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Yes I know that, and we are talking about winter. I am not so sure you Xmas day argument is valid with a 8 cubic feet fridge, but is irrelevant IMO. With a 1200 watt panel on average in December generates about 3 Kwh, with 2 Kwh usable which allows for decent recovery times and twice what the fridge uses normally. But again is still irrelevant IMO because one piece of equipment every off-gridder should have is a generator to make up and recover when you have a few cloudy days or extreme uses.
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MSEE, PE -
Not to be a pain, but could you show your work on how recovery from 2 days no sun happens with that equipment -- excluding the generator. Likewise, the "on-average" isn't going to be when it matters -- the NREL minimum is 2.2 hours for Evansville, which is a deficit (1.2KW * 2.9Hrs >> 1.2KW * 2.2Hrs ...) and if you don't design in the range of possible behaviors, you've got a client who is happy half the time -- the above average half.
And while I disagree with the OP's choice of 2 8.8 CuFt freezers (at almost 600KWh total, compared to 445KWh for my 25CuFt all-in-one), I'm unclear about how that choice makes it so people go in and out of a fridge more (or less) on holidays than otherwise? It's just what people "do" when they are at home (or a friend's) and enjoying themselves. Even the ISOs know it and put that into their day-ahead forecasts.Last edited by greenHouse; 12-03-2010, 01:06 PM. Reason: Clarify why I was asking for him to show his work.Julie in TexasComment
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nice! yall are awesome
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Hold the bus right there. 2 fridges? That maybe where we parted ways. I based my comments on 1 unit using 800 wh per day.
As for the rest of it yes i stand firm an 1200 watt panel in Southern Indiana is more than enough for 1 device that average 800 wh per day with ample overkill for recovery time for those heavy use days or a few cloudy days.
There are two extremes in a electrical design: Value and Mission Critical. A homeowner fridge is not mission critical unless they are diabetic or need to have refrigerated medicine, but even then there are less expensive measures to deal with those times of extended outages.
On the value side is is risk and for a PE who stamps specs and drawings is exposed to liability so we tend not to go that route.
Logically I shoot for somewhere in between. For consumer or home owner applications I am conservative and lean more toward value design with some reasonably economical overkill. So with that said I tend to go with 5 day battery reserve (2.5 days real time), and 150% more power than average daily use under shortest day or worse case conditions. In addition it is usually less expensive to add a generator rather than more batteries and solar panel wattage to cover the extreme cases where you are likely to have several cloudy days or extreme occasional uses. Smaller systems like 100 watt panel systems do not fall under those conditions.MSEE, PEComment
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Yeah, two of them.
Now do you understand better where I'm coming from? Because you're right about the one, but not about the two.
One is fairly simple -- 0.8KWh * 1.3 (safety factor) / 0.85 (DC discharge derating) = 1.2KWh / Day. 1.2 * 2 = 2.4KWh DC. 1.2KW (panels) * 2.2 (hours) * 0.70 (DC charge derating) = 1.84KWh / day DC. Even at 2.9 hours, that's 2.4KWh DC, which is estimated demand with =zero= left for recovering from no-sun days.
Double checking the derating factors, 0.85 * 0.70 = 0.60 full-cycle efficiency. I find 60% is more accurate for systems that don't include large amounts of float time built into the design.
Back-calculating the correct array size you get 1.2KW * (2.4KWh / 1.84KWh) = 1.6KW. The added cost is (1600 - 1200) * $2.50 / watt = $1,000. That's about two extra panels. Racking is minimal, along with wiring, DC combo box, etc. A bit expensive, but that should produce a system that almost never needs a generator. Not even during a winter holiday in a below average insolation year :becky:
As an aside, my first engineering field was Marine, and "safety factor" is all over the place there. Not including a proper safety factor is not a "value" proposition -- it means the system is guaranteed to fail in below average conditions.
(And not sure why the "Big Grin" icon is called "Becky")Julie in TexasComment
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Julie I am tired of debating. My comments were base on 1 fridge. I do not bother anymore with all the math and showing work. For this simple stuff I do in my head and round off. A fridge that uses 800 wh/day in Indiana only needs a 1000 to 1200 watt solar panel and a 4500 wh battery capacity.
If someone wants to know the math, there are hundreds of threads where I have done it, and even a Sticky which I will gladly direct them. I am one of those people when you are hungry, I will teach you how to fish to make make it on your own, but you gotta do the work.
Anyway I am too tired right now. Spent last night out late in OKC at Daughters graduation from OU med School.
MSEE, PEComment
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@ Sunking - Congratulations to your daughter, your wife and you!
The end of tough years of study for her and the start of some more tough years I believe![SIGPIC][/SIGPIC]Comment
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Thanks Russ. Yeah my Daughter has already had to make some changes in plans with the Healthcare Reform Act she is not happy with.
My wife is also a doctor and she is pretty fed up with the reform too. She has now quit accepting Medicare and Medicaid patients and is now considering just leaving the profession all together.MSEE, PEComment
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Mazel Tov on the daughter's graduation.Julie in TexasComment
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Beautiful daughter Dereck, thanks for sharing the photo and congrats to your family.Comment
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