Trying to get a handle on it

dolcesails
dolcesails Registered Users Posts: 11 ✭✭
in Solar Beginners Corner #1
Hi folks,
New to the forum, I been doing some reading here trying to figure out some of this stuff myself, alittle confusing at times, I now realize now I need to ask questions, I have a basic solar system on my sailboat, 2 120 watt 12 volt panels charge regulator and 4..... 6 volt batteries with a air-x wind generator wired for 12 volts runs my fridge no problems.......pretty simple... and a basic understanding of what is going on. Sun charges the batteries to run the 12 volt fridge, lights, pc, radio, and some tv.
Here's are my questions I have 20 of these 120 watt 12 volt panels and I have a small home in the woods, I'd like use these panels to for an off grid application. Having lived on my boat I understand about usage, and at cloudy extended times had to run my diesel to make up some of the power. Now I'm trying to figure out..... do I want to go with 12 or 24 volt system, if 2 of this 120's are wired up for 24 volt does the panel become 240 watts or does it maintain 120 watt @ 24 volts if that is the case I'd have a 1200 watt system @ 24 volts or would I keep the 12 volt value and have 2400 watt system @12 volts and how big a battery bank could they support, I have just learned about the MPPT charge controllers, do they really work that well, and I do understand some about pure sine wave vs modified sine wave, and wanting the better of the two running my fridge and sensitive equipment, so with being said any suggestions on an inverter, and how big? and will all the electrical for the house run thru the inverter. Again I understand a small gen set to run the washer machine, compressor, etc. or any of those heavy duty items when needed. Mostly heating with wood and once in awhile some propane. Am I dreaming...........when I think of a small window ac unit. At the present time I am in Florida trying to put this together and realize going north has a difference in the equation. Any thoughts would be appreciated thanks again......Nick
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Trying to get a handle on it

    The choice of 12 Volt or 24 Volt or 48 Volt nominal system Voltage is largely dependent on how much Wattage you use at any one time. As a rule-of-thumb, a 12V system is good for under 2kW. 24V under 4kW. 48V under 6kW. Above that and you've got multiple systems. It is very important to get a good fix on your load potential before designing the system. Use of a Kill-A-Watt meter for some "real time" numbers is essential. Don't rely on the power specifications provided by manufacturers; they often are higher than reality and sometimes lower. For instance they never mention start-up surges on motors and/or leave out the defrost cycle on refrigerators. Also, guessing at how long you use something for can give you entirely wrong answers about your daily kW hour consumption.

    Your 20 120 Watt panels will give you 2400 Watts (less derating, typically 77%), no matter how they're configured. The difference is that in a higher Voltage array the current will be lower. This is advantageous for a couple of reasons: 1). less Voltage drop over the wires (good for long distance between array and charge controller); 2). fewer parallel panel connections means fewer panel fuses/combiners (more than two panels/strings in parallel and you must fuse each); 3). better charging potential in low-light situations (arguable).

    With that much array Wattage available you could have quite a potent off-grid system!
    Something like this:
    2400 Watt array (configure for 48 Volts nominal, use 60 Amp or better MPPT to step-down to 24 Volt system). Delivers 65 Amps potentially which would charge ...
    650 Amp hours of battery (parallel sets of L16 320 Amp hours - 640, L16 390 Amp hours - 780)
    Which would give you up to 7.5 kW hours to work with daily (quite a lot!)

    Just a rough idea; not a specific recommendation.
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  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    #3
    Re: Trying to get a handle on it
    do I want to go with 12 or 24 volt system, if 2 of this 120's are wired up for 24 volt does the panel become 240 watts or does it maintain 120 watt @ 24 volts
    You are confusing current and wattage. 20 120 watt panels gives you 2400 watts nominal. It doesn't matter how you wire them, except if you go to a 12 volt system you will need two charge controllers, 10 panels on each one. You could probably get by with a single 80 amp charge controller with a 24 volt system and definitely with a 48 volt system a single 60 amp charge controller. Also the wire size gets smaller and cheaper with higher voltage battery banks. I would consider wiring panels in series of 4 and 5 strings into a combiner box. This will give you around 80 volts into your charge controller which most mppt controllers can handle. 2400 watts of panels can peak around 85 amps on a 24 volt system but you would be fine with an 80 amp charge controller.
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