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FAQs 

Electrical

Strings
The cartoon above describes how panels are clipped together in 'series'..which simply means a positive to negative connection of the MC4 plugs on the back of every panel. As each new panel is connected the voltage of the 'string' increases.

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A few years ago the current produced by most solar panels was only 4 or 5 Amps and this allowed us to 'parallel' strings of panels together. If the inverter accepted a maximum input current of 12A, parallel strings, which double the current were easy and any inverter could do it. These days, with the typical panel current more like 10 Amps, it's a very rare inverter indeed, at least for residential installations, that can accept parallel strings. (More on paralel strings further down).

How many solar panels can be connected on a single string?

We have to ensure that the combined voltages of all the panels on the string stay below 600Vdc. The voltage of a panel is highest when it's cold, when the sun is very bright and when the panel is 'open circuit'. There is a formula that makes sure a solar designer never risks going over 600V. It depends on the panel, but at time of writing, using a standard 330W panel, on the coldest recorded dawn temperature in Perth (3.2°C) the maximum possible on a string would be 13 panels, achieving a maximum of 570Vdc. In reality, the calculation used is erring very much on the side of caution, because in order to achieve that 570Vdc voltage, not only do you need a record low temperature, but you also need an intense 1000W of sunlight per square metre. Never going to happen.

Inverters allow two strings, so in theory you could have 26 x 330W panels = 8.58kW

Now if you happen to have a hybrid inverter and connect a battery to it, then the Clean Energy Regulator will allow you to connect that full 8.58kW of panels and pay you the STCs (Government discount) on all of those panels. Whether the inverter specs allow it though very much depends on the manufacturer datasheet.

 

Parallel Strings on different roof orientations.
If you have an equal number of panels on your East roof and your West roof then as the sun moves across (OK, the sun doesn't move, we do) the East facing panels will be producing more current first and then later on in the day, the West facing ones will make more current.

 

If those two strings are in parallel then the current has to be the same across all the panels in the strings, so all day long you end up with the worst result possible. Fronius wrote a fascinating white paper about parallel strings saying that the losses in an East/West parallel scenario was only 1 to 2%. Fascinating only in that whilst it might make sense to a scientist in a laboratory it certainly doesn't in real life. At the time, Fronius didn't have a 2 x MPPT inverter, just a single MPPT range, and it wasn't until 2013 that they released Primo and Symo, dual MPPT inverters.

 

Case in point, we had an installation of a 3kW inverter back in 2012 where we used a Fronius IG 30TL inverter that had a single MPPT. We put 8 panels on East and 8 panels on West with both strings in parallel. We monitored the result over the next year, and after twelve months determined that the output was 10% lower than we expected from that system. We replaced the Fronius inverter with a cheap Chinese (Growatt) inverter that had two MPPTs, and changed the parallel so that one string was on one MPPT and the other string was on the other MPPT. Removed the parallel. Again, we monitored it for 12 months and in the second year the system produced almost exactly what we expected... 9.5% more than the Fronius did in the first year.

 

In other words, same panels, same everything, apart from no parallel, nearly 10% more power. We've done a few parallels since when we had to, and the results always seem to come back to 5 to 10% less than expected, so bear that in mind if your installer recommends parallel strings.

 

 

 

 

 

 


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