6. How It All goes Together

6.1. The Panels

Most Photovoltaic panels are wired to support either 12VDC or 24VDC. A few panels used for PV Direct systems are up to 110VDC. At one time most PV power systems were wired for 12VDC, but there are problems with wire size, and sufficient voltage for heavy loads. It is entirely possible to have the voltage of your PV array be different than your batteries and household power.

In my own case, I wired up 4 panels in series at 12VDC apiece, to get a 48VDC array voltage. This was so I could run the wires from the array to my existing power shed 140 feet away, and not use insanely big wires. Big wires are expensive, hard to work with, and tough to run through conduit.

All the wires from the array go to a PV Combiner box, which has fuses and switches in a more convienient place than on the roof. From the combiner box, the #10 awg wires are all connected using junction blocks, and then it goes to two pairs of #2 awg wires that run through conduit to the power shed.

6.2. The Wiring

The advantages of 12VDC are that many parts are inexpensive, and commonly available. Automotive fuses, cigarette lighter sockets, lights, can all be adapted. The main disadvantage is the voltage drop on long wire runs makes you either keep everything near the batteries, or run larger (and more expensive) wires.

The advantages of 24VDC are that the higher voltage is better for heavy loads like refridgerators, water pumps, and light bulbs all start faster and run better on 24VDC. You can also use commonly available Romex (10 or 12 awg) for most of the wiring, and there is less problems due to voltage drop. The main disadvantage is that it's much harder to find 24VDC ballasts for lights and other components.

You really want to pay attention to correctly sizing the wires, because burning your house down would be a bad thing. Typically for the inverter to battery run, you keep the distance short, like 5 - 10 feet at the most. For this run the wires must be able to handle to full power your inverter puts out. So if you have a 4000 watt inverter, your wires need to handle that. There is special cable made by Trace and others for this (#0 awg, at $5/foot) which being multi-stranded, is easier to work with. Other folks use welding wire even though it isn't specified by the NEC code because it has the same performance at high voltage, and it's much cheaper.

The battery interconnects also need to be rather large, since they also have to handle large voltages. Most battery companies will supply with a set of interconnects if you buy more than 6 batteries. Otherwise you can buy them from a number of soalr energy suppliers, or make your own. Most do it yourself types are made using copper tubing, hammered flat on one end with a hole drilled in it, and the other end swagged, crimped, soldered, or welded to the end.

The panels themselves can usually be wired with standard 10 awg or 12 awg wire between them. The connections into the junction box needs to be water proof, and caulking just doesn't quite cut it... I've seen more than a few systems have problems when water got in and fried something. I prefer using 3 conductor tray cable, which is properly rated for outside use, and being round, watertight connectors are easier to find.

A charge controller is needed between the PV array, and the batteries. This will need to be 12, 24, or 48 volts, depending on your system. Some charge controllers will take 48VDC, and output 12VDC or 24VDC, which is great if you have a long wire run between your array and the batteries.

6.3. The Power Distribution Center

The distribution center is where all the wires come together, and then go out again to the rest of the house. The first component is the main shutoff switch, which is typically 250 amps. This is between the batteries and everything else, and shuts down *everything*.

From there, the 24VDC goes to the house, and to the inverter. It is also possible to add an equalizer to get 12VDC from a 24VDC system. Ideally theer are also cutoffs using 60 amp breakers for the 12 and 24VDC sub systems. It is perfectly fine to have a series of disconnects using 15 amp breakers to give you fine tuning of what can be shut off for maintainance purposes. I personally have a main disconnect on every sub system, and then the 120VAC, 12VDC, and 24VDC subsystems all go to breaker boxes where they get further sub divided for lights, pumps, fridge, fans, etc...

While you can build up your own power center from parts, it's a bit more work, and potentially more expensive than buying a pre-manufactured power center. Both Outback and Trace build power centers, all prewired for their inverters, charge controllers, and other components. My own system, which was built by canabalizing the old system, and splicing in new components, is probably more work than most folsk want to deal with.

6.4. The Battery Bank

Most batteries for off-grid houses are 6VDC, or 2VDC. A few are 12VDC, but this isn't very common. Most of the old telephone company, or old missle silo batteries with a 50 year life span, are 2VDC.

There are several types of batteries, but what it all comes down to is how many years life can you expect. Longer lasting batteries cost more, but may last 15 - 20 years. Cheaper batteries cost less, but then you have to replace them every 5-7 years.

There is also the issue of size. To get the same amount of power, you can have many little batteries, or a few big ones. While the many small ones may be cheaper initially, the wiring can be a nightmare. Usually batter banks with over 16 batteries should be wired with solid bars, rather than wire. So using fewer big batteries, you can reduce the wiring hassle to some thing more manageable.

6.5. An Example Wiring Diagram

This is a wiring diagram based on my own system. The original DIA files, and an updated version of this is available on the GnuAE web site. In this example, 4, 12VDC panels are wired in series to get a 48VDC string of panels. Each string of 4 panels went on a separate rack. All the wires from the panels go to a PV combiner box, which is fused, and that used junction blocks to wire them all into 2 pairs of #2AWG wires that run through conduit to the power shed. From there, the power goes to the charge controllers.

From the batteries, the power goes to the inverter, or fed directly to the house as 24VDC and 12VDC. The generator is also wired so it can use the inverter or the GenMaster to charge the batteries.