Originally Posted by Joee
If anyone has a good resource on sizing solar panels with mppt controllers I could use any info I can get.
John's point about conversion efficiency is a good one and also reason that a higher battery bank voltage (like 24V vs 12V) might be a better choice (stepping 50 volts down to 28 volts is more efficient than stepping it down to 14 volts).
"SCC" = solar charge controller
Other panel array to SCC "sizing" considerations include:
1. SCC minimum/maximum voltage. As John said, you can connect the panels in such a way to meet these requirements.
2. SCC total output capacity. This is related to battery voltage. For example, a Morningstar TS-MPPT-45 can output a maximum of 45 amps. If your battery bank is 12V, this is about 630 watts (charging voltage of 14V). If your battery bank is 24V, this is about 1260 watts (charging voltage of 28V).
3. The real world output of the array does not match the theoretical maximum. Solar insolation is the biggest factor but how the panels are mounted can also be a significant factor. Observing high voltage panels on my last two coaches over the last four years (all in the mountain west - south to north), I get ABOUT 60% of rated output during the winter months and 80% during the summer months. Factor this into your SCC considerations.
4. An MPPT SCC can be 'overdriven' to some extent. This means that a 700 watt array can be connected to a 45 amp (12V battery) without damaging it. Using the theoretical maximum, the array could produce more power than the SCC can output (45 amps) but the SCC will simply clip off the excess. However; this is a poor example as, in the real world, a 700 watt array is unlikely to produce a full 700 watts (#3 above).
Depending on how and where you plan to use your system, #3 can be a biggie. We often think that installing 600 watts of panels will get us 600 watts of power. That is not usually the case.
I don't claim that this is the best approach but I tend to build things with some future growth/expansion in mind - at least when it is possible/affordable. For example, one might conclude that a 30 amp SCC would be sufficient for a 600 watt array to a 12V battery bank. This would be borderline (undersized) and would have zero expansion capability. I would instead go with a 45 or even 60 amp SCC so I could later replace that array with a larger array. Same with the cables, oversize them a bit.