With MPPT charge controllers- series is best (in fact some higher voltage ones like the 250v PVmax ones might need upwards of 50v to even start working at all- so the old 'parallel your panels' (in itself a hangover from the PWM days) is a poor choice- series will give you up to 30% or more total daily power generated compared to a low voltage parallel string on MPPT or PWM charge controller....
In most parts of the world- what you want is get the panels Voc rating, multiply it by the number of panels in the series string- and that resulting number should be no more than 80% of the PVmax rating....
Using mine as an example- mine are 150v PVmax rating so 80% of that is 120v max combined Voc...
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My panels have a 38.17v Voc rating...
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Three in series is 114.51v combined Voc- just below that 'safe' 120v limit....
If I had the 250v Victron, then its safe working limit (for most of the world) is 200v (80% of 250v) which means 5 of my panels is the safe working limit in a single series string (5x 38.17v = 190.85v combined Voc- just under that 200v limit...
In colder climates (ie anywhere you get snow on a regular basis and sub zero temps) then allow 30% instead of 20%...
There is a very good reason for this- look at my panels spec label- and you will see that its specs apply ONLY at STC or Standard Test Conditions- the hotter they get above 25C the LESS power (and voltage) they make- and the revers also applies- drop the temps below freezing, and they make MORE power(and voltage)....
If you get an increase in light levels above 1000w/m^2- their voltage and power goes UP, if the temp drops they go up- so a nice white snowy ground reflecting extra light at the panels, with them making more because of the colder temps- can push the voltage up 15%-20% or more- which is why people in those conditions should leave that extra 10% safety margin up to 30% ...
Look at your panels specs sheets and you will usually find something like this...
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For every single degree the panel temp drops below 25C, the power INCREASES by 0.4%, and the Voc voltage increases by 0.27%...
If your temps are down say 15C below zero- your panels will be 40C below their STC ratings- so that 38.17v 250w panel of mine in say Canada will be up from 250w to 290w!!!! (thats with the same light levels even!!!) but more importantly for our charge controllers, the voltage has also risen to 41v each- with three in series it is still safe at 123.6v, but had I 'crowded the limit' and stuck four panels on the controller it would now be over 161v- ten volts over the 'let magic smoke out' limit of 150v....
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Lot more than just 'stick a panel on a battery' as far too many seem to think lol....
ever for a 250v like a Victron 250/60 or similar will net you a considerable increase in total daily generated power- the high voltage MPPT will start earlier and finish later.... thats where that 30% increase comes from over the low voltage MPPT....
