need help verifying size of charge controllers

[wolfcreek]

I bought three 255w solar panels with intention to install one on my camper and the other two as a small power outage backup for my house. To simplify my question I'll focuse on my camper application. The camper is a 12v system. The solar panel is rated at 255w stc power / 31.142 vmp /8.300A imp / 38.415V voc / 8.916A isc. If I'm understanding things correctly to calculate the size of controller needed for one of these panels is 255w divided by 12v = 21.25 amps and I believe you're suppose to go up to the next size since it's just over 20 amps? Now the curve - the solar company that sold me the panels are telling me it's a 10 amp controller required for each panel because they are rated for 32 volts. I'm thinking they are not hearing that I'm installing the panel to a 12v system ..... either that or they are telling me I can't use this panel on a 12v system. Thought I had this all figured out until the supplier didn't agree with everything online. I am at the understanding that panels work on 12 / 24 or 48v and you match your controller to system voltage, not the panel voltage rating. So I guess there's two questions here. 1) can this panel be used on a 12v system? 2) if it can, which is the correct size of controller needed?

[ButchDeal]

you will need an MPPT charge controller due to the module voltages.

[SunEagle]

To get the most out of your 255watt panel it would be best to use an MPPT type CC. Then get the proper Amp rating of that CC you are correct you would divide the battery voltage you want to charge into the panel wattage. You then go to the next size amp rating.

So with a 12volt battery and a 255 watt panel you calculate ~ 21.25amps so a 30amp MPPT CC would be acceptable for that application.

Now if you wanted to charge a 24volt battery you would calculate 255w / 24v = 10.625amps so you could get away with a 15amp MPPT CC.

[wolfcreek]

Thank you for the replies. The 30A MPPT CC was what I figured. The supplier insisting no a 10A didn't make sense to me. Glad you could confirm this for me suneagle, thanks

[Mike90250]

Because of losses and generally, quality MPPT controllers can be over-paneled a bit, you likely could get by with just a 20A, but it would be marginal.

[inetdog]

I suspect that the solar company guy was wrongly thinking that the rating of an MPPT CC was based on the input current rather than the output current.
That is, that a 300W panel with a Vmp of 30V would have an Imp of 10A and that would correspond to a 10A controller.
He is missing the fact that with MPPT, unlike PWM, Iout can be much greater than Iin.