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Solar electricity
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kathy in illinois    Posted 02-10-2003 at 10:00:47       [Reply]  [No Email]
I have a question for anyone who can help.

If I have 10 fluorescent light fixtures, each of which uses two 40 watt bulbs, how do I calculate what size and how many solar panels and batteries to use to run them. I had found a website that explained in very simple terms how to build your own solar power system, but I can't find it again.

Also, can you remind me what that formula is......watts X amps = volts or amps X volts + watts or what ever...I can never remember that one.

I will be starting my seedlings in the next few months and would like to run my lights on solar power.

Kathy in Illinois

Juiced NRG    Posted 09-08-2005 at 12:45:54       [Reply]  [Send Email]
Solar panels should pay themselves within 18 years and will usually last 50-100 years. is Canadas' premier Alternative Energy and Natural Conservation website as well as the countries least expensive solar supplier.

I hope you decide to try solar! Wind is great as well.

Bill    Posted 02-11-2003 at 09:38:06       [Reply]  [Send Email]
Couple of things you need to consider is that the sun is probably not out when you'll be wanting to use the lights and that the lights actually require more energy (read amps) to start than to operate. There's good and bad to that and it can be solved by use of storage batteries. You would want to either use 12VDC ballasts in the fixtures or 125VDC (this avoids an inverter and its inherent inefficency) and it would probably be better to relamp for 32W as they are more efficient.

The calculations necessary to size the solar panels and the battery plant are fairly extensive but basically you need to take into account the average amount of sunlight recieved on the worst day (like Dec. 22nd), the amount of time the lights will be used per day (watt-hours), any other loads that you will place on the batteries and a contingency factor (probably about 15-20%).

After making a rough calculation and costing you'll probably find that kerosene and LP are much cheaper. You shouldn't need solar panels sized extremely large.

Ludwig    Posted 02-10-2003 at 12:13:33       [Reply]  [No Email]
Well lesse, Salmoneye is right amps x volts = watts which means that watts/amps = volts and watts/volts = amps. /=divide by.

So 10 fixtures at 40wattsx2 each is 800 watts. At 110 volts which is normal household you've got 7.3 amps.
At 12volts which is what a PV system usually works at (24 is also common) thats 67amps. So if you assume you'll use your lights for say 4 hours thats 268 amp hours per day.
Now you only get 4 or 5 hours of usable sunlight per day so worst case scenario lets say 4 hours divide 268 by 4 and get 67amps that you need to generate to keep up with the lights add in a safety factor since you want the array to be able to keep the batteries charged if its cloudy. What I mean is that the batteries will have to be big enough to carry you through a cloudy day or two so the panels need to be able to make that up. So you really should have like 150amps of panels.
Big solar panels in the 12v world make 7amps so you'd need 22 panels, on sale right now thats $12430

Note: We're talking about alot of power here.

Remember that when sizing a battery array only plan on using 50% of each battery's capacity. So in this case if we assume that you need 268ah (amp hours) of capacity then your battery array needs to be large enough that 268ah is 50% of its total capacity. Otherwise you'll be shortening the life of the batteries dramatically this also gives you some excess in case of cloudy days. So your battery array will need to be like 1000ah. Last I checked Trojan T-105 batteries are 220ah @6v. So you'd need 10 @ $75apiece thats $750
That'll actually give you 1100ah which gives you enough excess that even if you had 1 day with NO generation you'd still be okay without damaging anything plus maybe 2 hours into another day of no generation. So since its pretty rare to have a no generation day you'll be pretty safe.

Whew, okay, unless I messed up my math here, or if 8 hours of light is too much then it seems that this is going to cost a BUNCH of money.
Figure maybe $13180. The acutal power cost from the grid, assuming $0.08 per killowatt hour is going to be like $.32 per 4 hour period.

So if you're looking for solar to be cheaper than grid or generator power your barking up the wrong tree...

Yah'but...    Posted 02-10-2003 at 12:58:11       [Reply]  [Send Email]
The system will pay for itself in 112.842 years...



Ludwig - yeahbut    Posted 02-10-2003 at 13:45:33       [Reply]  [No Email]
When I was in highschool one of my friend's license plate was "yeahbut"...

Another thing to think of is that when a yuppie buys an SUV at $25,000 they never think of when it will "Pay for itself" thats just silly right?

Anyway, solar is good if it'd be too expensive to run grid power to your location. Its also handy for things like hunting camps and such in remote areas where you'd only like to run a couple lights and maybe a battery charger.
Thats what I'm considering for our hunting camp, 2 of the trojan batteries I listed before and one or two of the 7amp panels. Be good enough to run a small electric cooler a radio and charge flashlight batteries. The only thing I fear is that somebody will want to have a danged television...

Camp rule #1: NO TELEVISION!

Salmoneye    Posted 02-10-2003 at 13:51:56       [Reply]  [Send Email]
I thought that too...Till I met my Wife...and her camp...

She still says I only married her for the hot shower and the electrical access...

Bob/Ont    Posted 02-10-2003 at 13:51:04       [Reply]  [Send Email]
Tell them to only bring a crankup radio, the one with a mainspring like a clock, run 30 min on a full crank and sound good too.
Later Bob

Ludwig    Posted 02-10-2003 at 14:31:12       [Reply]  [No Email]
I've got a good battery powered solar recharge radio.
Probably we'll never put in any power, we've got gas lights and stove, put in a good woodstove and some insulation (the big problem right now) and we'll be pretty good. Wife wants a shower, so I'm going to build her a platform and get one of those solar showers, if she wants the water hotter she can warm it on the stove....

Bob/Ont    Posted 02-10-2003 at 15:11:29       [Reply]  [Send Email]
We know people that retired to their cottage in Quebec and that's all they had. They added a big propane tank, honda genset and a windmill. They used the cottage that way for years though.
Later Bob

Salmoneye    Posted 02-10-2003 at 10:28:06       [Reply]  [Send Email]
Amps x Volts = Watts

40 watt lamp at 120 volts = .33 amps/bulb...

40 watt lamp at 12 volts = 3.3 amps/bulb...


I think...


Salmoneye    Posted 02-10-2003 at 10:28:06       [Reply]  [Send Email]
Amps x Volts = Watts

40 watt lamp at 120 volts = .33 amps/bulb...

40 watt lamp at 12 volts = 3.3 amps/bulb...


I think...


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