Help Sizing Small Off Grid System

[inetdog]

Ok cool, thanks for the clarification. Sorry for the competitor's link, wasn't thinking. Good to know about the panels, I just assumed all panels were nominal 12/24/48v etc but now I know better. I was planning on going MTTP anyway though so not too nasty a surprise.

I just found a stupid good deal on a new Xantrax Prosine 1800/24 so I went ahead and got it. It doesn't have a charging feature but I'll figure that out later, I could buy a brand new DC gen for the money I saved. So I'm locked into 24v now but I feel good about that.

One thing about charge controllers is kind of confusing me. My book has a footnote that MPPT feature will only work well if the "input amperage to the controller is lower than the rated output amperage to the battery." I've calculated my Array short circuit amps at 60.9 which I guess is the max "input amperage to the controller" bit. So does that mean that a 60 amp charge controller would be too small? I kind of had my eye on the 60 amp TriStar MPPT. Would that really be undersized? It looks like people routinely use it for larger arrays than mine

I also tracked down one of the teachers from a solar course I took a while ago and he's going to help me out with wiring everything up. The fuses and breakers and battery monitors and wiring is still fuzzy to me but everything is slowly coming into focus.

If your panel output at a reasonable voltage (which is not too much less than Voc) adds up to more than 60Amps, then the extra power the MPPT CC will give you to the batteries (by transforming current A at high voltage to a larger current B at lower voltage) will not be usable since the output current of the CC is limited to 60Amp.
But during part of the times of day/year when your panels are producing less than 60Amps, you will still be getting 60Amps of output to the batteries. Just don't go whole hog and put in a panel system which produces a lot more power than the CC can transfer to the batteries when they are discharged.

People who use it for larger arrays than yours may be trying to get all of the power they can from limited sun hours. Or they may be using a higher voltage battery pack which can take more power from the arrays at the same maximum current (60 Amps.)

[Photowhit]

One thing about charge controllers is kind of confusing me. My book has a footnote that MPPT feature will only work well if the "input amperage to the controller is lower than the rated output amperage to the battery." I've calculated my Array short circuit amps at 60.9 which I guess is the max "input amperage to the controller" bit. So does that mean that a 60 amp charge controller would be too small? I kind of had my eye on the 60 amp TriStar MPPT. Would that really be undersized?

I like the prosine units and I think you'll like them too!

The Charge Controller(CC) should be fine. They work on the amperage and for the most part you need only worry about presenting (inputing) too much voltage, Voltage Open Curuit (VOC) rather than Voltage Maximum Power (VMP). Likely you will need to run the panels in strings of 2 or 3 for the Tri Star MPPT Charge controller (be careful Morning star has similarly named PWM CC's)

Since watts = Amps x Volts and your running a 24 Volt system... your looking at inputing 2 strings at 7.43A at 3x29V=87V or @15Amps max and outputing 215 watts x 6 for an array of 1290 watts / 24V = 53.75 Amps (your batteries may actually drop lower in voltage and your panels can produce more current on cold clear days, but you have a safe margin)

You can check the CC to see what the max input voltage is, this will be the VOC X the number of panels in the string. 35.6V x 3 in this case might be too much.

Other factors before you buy a CC will be how far your CC will be from the batteries, since higher voltages allow lower voltage drops on the same size wires.

[nch18]

Thanks inetdog, that make sense. I think I've got it figured out now.

Photowit, I actually saw that you had the same Prosine after I bought it and was pretty pleased with myself, that's gotta be a good sign.

This is probably a stupid question but what determines how many strings I have. What made you choose 2 strings of 3 instead of using 3 strings of 2 to decrease the voltage somewhat? I get what you're saying in general though and it's made me realize that I was thinking of things wrong in terms of the input side of things. I didn't realize the freedom I had to play around with different voltages coming in.

If anybody has any suggestions for a good CC for me I'm all ears as well. I just picked the morningstar out of a hat more or less. Anything cheaper that would do the job well?

[Photowhit]

This is probably a stupid question but what determines how many strings I have. What made you choose 2 strings of 3 instead of using 3 strings of 2 to decrease the voltage somewhat? I get what you're saying in general though and it's made me realize that I was thinking of things wrong in terms of the input side of things. I didn't realize the freedom I had to play around with different voltages coming in.

Always nice to have 1 or 2 strings, this makes a combiner box(a fuse box for the panels) not required as one set can't overload the other in case of a short. It also cuts down on the wiring needed. In truth though MPPT CC generally(always?) work most efficiently with about double the voltage, but the savings in wire and fusing for a small gain wouldn't be worth it for most.

I recently bought 2 midnite classic 150 lites, likely the best bang for the buck right now if your are computer literate, they have all the features of the regular Midnite classic's accept for arc fault protection (contact switches?) but with out the display. for standard setups you can use dip switches, you can display whats happening or do custom settings with a network link to your computer. At $500 it would also give you room to expand, as it will handle up to 94Amps.

[nch18]

I think I've got it narrowed down to either the 150 or 200 MidNite Classic Lite. I've got my max volts at 126v (OC with all the safety and max temperature factors included). Is the 150 a good bet or should I go with the 200 just to be on the super safe side given that either one puts out more than enough current for me. Thanks for the tip, these look really good, like that they offer a stripped down version and like that they're made in usa. Thanks!

[ywhic]

I went to the Midnite Classic 150 (not lite).. the difference from NAWS is like $120.. and you'll have the graphic display on the front in-case you can't/don't have a computer near by to monitor with the Midnite 'Local App'.

And it looks nice and neat..



Granted at $500 flat (for the Lite) the performance and ability to download firmware and monitor locally with the 'Local App' or via the internet cinches the deal.. it has dip switches to set it all up.. so really no computer is needed.. just the Lite and a Battery Tester.. LoL.. or you can later by the display if you want..

For me I went from a Morningstar TS45 (PWM) to the Classic 150 for the GROWTH possibility and the meter front that showed in/out V/A at the same time.. and the MPPT to try and get more from the panels then the PWM was allowing..

The Morningstar TriStar PWM and TriStar MPPT requires you to dig into the diagnostics menus to get the PV input voltage.. and its doesn't refresh unless you go in/out of that screen.. (so it stinks IMHO)..

[nch18]

That's a nice clean install there, hope mine will look as good. I'm on the computer all the time so I think I'd probably do 99% of my interfacing via comp anyway whether I had the nice display or not. So I think the lite will be fine but I do like that I can add on the display later if my computer addiction wanes in the future.

I guess I'll just go ahead and get the 150, seems to be the popular choice and I guess the extra amps can't hurt, maybe I'll add some strings in the future.