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:
- 15 Amp DC circuit breakers for the 130 W PV panel and charge controller
- 8 Amp DC circuit breakers for the 64 W PV panel and charge controller
- 30 Amp DC circuit breakers for the inverters of both systems
- Outback “Combiner Box” and extra bus bar to house breakers and shunts
- Pentametric system monitor: measuring three currents and two voltages
- Indoor display (LCD) for Pentametric
- Outback MX60 MPPT charge controller (massive overkill for single PV panel)
It is useful to get breakers that are bi-directional (not uni-polar) so that the orientation in the breaker box is not determined by the direction of current—also providing fault tolerance for improper installation of the uni-polar type (there is no wrong way with the bi-directional sort).
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.
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.
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.
- Outback VFX3524 3500 Watt, 24 volt inverter
- Outback “Mate” for indoor display and access to advanced inverter settings
- Outback Hub to link charge controller and inverter with the “Mate”
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 itthat 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.