Solar PV Cable Size:

It is best practise to use a large cable size when running to solar panels, especially in a 12V system. Smaller cables tend to lose power in conducting the DC voltages over longer cable runs. In this case I am planning to run the same size cabling as I used for the turbine, i.e. 6mm². 

Solar PV Panel Orientation & Location:

Ideally a 45 degree south facing roof is best although there is nothing stopping you from mounting them in your back garden, or to a flat roof, side wall and so on. If you don't have the optimum position, then use whatever position is best, and plan to add more panels over time to compensate. The panels I received were photovoltaic polycrystalline panels, measuring appx 3.8 x 3.8 feet. They are framed in aluminium and have some pre-drilled holes at the back of the panel. There is a weather protected box at one end of the panel. The first thing I did was open up the box and I found 3 connectors, a left, middle and right. A quick chat with the supplier established that I only need to use the left and right connectors; the middle one is not used for off-grid use.

Solar PV Cable Connections:

If you have more than one panel what is best way to cable it all together, because the controller has only one positive/negative input for connecting the panels? Should you run the cables from the controller to the first panel and then from the first to the second, second to third etc, in a daisy chained parallel fashion? Or should you run separate cables to all panels? I decided to run separate cables to each panel and bring them all back to a junction box, where the cables are joined together before going to the controller. There are a number of reasons for doing this, namely:

• running separate cables from each panel back to the controller area gives more flexibility
• separate cables allow me to easily disconnect a single panel should I need to
• separate cables also ensure that each panel junction box only handles the current it is rated for
• daisy chaining is simpler and cheaper but may result in damage to some panel junction boxes due to high current

So using individual cable means I can test the output from each panel in order to prove it was working and more importantly, if for any reason a panel stopped working I could simply remove that particular cable from the junction box, without having to disturb other panels to reroute wires etc. However this method does require large amounts of cable, which adds considerably to the overall cost, especially if the panels are some distance from the controller.

Installing Solar PV Panels On The Roof:

I spent ages thinking about this. I am not a roofer, have no experience with roofs and was concerned enough to bombard the customer care team with questions galore. I found a system on the web called Click-Fit, which is designed to fit solar panels onto your roof without any drilling though tiles etc. But that proved to be quite an expensive method, and was difficult to source here in Ireland. Then I was told that many people use something called "galvanized unistrut", which looks like a long steel housing, and is bolted through the roof to the rafters and then the panels are connected to it. So off I went and bought some. The night before I was planning to put the unistrut on, I had another idea. I was in the garage pottering about and noticed the galvanised wall ties at the top of the interior wall (sometimes call frame ties); these are used to bind the roofing structure to the top of a masonry wall, or to reinforce a wall. They cost appx EUR1.20 each, so I bought a few in my hardware shop, then bent them into a roof hook shape such that a) it would allow me to screw them into the rafters b) it would then protrude below the roof tile once the tile is pulled back down and c) it would be bent up at the tip by appx 2 inches to hold the panel frame. The following pictures illustrate what I did:

Well this seemed to to work, so I want back and bought some more so that I would have enough for all panels, plus a few spare if I made some mistakes. Although it probably took a lot longer to do, I figured that by making my own roof hooks I would also save a lot of money. And most important of all - I did not have to compromise the integrity of the roof. The actual install of each panel was still quite a challenge however, and here is a breakdown of each of the steps I took:

• buy rook hooks (wall/frame ties)
• bend each one into shape, cut away excess
• make up a roof ladder by buying 2 16ft 2x2 inch timber and 2 16ft 4x1 inch timber
   
  then for each panel:-
  
• push up the roof tiles to uncover the rafters
• put three evenly spaced hooks along the rafters, ensuring they are level - these will hold the panels
• screw hooks into place and pull down the roof tiles - leave one exposed for cable
• offer up the panel so that it sits along the bottom hooks
• use the top of the panel as a guide for marking the top 3 rook hook positions
• take the panel down, and screw the top three hooks into place and pull down the roof tiles over them
• offer up the panel again, this time mark all 6 points on the aluminium frame where the hooks meet the panel
• take the panel down again, drill small holes in the frame using a slow speed 3mm steel drill bit. 
• make a small incision in the roofing felt, push the 2 cables though so that about 8-9 feet of cable is available.
• connect the cables to the panel on the ground (that's why I pulled the extra cable length through!)
• offer up the panel for a final time, feeding the excess cable back though the felt as you fix into place.
• pull down last exposed tile. Screw hooks to panels using M5 self tapping screws and spring washers to ensure a tight fit.
• paint tips of hooks with rust preventer.

Ok it all sounds really painful, and to be honest it was tough work. There was a number of unknowns in the process, for example: would the frame ties sit flat enough not to push the roof tile too high, would they be strong enough to hold the panels, would drilling the panel frame be ok, how would I fix them to the hooks, and so on. It was only by experimentation that I found all the answers.

There are no doubt much much simpler ways of doing this, but for me I was happy knowing the integrity of my roof was kept intact, and not having to drill holes in the roof means not having to worry about rain getting in later. Note: although I have concrete tiles, there is no reason why this would not work with slate roofs also, the only thing I would say is that the roof hooks would have to be more precise in terms of angle so as to sit completely flat and not push the slate tile too high.

Two final points merit specific mention: the picture below shows the panels in place - you may wonder why I have spaced them out along the roof, rather than simply place them side by side. I decided that it would be much easier to leave gaps at both sides of each panel, as it makes both the installation and future access much easier. I measured the distance between each one in advance of positioning the hooks to ensure that the roof ladder would fit between them. This may not be an option if you only have a small roof area. Also, having panels laid flat on the roof raised questions in terms of air flow and heat dissipation. I have been assured that these panels do not require specific clearances underneath for air flow etc. In fact, due to my particular roof construction and thick concrete tile, there are gaps at each tile ridge to allow some movement of air. Should you be installing panels yourself, be sure to check with the supplier that they can be laid in this way.   

Linking Solar PV Panels To The Solar Controller:

The wiring from each panel is brought back along the rafters to the renewable store room. Here I originally considered a simple junction box to handle all the solar panel connections, but that proved a very clumsy way to connect and made it very difficult to disconnect panels when I needed to (e.g. when going on holiday). I noticed a number of very expensive solar panel combiner boxes and circuit breakers that could be sourced online; these boxes, costing in the region of EUR200 to EUR450, appeared to be nothing more than a glorified DC version of the standard Consumer Unit which handles your house electricity distribution. 

So I decided to use this principal and do it my own way, by obtaining a simple Mini Circuit Breaker Enclosure box and some cheap 40amp AC Mini Circuit Breakers (MCBs). The enclosure box cost GBP20, and the individual MCBs cost GBP1.50 each. I bought 8 at the time to allow me to expand to a maximum of 8 panels, and they are shown below in a split configuration of 5 and 3. The 6mm² positive and negative cables from the panels are routed into the rear of the enclosure. All that is required is to join the negative cables from the solar panels to one terminal bar, and then connect each positive cable to an individual MCB. The MCBs themselves are mounted on a metal DIN rail which allows them to be snapped on and off as required. The outgoing wire from each MCB is connected to a second terminal bar. Larger gauge 10mm² cables are run from both terminal bars onwards to the solar controller.

The important point about this enclosure is that it is acting as an elegant method of both combining and turning on and off individual solar panels, by flipping the toggle on each MCB. So if I want to shut down all power from the solar side, I toggle all the switches to the "off" position. It is also very easy to check the current coming from each panel by selective switching.

This system is not being used as a circuit breaker to handle high current or short circuits. I have used 40amp AC MCBs in the enclosure, to ensure that they can easily handle the 5-7 amps DC current flowing though from each panel. If I wanted to prevent panels from sending too much power to the controller or using them in true circuit breaker mode, then I would opt for a single correctly rated DC MCB on the outgoing 10mm² cable which would trip if overloaded. These can be bought in the same configuration as AC ones here, but are up to 10 times more expensive than the AC MCBs. Eh, no thanks!