how to size a mppt charge controller

[keysbottles]

I have combed the solar threads and cannot find an answer that will penetrate this thick skull of mine.  Is there a chart somewhere that states that for X# of watts you need X# Charge controller?  Each time I think I have it figured out I read something that changes everything.  Help!!  Thanks  Don

 

[B and C]

I have 350 Ah of AGM batteries as my bank. I have 200 watts on the roof running to a 30A MPPT charger (Midnite Kid with Whiz Bang Jr.) 200 watts is all I could fit on the roof. I also have 2-100 watt panels that are portable that I can put out when needed as in when in shade or I need the charging help. I only discharge my batteries about 20% over night so the 200 watts is plenty most of the time. I also installed a Trimetric RV-2030 battery monitor. I would not be without the Trimetric. Really lets me gauge the state my batteries are in. Being your own power company has its' pitfalls/costs.

AFAIK, there isn't a chart. The general consensus is to install as much solar panel as you can afford/fit and get a charge controller that will handle that wattage and voltage combination of the panels.

How many watts of panels are you planning on installing and will that support recharging the batteries from what you used overnight the next day?

 

[keysbottles]

Everything is speculation at this time.  My MH, 2014 Freedom Elite 24' has the battery compartment under a entry step and I'm trying to figure out how to add more batteries w/o major overhaul.  Someone said, concerning solar, more/bigger is better if you have deep pockets.  I don't, so I'm trying to the biggest bang for the buck.

I assume that since charge controllers are sold in different sizes, there must be a relationship to panel watts and SC amps.  For example what is the minimum amps for 400 watts of 12 volt panels? 
This stuff makes me nuts.

 

[B and C]

That is exactly what I have, 400 watts of solar. The two panels on the roof are wired in series and my two portable panels are wired in series. I then parallel them. A 30 Amp MPPT controller will fit well but may be small if you wish to upgrade to more panels later.

 

[jimindenver]

400 watts...30 amp MPPT controller

 

[Surly Biker]

W/V=A

Since most of our systems are 12V, simply divide panel wattage by 12. That’s approximately a minimum current for your controller.

So a 360W panel needs a 30A controller.

 

[Ticklebellly]

There is a big difference between a calculated decision and a decision made on what is realistic. A useful rule of thumb for sizing Panel watts to Ah is 2:1 Rough as guts but positive real life outcomes are reported most of the time. A positive outcome might be measured as "Usually the battery bank does to float about lunch time". The other confusion comes when people talk about theoretical panel outputs and what is real. Panel wattage is determined in laboratory conditions. Peak Sun hours with a clear sky and clean panels and 20 Degrees C will deliver close to the panel rating. Outside of those conditions, the panel output falls off a lot.

Small system, 360 Watts of panel, 150 Ah battery, and 20-25 Amp MPPT seems about right, in my experience.

 

[keysbottles]

I watched Bob's video about Santan Solar and their cheap panels.  Someone have opinions about those?  I've read that solar panels without damage will last a long time.  How would higher voltage effect size of controller?  Would dividing the watts by volts still work?  Sorry for all the questions, I'm trying to understand this and hoping to avoid costly mistakes.  I really appreciate everyone's information.

 

[wayne49]

You'd think that the wattage of solar input would be plainly stated on the product, instead of requiring one to know W=V*A and the correct values to use.
A 100 watt panel puts out 5 amps, but that is at 20 volts. 4x5amps=20amps. But a 20amp controller would be overloaded.

 

[nberna19]

Got this from the ParkedInParadise van conversion blog:

How To Size Your Campervan Charge Controller:
[font=Roboto,][font=Roboto,][size=large]At least 7.5 Amps for every 100W of solar[/font][/font][/size]
[font=Roboto,]This is easier for campervans because we already know our batteries are at 12V. Vandwellers are not usually using above 1000W of solar, so [size=medium][font=Roboto,]use the above for a good rule of thumb[/font]. Going bigger isn’t an issue with this, other than price, and allows you to add more panels later if your setup proves to not be enough.[/font][/size]
[font=Roboto,]Example: if you’re getting a 300W panel setup, you need at least a 22.5A charge controller. No reason to cut it close so get the common 30A charge controller size. This is big enough to add another 100W if you find you’re not getting as much sun as you were expecting.[/font]

 

[Willy]

I'm running 662W of solar, charging 330Ah of AGM's, w. 400W going through 1 MPPT controller and 262W through the other MPPT. I use 2 in case 1 bites the biscuit. It also allows me to use thinner wire (tho I don't) and I've got 3 backup PWM controllers kicking around. ..Willy.