Charge controllers: Rated by input or output amps???

I'm finding conflicting information on this... I'm looking at using an MPPT controller to use some cheap used 24v panels and feeding a 12v battery bank on an RV. If controllers are rated by input amperage, a 20 amp controller should be fine. If rated by output amperage, I'll need a 40. There must be a definitive answer available from this group... help! Many thanks.
Comments
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
Ok... so if I'm running 2 * 275w panels in parallel that spec at ~9 amps each, not accounting for de-rating (in order to be conservative on controller needs).... I'd have about 31v at 18 amps going in, and would have about 46 amps at 12v coming out... so I should go with a 60 amp MPPT controller?
Thanks again - the help is really appreciated.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
- 2 * 275 Watt panels * 0.77 panel+controller deraring * 1/14.5 volts nominal charging = 29.2 Amps on cool/clear day/noonish
That would be about the best I would expect to see spring/fall clear days.- 2 * 275 Watt panels * 1.0 cold panels * 1/14.5 volts = 37.9 Amps winter peak
Given that you are in/around Montana (?), then in very cold/sub freezing weather, you could see more.It is one of those things... You probably get 20% more power in deep winter, but your actual harvest time is pretty short in winter.
And your maximum current is during cold weather--So that is "better" for running near full power...
-Bill
PWM controllers rated INput amps
MPPT rated by OUTput amps Enjoy
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,