DIY Solar Power

I was setting up new equipment in the house and decided to look at the transformers that were included with the equipment. I noticed that most of the items I was connecting were 12 volt DC output. The DOCSIS cable modem, Linksys router, and both of my external USB drives from Seagate. As these items were going to be running all day, for the most part, why not run them from a solar panel?

I will be publishing all the information that I gathered and my results. Also, I will be publishing my errors and mistakes. As someone said to me recently they liked that I instructed “warts and all", as in learn from my mistakes.

Power Requirements:

Device Volts Amperes Watts
Motorola Cable Modem 12 2 24
Linksys E3000 router 12 2 24
Seagate External USB drive (Quantity 2) 12 3 36
Uniden wireless phones 8 1.2 9.6
HP J6480 printer 16/32 .6/1 41.6
Totals 135.2

I wanted to point out that some of these items are not running 24 hours a day. If I calculate my power requirements as if these items were running continuously it would give a worst case scenario and leave a little wiggle room. I will not get into all of the math but it turns out that my cable modem and router each consume 8.64 kilowatt hours per month, or about $2 (USD) each per month.

Okay already it does not sound like a lot of money but start adding it up! I did various calculations including the desktop, laptop, another printer, etc.… and the normal duty cycles (how many hours everything runs).

It turns out that if I convert my office to solar power only I can save about 12% ($110) of my energy costs annually.

OK, I still sound like a tight wad (OOOHHHH, I can save $100 dollars a year!). What I forgot to add in was something called the ‘derate factor’ !

Derate factor is the power lost converting from Direct Current to Alternating Current and vice versa. The normal derate factor is 23%, so your utility loses roughly 23% of the power generated and transmitted to your house.

Ok, I still sound like a tight wad! Now I am up to $137 a year! Hmmm that is about 25% of my Dunkin Donuts spending per year.

Here is the first major decision point in the process.

  • Sell excess production back to the utility – NO
  • Do I need a 25 year warranty – NO
  • Professional installation costs are very high, if you can get someone to install this small – NO
  • Install yourself but pay for a survey from the utility – NO

I decided right away that this was a ’hobby’ installation. I am currently underemployed and living with a relative, so a permanent installation is out of the question. This implies that I am not selling excess power back to the utility. I will not be living here for 25 years and such a small installation is not worth the resale. A site survey from the local utility was listed as $900.00

So what will I need: Solar panel(s), batteries, charge controller, wiring, fuses, installer (me).

Solar Panels (175 Watt) $800 each
Charge Controller (30 Amp) $ 65
Batteries (gel-cell) 105 Ah $300
Wiring, fuses, etc. $100
Installer $0 (works for coffee)
Total $1,265 (for one panel)

Hmm, not economically feasible for a purchased panel, etc. This give a ROI of 10 years for a one panel installation. ROI is an acronym for ‘Return on Investment’ in other words how long will it take to recover the money I spent in the first place.

I’m a Handy Guy

Well I am. I know how to use various power tools and know a few things about DC electricity. So I looked at what other people had done on the Internet. I initially wanted to go for the ’broken sliding door’ installation. You find a sliding door panel with one side of glass broken out, then use the remaining door panel and glass as the enclosure for your solar panel. Unfortunately, good luck finding a broken panel to use.

So I had to decide how large a panel I was going to create and then decided on the frame. I noticed that most of the commercial panels are 72 cells. This is interesting. A single cell generates 1/2 volt so a 72 cell panel generates 36 volts maximum. Since your charging voltage has to be higher than your battery voltage, this would imply a 24 volt battery system.

I decided to build a 72 cell panel but I split it into 2 36 cell strings. Strings are soldered in series with the top side of each cell (Negative) connected to the adjacent cells’ bottom side (Positive). Each string would generate up to 18 volts.

Now it was time to determine what solar cell to use. I decided on the Plastecs WB-88 monocrystalline cell. This is a 6"x6" cell that generates 6 amperes. So I would wind up with 18 volts and 12 amps or 216 watts of power. I looked at the website and no longer saw ’buy by the pound’. You would wind up with some broken ones but it still made it a lot cheaper. You might want to email them and ask for a deal if you are buying this quantity of cells. Make sure you order extra cells! Solar cells are actually just coated very thin glass so they are extremely fragile. I will talk more about this when I publish about assembling the panel.

I looked at a plethora of charge controllers. You could get charge controllers in just about ANY voltage and current you could possibly want. I used the SCC3 charge controller from CirKits. There are 2 models, a 12 volt 20 amp model and a 24v 15 amp model. The kit was very very easy to assemble.

For batteries, I purchased 2 marine batteries from WalMart for $80 each. These are 105 Amphour (Ah) batteries. Battery Amp Hour ratings are for either 10 or 24 hours. Unfortunately, I was not able to find out which.

For wiring I settled on 14 AWG solid core wire with THHN insulation. I was able to purchase this locally at Lowes. When selecting wire the insulation has to be considered for the current rating of the wire. This particular wire will transport 40 amps without approaching the insulation breakdown current, which is when the insulation will melt off of the wire.

As I was using 12 volts for my system, I decided to use automotive fuse blocks and fuses for my system. I purchased these items at local automotive stores. I used 3 amp fuses for the cable modem, router, printer, and cordless phones.

Well that is all for now.

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About KC2ZRC

Mad Scientist proving the nay-sayers wrong!
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