my expenses are not representative of an installation where saving money on electricity is the primary focus. A small scale (few-panel) off-grid system is bound to come out less favorably than a large grid-tie system from a financial point of view. The latter type can certainly accomplish a financially-motivated quick payback, but my entry was more of the hobby variety.
The two panels I started out using in my dual PV system. At left is a 130 W polycrystalline panel, and at right is a 64 W thin-film flexible panel. Both are mounted on frames allowing tilt adjustments to follow the seasons.
First cut at the electronics for my dual PV system (June 2007); keeping it simple at first. Two golf cart batteries are in the back, and mounted on the plywood are two different charge controllers, two 400 W inverters, a couple of shunts (for current measurement), and fuses here and there. The class-T 110 A fuses for the batteries are hidden on the back of the plywood.
The following list isn’t quite at the level of detail you would need to replicate my initial system without a bit of your own design/thought. But it certainly fleshes out the principal components.
Trojan T-105 batteries (6-volt 225 amp-hours) are more frequently seen in PV systems than are T-1275 units. I opted for the T-1275 because I favored a single 12-volt unit for convenience. Later, in researching battery details for the nation-sized battery post, I learned from a Trojan engineer that the T-1275 and T-105 cells use exactly the same lead plates/grids. So they should have the same cycle performance—just packaged differently and with differing capacity. Incidentally, a battery’s storage capacity in kilowatt-hours can be obtained simply by multiplying voltage and amp-hour capacity (then dividing by 1000). So the T-1275 battery, at 12 V and 150 Ah, comes to 1.8 kWh, for instance.
Assembling the parts into a working system was not terribly difficult. It really comes down to lots of stripping and crimping large wires. Heat shrink (especially the kind that oozes sealant/goop as it melts/shrinks) is useful to protect the crimp joints from corrosion.
At the time, I was renting a condominium, and could not make arbitrary alterations to the place. Because I was simply running extension cords inside, I did not need to mess with the condo’s electrical system—and I found a way to get the extension cords inside without drilling any new holes. I was even able to follow existing holes in kitchen cabinet partitions made for supplying the refrigerator with water. I only needed to drill through one wall, with the landlord’s permission, to get into the living room. The panels sat on the carport, and as such (not being attached to a dwelling), I did not have to provision the system with a ground-fault protection device (GFPD). The rest of the electronics and batteries occupied a space outside in a protected alcove of our patio, safe from rain.
Having such a small system, I had to be vigilant about energy use in the living room. The energy I was using had become very personal. I felt it was my energy in a way that I had not remotely felt before. I paid more attention to the weather, and to the forecast (boring as this tends to be in Southern California). Cloudy periods meant we should ration our television watching. A sunny afternoon when the batteries had reached full charge meant “free” energy that otherwise would go unused. Break out the movie!
The kind of energy awareness that accompanies personal on-site energy production—even if representing a small fraction of total use—turns out to have tremendous leverage. That’s because increased awareness and the resulting behavioral shifts transfer to all sectors. You’ll never look at energy the same way. Energy becomes personal; precious. Once you’ve experienced horror at realizing you’ve left the solar-powered lights on while out of the room (unnecessarily draining batteries and making the system’s job that much harder the next day), you’re unlikely to ever do it again, and that much less likely to perpetrate the same crime on any lights anywhere. Similarly, I have found that energy monitoring (as with a TED system) is another effective way to personalize energy use.
Buoyed by the proof-of concept, I settled into fleshing out the dual system a bit, doing things “right.” I added circuit breakers and a monitoring system.
I also tried several different charge controllers, learning the pros and cons of each type. The Phocos charge controller—despite being very affordable—had no equalization mode, so did not seem like a good long-term solution for keeping the battery happy. At first, I added a low-cost maximum power point tracking (MPPT) charge controller for the TV system, moving the Xantrex to the lighting system. A MPPT unit typically recovers 30% more energy than a simple charge controller by performing a DC-to-DC conversion at high efficiency so that the panel can be operated at its optimal voltage—while the battery is fed a reconfigured voltage compatible with its state of charge. The MPPT was useful and good, although it had trouble in lower-light situations, and also had a maximum power input capability of 250 W. With an eye on expansion, I upgraded to a serious charge controller: the Outback MX60 MPPT (affectionately called “the muppet” in our household), capable of 60 A of output current.
The added components to the (still dual) system were:
PV system after “finalizing” the two-panel system (January 2008; now disassembled). Clockwise from left is the ground connection to pipe; Xantrex charge controller; Outback MPPT charge controller; connection box with breakers, bus bars, and shunts; Pentametric monitoring unit; two 400 W inverters (extension cords lead from these to inside); unused MPPT charge controller; class-T fuse; batteries; class-T fuse.
Wiring diagram for my initial dual system. The shunts are 0.001 milli-ohm resistors that produce 1 mV per Amp of current running through them (connections to Pentametric not shown). It is useful to have polarity-ignorant breakers so all can be oriented the same way in the breaker box independent of current direction.
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.
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.
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.
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.
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.
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.