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Small Solar system kit help

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Posted by Travelbike on January 22, 2004, 12:01 am
We're helping an elementary school assemble a small solar system kit to get the
kids interested in alternative energy sources.

The plan is for the system to power a desktop computer and printer 8 hours
during the day.  We're new to this and would like to tap the minds of the
newsgroup member.

SOme of the questions are:
1) How many solar panels - (if you have a specific brand/model/output, pls let
us know.
2) Do we need a charge controller to control panel output to battery to prevent
3) Size inverter
4) How many batteries

Since cost is critical to schools right now,  the lower the cost the better.  I
think we can use boat batteries if that would lower cost.  The simpler and
cheaper the system the better.  If you have an idea of what the total cost
would be, please let us know.

AN advance Thank you to contributors.


Posted by ptaylor on January 22, 2004, 1:29 am
Travelbike wrote:

Cool idea! But,well,that doesn't sound very "small".
Assuming a 300W draw (PC,monitor and printer) you'll need atleast 300W
of panels,actually more,to compensate for inefficiencies in the

Well, 4 100W panels should do it,but that alone is probably alot more $$
than the school will dish out.

Erm,you *might* be able to get away without one,but I'd suggest atleast
a simple one..more $$$

atleast 400W,cheap ones can be had for $5! (check out the Coleman line)

A couple of 6V golf cart batteries in series will work nicely. 4 might
be better.

I'd estimate maybe $-800.

Using a flat panel monitor instead of a CRT monitor might be good
also,as most I've seen run from a 12Vdc "power pack".

Now,The trick is to *conserve* from the beginning! ditch the desktop for
a laptop,much lower power consumption! don't let it run overnight,turn
the printer off when it's not in use,etc. Thus you can get away with a
much smaller (cheaper) system.

If you're lucky,the laptop will run directly from the 12V source,maybe
the printer too.That would eliminate the cost,and inefficiency of the
inverter alltogether. But you'll probably still need one,as not all
laptops/printer will run from 12Vdc.A smaller inverter would be fine,but
for about $5,the 450W(?) Coleman is a good deal.

One panel,maybe 75-100W,a single 12V battery,and you're set!

Probably still a couple hundred bucks,But if you'r resourceful,you can
come up with something suitable for much less. (look around at local
surplus shops.)
I got a couple 17AH 12V batteries for $0 each at a local surplus place
for a similar project,sometimes ya might just get lucky and find
something good!

Posted by Solar Guppy on January 22, 2004, 2:06 am
 Well then use a PC that isn't a power PIG ...

mini-itx PC's draw about 11 watt's using a laptop harddrive

Read about them at http://www.mini-itx.com/faq.asp

The solar-guppy servers are mini-itx base , verified maximum draw is 11
watts running disk benchmarks ...

get the

pls let


better.  I

Posted by Roger Gt on January 22, 2004, 3:13 am

small solar system kit to get the

computer and printer 8 hours

like to tap the minds of the


you'll need atleast 300W

inefficiencies in the

specific brand/model/output, pls let

is probably a lot more $$

panel output to battery to prevent

one,but I'd suggest atleast

(check out the Coleman line)

will work nicely. 4 might

the lower the cost the better.  I

lower cost.  The simpler and

idea of what the total cost

monitor might be good


beginning! ditch the desktop for

it run overnight,turn

you can get away with a

from the 12V source,maybe

cost,and inefficiency of the

need one,as not all

inverter would be fine,but

you're set!

you'r resourceful,you can

(look around at local

at a local surplus place

get lucky and find

I'll let you do my shopping!   I just equipped my
Shack with a Solar back up system.  4 each 110Watt
PV Panels, one charge controller, four marine
batteries (220AH).
Total cost, Panels and Charge controller $,642.00
Batteries 4 at $8.00
Cables and connectors, $5.00.
Inverter (400Watt) $1 inc. Tax.
Mounting hardware $4.

Or about $,900.00
I would have loved to get in for $-800

Posted by William P.N. Smith on January 22, 2004, 1:41 am
 travelbike@aol.com (Travelbike) wrote:

You need to define your load better first.  Your best bet is to
actually measure the power consumption of the computer, monitor, and
printer during the course of a busy day (not an average day, you want
a worst case) and determine what your actual load is.  _Everything_
else is determined from this number, so you must get it as close as
possible the first time.  Buy, borrow, or rent a watt-hour meter like
a Watts Up and measure the actual power consumption for a week, then
look for the highest daily power consumption.

Those are all easy to answer once you have your load numbers, though
you'll need to be able to guess at solar insolation and number of
sunless days in a row you might expect in your location.

Not really, golf cart batteries from Costco, Sam's, BJs, Napa, or
equivalent are going to be pretty cheap, unless you happen to have a
couple of boat batteries lying around that you know the state of...

Again, without the load numbers it's impossible to tell.  The rest of
this is just talking off the top of my head:

My computer uses 200 watts when running, my monitor is (lessee) about
75 watts, and we'll throw in another 25 watts for a printer (WAG, I
use a color laser!).  300 watts times 8 hours is 2.4KWHrs per day, or
about 25 cents worth of electricity.

In order to get 2.4KWHRs/day from solar, assuming 5 'peak' hours per
day insolation (a rather good number), you'll need 480 watts of solar
power system (times a couple of correction factors for
efficiency), guess somewhere around 650 watts
of solar panel.  Note that at $/watt installed, that's going to be
over $,000 worth of solar power equipment.  Is this still a feasable
project for you?

Golf cart batteries hold about 1200WH, and you don't want to use more
than about half of their capacity, so for one day of backup you'll
need a bank of four of them, which ought to cost you around $00.

Sure, a lower powered computer could save you considerable amounts of
this  (most laptops will run on a 70W power supply, which gets your
costs down to the $,000 range), so (again) you need to know your load
numbers to a good degree of precision.  In fact, it may be cheaper to
buy a lower-power computer than to build a solar power system around
the computer you have!

Having the weekends off will help your numbers, as you'll have time to
top off or recharge your batteries if there are dark days during the

You'll probably need a charge controller, but those aren't expensive
compared to the panel costs.

Your inverter doesn't need to be huge, 300-500 watts would be more
than enough, and a low-power computer might get away with 200 or so.

Hope this helps, I didn't mean to be negative, but there isn't any
such thing as a low-cost solar power system...

William Smith
ComputerSmiths Consulting, Inc.    www.compusmiths.com

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