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Are my calculations correct
http://solar-guppy.com/forum/viewtopic.php?f=6&t=617		 Page 2 of 2
Author:  tbottger [ Sun Mar 09, 2008 4:21 pm GMT EthGMT ]
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Thanks niel and Bill for some great things to keep in mind.Thanks also for the explaination of MPPT and why it is good.

It crossed my mind early on to use the panels on the roof that I already have to build a hybrid system, but I tossed that out only because of my lack of experience at this point. The real purpose here is to learn by building and if I just use what I already have (even though it would be more cost effective and better all around) I wouldn't probably learn as much as starting small and building out the whole thing. I know that may sound stupid, but the more I learn from this project the more I hopefully be able to contribute to this group in the future :D

As for my small system, thanks to this discussion, I think I now know that I need it to be about 900 W (5 or 6 Evergreen 170's) or so and I need a bigger battery bank. I was a little concerned about just hitting the minimum configs of some of the MPPT converters, but at 900 W I think I am well in range and will probably go to a 24 V battery bank so that I can stay with 1 controller (this is correct right? (900/24=37.5 vs 900/12=75). All of you have given me lots to think about (which was the point of my initial question) and confirmed that at least I have a basic understanding (with lots more to learn). Thanks!
Author:  niel [ Sun Mar 09, 2008 8:52 pm GMT EthGMT ]
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i think i'll go along with you that 900w in pv would be good and more capacity in the battery bank of around 400ah at 12v, but you could go to 24v and 200ah, however i don't see it to be necessary if you input less than 48 amps or output less than 60 amps from the controller at the 12v level. it is modifiable to 70 amps out and 56 amps in for the mx60. the mx60 is ok, but you have my curiousity if you had ruled out the xantrex xw60 and why?
if you don't mind saying why you picked the ah requirements that you did as i would like to know what it is to be powered. in fact, i am confused here as you already have a system in place so why this scenario?
Author:  tbottger [ Mon Mar 10, 2008 7:52 pm GMT EthGMT ]
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I actually haven't ruled out the xw60 and am actually considering it over the mx60 at this point. Early on in the process I was looking at the lower end charge controllers and eventually I started thinking MPPT and remembered seeing the MX60 at some shows I had been to. I hadn't heard much about the xw60, but after doing a little reading here and considering that I have had good luck with Xantrex thus far, I will probably stick with them.

Sorry to confuse you. I should have stated where I got my ah requirements. I was looking to power a regular size fridge 24/7 if the power goes out and the majority of the time I will use the system to power things in my basement (shop lights, radio, small tv etc.) Ultimately it is being built to keep the fridge running when the power goes out though but I wanted it to be a usable system everyday which is why I plan on using it to power the basement stuff. I know that I could buy a generator for a lot cheaper, but then it just sits there and cost me money to maintain and fuel to run so I opted away from that. I also could expand my existing system (which was designed and build by a professional) to include batteries but I don't think this is a job I can handle on my own (at least at this point) and I am in this to learn.

Anyway I metered the fridge and came out with a number of it using about 1306 watt/hours a day or about 108 amp/hours a day. To give myself a little room I rounded that up to 150 amp/hours a day.
Make sense? Probably not huh? :)

If something seems wrong here, please let me know or if I really am going in a stupid direction just tell me. I have yet to invest any money in this project so I will be out nothing if I stop or change directions.
Thanks in advance for your advise.
Author:  niel [ Mon Mar 10, 2008 9:22 pm GMT EthGMT ]
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ok now it's making sense and thanks for tying it together. seeing as you already have a batteryless grid-tie do you intend this to be battery backed grid tied? at least when the batteries are full some of that excess power could be sold.
Author:  tbottger [ Tue Mar 11, 2008 6:47 pm GMT EthGMT ]
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I suppose at at some time in the future I will tie the grid-tie to this system, but that will not be anytime soon.

Thanks again! :)
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