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How to decide on a system voltage? | 14 comments (14 topical, editorial)
Re: How to decide on a system voltage? (3.00 / 0) (#2)
by electronbaby (roy<at>windsine.org) on Sat Aug 9th, 2008 at 02:18:09 PM MST
(User Info) http://www.windsine.org

How old is your fridge? Are there newer / more efficient units?

The cheapest part (and most misunderstood) of an RE system is conservation. If you can figure out a way to reduce your power requirements, you will not have to spend an arm and a leg on your PV array.  However, everyones needs and habits are different and there is no "one size fits all" solution. Only you can figure out your power usage. (it sounds like you are well on your way).

It looks like your heading in the right direction (although I didnt go over your math too detailed).

One thing to remember is that with batteries, you have to start out with an inherent 80% - 85% efficiency. (roughly 20% averaged loss from chemical storage)

Solar PV you have to start out with a 75% efficiency from nameplate rating on modules. This is not including your hours of insolation (lat.) or your declination. (a 130 watt module will only put out 97.5 watts max under "most" conditions).

One thing you will see if you look close enough (and I know you want to start this out as a hobby) but you will find it is much more efficient and easier on the wallet to go with a grid tie system if you live in an area that has reliable electricity delivery. You will need to purchase minimal equipment, and you can use all your existing appliances with knowing you will get the quickest return on your investment.

If you need to design battery storage in from the beginning, make sure you design the system so it is scalable and made to be grid tied some day. That way you can have best of both worlds, and most importantly, it serves your needs well (keeping in mind they might change in the future).

Good luck and welcome aboard. :-)
Have Fun!! RoyR KB2UHF



Re: How to decide on a system voltage? (3.00 / 0) (#5)
by RandomJoe on Sat Aug 9th, 2008 at 03:34:29 PM MST
(User Info)

Ah yes, after I posted I was doing a little more figuring and realized I need to throw in the inefficiency numbers.  I'll have to revise my figures a bit there.

The fridge is a pretty good unit, I think.  It's only 5 years old, and I could probably make it even more efficient if I'd get around to filling it up!  I'm single, and there's not much in there...  I do drink a lot of water, so have thought about buying a bunch of water jugs to take up the space and help hold the cold.  (Then I just have to remember to keep them filled!)

I've done quite a few of the usual conservation measures.  CFLs everywhere, the house was already well insulated, programmable tstat to set back the A/C during the day (usually never runs when I'm at work), things like that.  

I'm not terribly keen on grid-tie for a couple of reasons, first is that my utility company doesn't give that great of a deal on it - the very best I could do is reduce my electricity charge portion of the bill to zero (would still have the usual fees), I can't actually make money on them!  Considering my monthly bills average in the $70-75 range, we aren't talking much savings.  There are also no installation subsidies, according to findsolar.com the only thing I could get is the federal rebate.

Also, a major motivation here is backup power - while extended outages are still relatively rare, they are becoming more frequent.  Worse, the two extended outages that have affected me were both mid-winter during ice storms (also getting worse in recent years in my area) which is a miserable time to have no power.  If I put in an RE system, I darn sure want to be able to use it in an outage!  I understand it's possible to have batteries on a grid-tie, but it does seem to be the exception rather than the rule.

Is the derating of the solar panels really that bad?!?  I know I've been seriously unimpressed with the "45 watt" panels I got from Harbor Freight, but just figured that was more due to the usual HF quality than anything else.  (Best I get is usually 25W.)  I remember reading that there is some derating due to the difference between "test conditions" and "real", and there's also the loss due to voltage mismatch - for which I intend to use an MPPT controller.


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How to decide on a system voltage? | 14 comments (14 topical, 0 editorial)

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