First time solar dummie

System

[/RIGHT]I live full time in a motorhome in the arizona valley, and i want to install a solar array on my motorhome. I only have room for 6 panels on the roof, and 4--12volt battery's in the box. I'am looking to buy 6--275watt 35volt that putout 35.1Vmp with 7.84Imp and 44.7Voc with 8.26Isc, useing a Xanterx xw-mppt-60A-150V charge controller, to charge 4---12volt Concorde PVX--258 OL solar battery's with 24hr rate of 258 amp hours. What would be best, leave them at 12volt or double them up to 24volt By wireing the solar panels in different series/parallel arrays, do i want more volts or amps going in to the charge controller. what array set up, not figuring in power drops of time of day, temperature, wire size, etc. etc. etc. would be best? 6 panels in series , 6 panels in parallel or a combination of series and parallel array. The trouble i have is figuring out the cross referenceing of the electrical specifications of the different solar components to achieve the best match. Some one should wright a book on how to cross reference solar components for dummies, they would make a fortune!!! thank you for any help you can give me.

Cariboocoot

Re: First time solar dummie

Welcome to the forum.

Let's take a look at the numbers first.
Six 275 Watt panels is a 1650 Watt array. That's quite good sized for an off-grid application. Roughly speaking you could expect that to produce 88 Amps of current on a 12 Volt system. That's more than the charge controller will make use of, so you might want to go 24 Volt system here (it has advantages). It should also yield somewhere around 4 kW hours per day on the AC side, which is a good amount for an off-grid application.

As for the batteries, using the 24 hour rate of 258 Amp hours you could have a 12 Volt system @ 1032 Amp hours, which would require a lot of current for charging and big wires, or you could have a 24 Volt system @ 516 Amp hours which is much more manageable. Again I'd go with 24 Volts. The panels should have no trouble recharging that, and in a stretch you could pull nearly 6 kW hours from them. Should be plenty.

Whether to wire in series or parallel is another thing. Usually the standard with an MPPT controller is to go with a nominal array Voltage of 2X system Voltage, so a 12 Volt system would have a 24 Volt array and a 24 Volt system would have a 48 Volt array. There are circumstances where you want to go even higher, namely a long wire run between the array and controller. Since this is on a motorhome that is not going to be an issue.

Shading can be a problem with panels in series, as in on sailboats where there are lots of lines to cross the panels with everything moving all day. In that case parallel wiring is typically preferred. Again, not a situation likely to arise here.

Your panels are already "24 Volt" if they are Vmp 35. You could have them all in parallel, which would require one fuse per panel and wire hefty enough to handle 50+ Amps of current (based on Isc * 6). That would be silly when you could just as easily put three in series and then parallel them up. Then you'd have: Vmp 105, Voc 134 (not a problem unless it gets really cold), and Imp of 15.6 which is much easier to deal with. The Isc becomes 16.5, which is much easier to handle than 50 and requires no fuses.

For precise wire size calculation you need to know the length between the array and controller, but I think you'll find that's going to be less than 20 feet and at 16 Amps max it will not require very hefty wire. 12 AWG at most.

simmtron

Re: First time solar dummie

Remember most motorhomes are 12 volt.. We live in ours full time and use only solar (small amount of generator). If you make a 24 volt system then you have to reduce the voltage to 12 volts for use in the motorhome.

Cariboocoot

Re: First time solar dummie

simmtron brings up a good point: are you trying to run an inverter or is this for 12 VDC equipment?
I touched on some of the problems you might run in to with a 12 Volt system and the panels/controller/batteries mentioned but perhaps we need to go into more detail.

mike95490

Re: First time solar dummie

also in a MHo, your roof will be flat, and you will not get good sun angles, unless you build angle racks, and remember to flat them before driving off...

DavidOH

Re: First time solar dummie

Better check on what you want to power with that system.

Might want to look at 12 volt appliances and keep it all 12 volt.
There are lots of 12 volt TV's DVD players and other things.

For myself I am trying to avoid the 24v - 12v conversion.

simmtron

Re: First time solar dummie

n the motorhome you need 12 volt for all lights, fridge electronics) , propain and carbon monozide sensor, water pump, So if you use 24 volt system evrything electric you use must be converted to 12 volt or 120 volt. You would have to buy 2 inverters one 24 to 120 volt and 1 24 to 12 volt. Can't see any advantage to go with 24 volt system. Any saving on loses from your runs you would lose in converting from 24 to 12 volts for most electrics in the rv.

icarus

Re: First time solar dummie

Just to state the unstated obvious,,,all calculations start with the loading. If you don't know the loads, you can't properly spec a battery bank, with out a properly speced battery, you can't spec a PV regimen etc.

Tony

PS RV pose a whole series of other issues, particularly shading issues, since if you park in full sun, you are likely to need more A/C loads, defeating the purpose of,,, parking in the sun.


t

Cariboocoot

Re: First time solar dummie

simmtron wrote: »

n the motorhome you need 12 volt for all lights, fridge electronics) , propain and carbon monozide sensor, water pump, So if you use 24 volt system evrything electric you use must be converted to 12 volt or 120 volt. You would have to buy 2 inverters one 24 to 120 volt and 1 24 to 12 volt. Can't see any advantage to go with 24 volt system. Any saving on loses from your runs you would lose in converting from 24 to 12 volts for most electrics in the rv.

No, you would not need "two inverters". Inverters convert DC to AC.
It is entirely possible to either run 12 Volt DC loads from 24 Volts with a DC to DC converter or to charge the separate 12 VDC system from the 24 Volt. The question is: do you need 24 Volts to run the inverter that supplies the AC? And that comes back to Tony's remarks about figuring out the loads.

If you don't need 24 Volts, don't go for it. A lot simpler to keep everything 12 Volt. That generally means keeping the total concurrent loads under 2 kW and the daily Watt hours below 2.4 kW (just as a rule-of-thumb).

My previous post is still accurate: he's got more panel than the charge controller will handle at 12 Volts, and more battery than is practical for the same.