# MorningStar TriStar MPPT 30 or 45 amp charge controller .

pyana1
Solar Expert Posts:

**39**✭✭
please correct if i'm wrong, lol, i know you guys will, nevertheless...

i'm on the verge of purchasing 2 panels tomorrow at (300 watts) totaling 600 watts.

the panel's vmp reads 37.58, so to find the proper amp, to determine which model mppt controller i'd need.

i would simply divide 600W by 37.58 vmp and that'll give me the amps, right? 600/37.58 = 15.97 amps?

i'm on the verge of purchasing 2 panels tomorrow at (300 watts) totaling 600 watts.

the panel's vmp reads 37.58, so to find the proper amp, to determine which model mppt controller i'd need.

i would simply divide 600W by 37.58 vmp and that'll give me the amps, right? 600/37.58 = 15.97 amps?

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## Comments

17,615✭✭It doesn't actually work that way on an MPPT type controller.

Technically the panel's Imp is its Wattage rating divided by its Vmp so 300 Watts / 37.58 = 7.98 Imp.

But when using the MPPT type controller it's more like this:

Watts * derating (usually 0.77) / nominal system Voltage = maximum current out, which is what controllers are rated for.

So 600 Watts * 0.77 / 12 Volts = 38.5 Amps.

Half that for a 24 Volt system, 1/4 for a 48 Volt system.

39✭✭thank you Cariboocoot, so i'm assuming the 30 amp model for a 600 watt pv system wouldn't be too efficient?

and lets just say i decided to go with 500 watts instead, so i'd get : 500 Watts * 0.77 / 12 Volts = 32.1 Amps. i'd definitely be able to use the 30 amp model then right?

the technical specifications for the 30 amp model also states that for 12 Volt set up the Nominal Maximum Solar Input is 400 Watts therefore it wouldn't be a problem if i exceeded the amount by 100; totaling 500 watts instead of the 400 watts listed?

i know this charge controller wouldn't leave too much room for growth in the future, but i'm willing to deal with that for now, as it will give me all the solar energy i'd need for now!

17,615✭✭What happens is the controller will only pass its maximum current rating, so any power potential above that is lost. In general going more than 20% over is a waste of PV.

So for a 30 Amp controller on a 12 Volt system that's about 560 Watts maximum from which you'll only get the 30 Amps. That applies to Voltage throughout the charge range (12-15 generally). Kind of a wide range in Watts: 360 to 450. 500 Watts wouldn't hurt, it's just a bit of a waste of PV money.

Keep in mind that things affect that efficiency ratio. Anything that has bearing on insolation such as elevation or atmospheric clouding, as well as temperature.

39✭✭ok i see.... guess i'll just lean towards the 45 amp model then, do you agree?

17,615✭✭How much battery are you trying to charge?

39✭✭4 Trojan T-105 6-Volt Deep Cycle, 220AH.

i'll simply go with 500 watts pv!

17,615✭✭On a 12 Volt system that would be 440 Amp hours (I think they're actually 225 so that's 450).

To charge that you want to try for 45 Amps peak current, which would need a 45 Amp controller at least and around 700 Watts of PV.

And it leads to the question "why 12 Volt?"; the same batteries could be configured for 225 Amp hours @ 24 Volts which is the same amount of power but could utilize a 30 Amp controller (and the same 700 Watts of PV). Slightly more efficient too.

39✭✭honestly, haven't thought about it that way, the 30 amp controller would definitely be a lot cheaper too

initially i thought a 12 volt system would be ideal, based on all the youtube videos i've seen so far, but i'm definitely willing to switch it up to 24 volts if it'll mean my system would be a lot more efficient.

i'm just trying to power one Haier 7.1 Cu. Ft. Chest Freezer that runs on about 50 watts in Haiti.

17,615✭✭In my opinion you only use a 12 Volt system when there is a specific need for 12 VDC, such as in a mobile application where the vehicle will be 12 Volts as well. This allows you to 'borrow' power between the two systems to keep things going. Otherwise 24 Volts is worth the small amount of extra money to get a more efficient system that is somewhat easier to work with.

Chest freezer in Haiti? I would expect a lot of cycling there! 50 Watts is not a constant thing.

Notice I practice what I preach; my own system is 24 Volts and it runs a full size refrigerator-freezer (as well as everything else) on about the same size battery bank (232 Amp hours @ 24 Volts). I ran it from 700 Watts of PV for five years and only added the additional 520 Watts this year, mainly to start charging earlier in the day. I could get about 2.4 kW hours AC from the system before the panel addition, although I have a slight elevation advantage.

I probably also have a temperature advantage over you: even though it can get to 40C here it is not typical for Summer; we're usually around 25 most of the time.

39✭✭guess i'll be switching up to 24 volts, it makes plenty of sense, plus i'll be able to save a little more , especially in the charge controller department.

solar is new to me, but i love the idea of off grid living. i'll start off with 500 watts then add more with time.

5 years is very impressive, how often did you have to change or upgrade certain components within your system?