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Old 09-16-2012, 08:32 PM   #1
Join Date: Jun 2012
Location: Cherry Hill, New Jersey
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Year: 1997
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Solar panel/array sizing?

I was wondering for off-grid folk who put solar panels on their roof, how many panels do they use and how many total Watts do the panels produce at one time?

Is it possible to be totally self-reliant energy wise (never on-grid), all year around? Even when living in harsh winter climates?

I was thinking that I'd want to supplement any solar setup with small vertical-axis wind turbines. maybe have mounts on the rear of the bus that can be on while I'm driving (may not make sense) or when I'm stationary in rainy, stormy, or snowy weather. I figured that the weather conditions that cause a loss in solar energy gain may involve winds of some sort, that I can take advantage of.

I am a bit on the frugal side with energy consumption. At the moment I only really plan to power a laptop, water pump, lights, and a few other gadgets, etc. Maybe at most 100W in a given moment, so I'd say 2.4kW in a day (but probably less, especially given that I'm sleeping 8 of those hours, so probably more like 1.6kW). I probably won't have a fridge for now and just cook food fresh (using a woodstove) from scratch. I have 3 kids so leftovers is usually not something we have to worry about. We may end up using traditional food storage methods, ie canning, drying, etc to see if refrigeration can be avoided altogether.

I'm new to solar, but I'm assuming when you say 'Watt' or 'Ampere', what's really meant is 'Watt-hour' and 'Amp-hour'.

I recently found a set of deep cycle marine batteries rated at 109 amps each for $75 at WalMart. They specifically have Amp-hours listed (separate from cranking amps) so I'm assuming they're genuine deep cycle. I'm starting out with 4 and will buy more as the weeks/month progress until I have at least 8 (ie 850-ish amps worth). At $600 I don't think I can do much better, especially when they're at a place like WalMart being freely findable while on-the-road in case of replacement, etc.

I have the opportunity to make my own panels using approximately (up to) 1000 cells that I bought for $400 on ebay. They are grade B cells, but are guaranteed to have at least 90% purity and produce 1.6W each. I'm imagining it's a lengthy project, but again, I can build as I need them, ie do a few new panels each month. So I figure I have up to 1600 Watts available. Does that mean 1600 more watts per hour of charging (in theoretical perfect sunlight)?

And would that then mean that the 2.4kW I use in a day would be replenished in an hour and a half of good sunlight (in theory)?

I realize that the batteries should never go below 50% so 4 batteries really provides only 200-ish amps, ie 2.4kW. Am I correct in assuming that this means I can go only a day (or so) using batteries alone without them recharging? (at least to keep it at/above the 50% charge minimum). And that 8 batteries would allow me to go 2 days without sun? (And 16 batteries 4 days, etc). Of course I can always be conservative and stretch things, but I'm planning worst-case scenario.

I'm also curious how realistic it is to get that amount of energy - whether cells produce their rating or if it all depends on other factors - ie angle of the sun, angle of the panel, cloudiness/overcast sky, etc. And how well cells produce in less than ideal conditions - ie on a cloudy, rainy or snowy day (if anything). Or how realistic solar energy is in harsh winters (upstate NY). Is it likely that even in snowstorms (2+ ft of snow) and constant overcast conditions that I won't get any (or enough) sun? I'm curious if there will be a point where it's storming for several days (ie a blizzard, etc) if I'm SOL without some sort of backup power, ie wind, on-grid, etc. Maybe there's a general rule of thumb on how many days of power I should plan for assuming I'm living in the northeast in the winter.

Is there's a general rule of thumb on how much current drops based on the angle between the sun and the panel (ie I'd imagine a panel pointing the wrong way, even slightly, would have a current drop). Do I really need to worry about positioning, and what can I really do about that?

I somewhat figure that I have more chance at enlarging my battery bank than getting more panels since once the roof is filled, I can't improve that further. Is the bottleneck usually the panels or the batteries?

I was also curious about mounting panels to maximize energy gain. Are the panels parallel to the roof surface? Each side slightly outward (45 deg)? Both pointing the same direction? Or are they made adjustable through the time of day or season to maximize current input? And do some people park in such a way that the panels face south better?

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Old 09-17-2012, 12:23 AM   #2
Bus Crazy
Join Date: Mar 2011
Location: Oregon/Alabama
Posts: 1,378
Re: Solar panel/array sizing?

i think the only batteries (which i do not have at the moment) is the tall trojan deep cycle batteries which cost around 330 each at the battery wholesaler.. is the only ones to use. For trojan batteries, you can google their website and see what i am talking about. These batteries last a very long time, and you need at least 2 of them, but 4 is way better. They last for 10 yrs or longer. One person i know bought 16 of them and runs his whole house off the grid, ac and everything. it took him a long time to build all his solar panels.
just be sure to check the voltage of each string as you build your solar panels esp when using the B cells. you dont want to put a panel together and have a bad row without checking them first.
i forgot where this was, but somewhere you have the amount of sun for each day for a given area, and time of year, etc.
i think 1200 or so watts is sufficient for most bus applications, and should even run the small ac unit in the heat of the day.
the panel mounting i think is best is having one side hinged so you can prop up the other side. a really trick way of doing it would be to have the panels double hinged so that either side could be propped up (think 1950 international hood or same year buick hood)
i dont think you are going to have long enough runs from your solar panel to the controller to worry much about current drop.
for bus mounting, i assume you are going to use the thick plexiglass to cover your panels...
Jesus Christ... Conversion in progress.
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