Solar expansion planning

Having decided to up our solar power on Herbie, there’s a bit of technical/ electrical planning to do, so for the record and in case it helps anyone, I thought I’d set out the considerations here.  Also, people cleverer than me might spot a flaw in my deliberations and put me straight.  Please feel free to correct me if am wrong.

Our current (gettit?) arrangement is one Kyocera 95w panel charging through an Emponi MPPT controller which is OK up to 100w but no more.  I also have a shunt type ammeter in the negative line between the controller and the batteries.  This shows we can make anything up to 6 amps when the sun is strong and in the right place.  More often we are averaging something like 3 amps.  we would like to double that by adding another 100w panel.

So if we have two panels, should we connect them in series or parallel?

I first decided series might be best, so the voltages are additive, and higher voltage in the cables means less resistance losses. Also, daisy chaining up the panels would be simple.  As I understand it, the down side is that if one panel creates more current than the other, there would be equalisation losses.

Now I’m more inclined to connect in parallel,  This way, as long as the panels generate the same or very similar voltage (much more likely), the currents are additive.

So what do I have to check in choosing a new panel?

Voltage is the main thing.  Panels specs should quote  the open circuit voltage Voc, typically about 21.5 volts and a maximum power point voltage Vmpp typically between 17.5 and 18 volts.  For parallel connection a pair of panels need to have these values as similar as possible.  Our current panel has a Vmpp of about 17.6v which I have checked with a voltmeter to be sure it hasn’t deteriorated in this respect.  I’m not sure but I suspect panels deteriorate more in amps than volts.

Type of cell is another issues. Monocrystalline or polycrystalline.  Poly is cheaper, mono is a bit more efficient so panels are a bit smaller for the same output.  They also tend to look a bit nicer. This is where you have to consider space on the roof.  I haven’t made a final decision on this yet, although the difference of up to £50 spread over ten years isn’t a lot. The narrower the panel, the less chance of tangling with Herbie’s centre ropes.

The panel frame is a bit of a consideration, but most of them are ok.  I just need to check the side of the frame is deep enough to take the screws for my tilting stands. 30mm plus is ideal.

Quality.  This is a tough one.  Panels come in a variety of prices for similar specs.  How do you avoid getting inferior quality without paying too much?  My gut feeling is that most panels are OK irrespective of price, but for something I want to work well over many years, I’ll stick to suppliers I think are reputable.  Our last one came from Midsummer Energy, not the cheapest but still reasonable and they are more than just box shifters.  I’ll probably stick with them.  Bits of cable and plugs etc I’ll source from ebay I think where they are a lot cheaper.  Little bits like that can soon add up.

Controller.  I’m going to have to buy an uprated one as the Emponi won’t take 200w.  Choosing MPPT is important, dearer but much more efficient.  Google it if you don’t know why.  Size matters too.  I want to fit the new one in place of the old and space is tight just there. Some of these controllers are at least twice the size of others.  And then there is cost.  Some come with more bels and whistles like remote monitoring etc.  I just want one that is efficient, has the right overload etc protections and from a good manufacturer.  Victron (well known in boat electrics) do a good small 15A MPPT controller for about £80.  That’ll do nicely.

Connections – how best to wire up in parallel.  I am pleased to discover than using the industry standard weatherproof MC4 connections, you can get natty branch connectors to plug two cables into one, which is what you need.  Lots of videos on YouTube show how to do it.

Cables.  Cables get hot and waste power if they are too thin.  They could even catch fire. I checked our existing cables. They are 4mm squared which will be OK to carry the increased current. Phew! I didn't want to have to take the boat ceiling down again for a new run of cable.  I need to check the cables between the controller and the batteries and the fuse too.

So I’ve done my homework.  If you can spot any flaws or omissions I’d be glad to hear about it.  I’m reckoning the panel, plugs, cable, controller, new tilting frames (see previous post) are going to add up to in the region of £300+.