Solar for off grid cabin - how to size properly?

[DThames]

The max input voltage of a MPPT charger is often stated as max at a given temperature and somewhat lower if it is hot, like hot weather. So be very cautious to stay away from the max within your operating temperature. What might be good in the winter may be death to your charger in hot weather if the system was unloaded and the charger say the open circuit voltage.

Also, the way most panels are wired (internally) any shadow/shade will take their output to near zero. Put 3 panels in series and put that same shade on one panel and the output for all three drop together. Shading is a big factor that can greatly reduce your output and shading on series panels even more so.

 

[Forester]

That was an interesting exercise, hope it helps

Thanks @ianganderton , yeah definitely it does. I was actually calculating it wrong, as I suspected. Hence ending up with 250/100.
Well, that's what happens when you sleep during your math class ?

So be very cautious to stay away from the max within your operating temperature.

Think that's taken into calculation, at least with that Victron calculator. That are those temp figures found in the specifications.

Anyway, one more MPPT question. Is it, technically speaking, ok to use larger MPPT than actually needed? So that you don't have to get a new one if adding more panels later on. If it is alright, is there any limit, when it becomes too "big" for the system?
Thanks!

 

[Steve_S]

Anyway, one more MPPT question. Is it, technically speaking, ok to use larger MPPT than actually needed? So that you don't have to get a new one if adding more panels later on. If it is alright, is there any limit, when it becomes too "big" for the system?
Thanks!

There is nothing wrong with getting a larger MPPT controller. The controller can only output relative to what is feeding it (panel wise) and how it's charging can be limited by the programming if need be. This would simply allow you to add more panels and UP the output potential of the SCC.

Also depending on what battery system you finally decide on will have an impact. For example, a typical prismatic 280AH LFP Pack can accept up to 280A for Charging where a 100AH pack would take up to 100A. (provided the BMS used to manage the pack can handle it) That is the 1C Charge rate. Now that get's divided between packs if you have more than one pack in a bank. Most residential / storage systems don't charge at 1C rate but more commonly at 0.5C or lower.