I thought I understood the math behind sizing a charge controller, but now I'm not sure

I'd say at first glance it might be a bit over paneled for a 60 amp MPPT charge controller. Lets' look at the numbers.

The 3 in series and 3 breakers looks right, Don't need that high a amp breaker once combined. but perhaps room for expansion. You'll have less than 30 amps coming in...  It is also under sized for 10 gauge wire. perhaps he had 4 gauge between the combiner and the breaker?

You have 9 panels at 250 watts or 2250 watts of array, at 24 volts you have the potential at charging voltage of 2250 watts ÷ 29 volts (charging voltage for 24 volt bank) 77.6 amps. and a realistic expectation of about 75% of that on normal days or 58.2 amps. So it's a little under what I would like to see, but within the 'cost effective' range.

I would call a foul on a maximum of 60 amps coming into a 740 amp hour battery bank. That is less than 10% this would be okay if it's a stand by/weekend use type situation. It's also okay if you are in a particularly sunny area, like the desert southwest. But undersized in general for daily cycling. Are you sure that's the '20 hour rate' for the batteries?

If you are in the desert southwest, it would make more sense. The voltage drop would be even greater in the heat there making the 60 amp charge controller make more sense. and the long sunny days would give you the extra time to charge making up for the lower than normal charge rate.

ryan112ryan said:

Had a professional installer come out and help me setup a system.  9 panels, 250 watts, VMP 30.1V,  IMP: 8.30A,  VOC  37.2V,  ISC  8.87A.  I don't have immediate access to check how he wired them, but I can't even figure out how any combination would not over drive the charge controller.

The panels come into a combiner box, three breakers (each 15 amp, 150v) and then it combines into a single breaker 80 amp 150v.  That makes me thing they did 3 panels in a series and then paralleled the 3 series via the combiner box.  Does that seem about right?

That goes to a Schneider MPPT 60 amp 150 volt charge controller.  That's connected to a a 740 ah battery bank that's 24 Volt.  Goes to a Schneider SW 4024 Off Grid Inverter. Distance from panels to all the equipment is 30 feet.  Pretty sure it's all PV wire from panels to Charge controller, 10 awg (don't quote me on that). 

Questions:

• Does that seem right, how is it not over driving charge controller?
• What is the correct formula to crunch the numbers for sizing a charge controller?
• Does the combiner box impact things?  Is it like running them in parallel? 
  

That's about right.  At a charge voltage of 30 volts that's 75 amps.  Using the normal 80% of STC number that's 60 amps, so that's OK.

To calculate:
Take total wattage, divide by output voltage.  That's max amps at STC. Now multiply by .8 or so to account for normal performance, and multiply by the efficiency of the MPPT controller.  That gives you max amps.

Combiner box just combines the leads.  You could do the same thing electrically by plugging the panels directly via 2 to 1 MC adapters; the combiner just gives you a place for fuses/breakers and makes wiring easier/safer/legal.

BTW for 740 amp hours you want at LEAST 60 amps charge current, so you are close to the minimum for that bank.

The system was well calculated, the added PV will be useful during non peak periods, the 10% charging versus battery capacity, really depends on overnight loads, assuming off grid, if dipping into capacity is low, then a few days of reduced output can be made up during the next few days of normal output, if all else fails a generator is a helpful addition. This has been my case, there have been more overcast days in the last 3 weeks than the last 3 years combined , climate change perhaps ? Getting to know the system will take some time, a year minimum, but nature has its ways of throwing curve balls, every once in a while, if there is no generator I would highly suggest getting one to allow the CSW 4024 to charge, should the need present itself. The 740Ah battery I'm assuming, again, is lead acid in which case the depth of discharge should be kept to 50% maximum as a general rule, the deeper the discharge the harder the recovery.