Tuesday, December 17, 2013

Emergency Solar Generator

How I Built a Small Solar Generator for about $1000

Last year I purchased a small 1600 watt power generator to run the fridge, freezer and furnace motor in case the power went off for more than a day. Storing quantities of fuel was going to be a problem though.
Some months ago I began checking into a solar backup generator for emergencies. I wanted to spend no more that $1000, and I wanted it to be more of less portable. Maybe it could be used when camping or take the solar panels only to charge the batteries in a travel trailer or camp trailer when dry camping.
I was very excited to find out that prices for solar panels had come down recently so I wouldn't have to build my own panels. Internet searches and youtube videos were very helpful in trying to figure things out. Basically you have to decide what you want to power up during an emergency. I decided that I needed to power a freezer or fridge, a few lights and my internet modem and router along with my computer for about 6 hours a day.
The freezer uses about 135 watts when running and the computer and modem about 175 watts. Through in a few LED or CFG lights for another 50 watts and I figured I needed to produce around 360 to 400 watts for about 6 hours a day. This comes out to be about 2 to 2.5 kilowatts.
So I purchased a 600 watt pure sine wave inverter for $160.00. The batteries are the next thing I purchased. Determining how big a battery bank I needed was the hardest to calculate so I basically decided to see how much I could buy for the money I had available and it turned out to be six 35 amp hour batteries for just under $70 each. Rather that use regular marine deep cycle batteries I chose to use a sealed AGM or Absorption Glass Mat battery that doesn't spill or leak. They are commonly used in motorized wheel chairs. I wanted small batteries that were easier to lift rather that a couple of large heavy batteries. In the picture to the left you can see the three in front. There are three more behind them. They will give me (6 x 35ah x 12V) = 2520 watts over a period of 24 hours. However you can't discharge them all the way before charging them up again or you drastically reduce the number of times they can be recharged. So using the 50% rule, I can get about 1200 watts a day or 1.2 kw, or dischare about 100 Amps from them and I can always add more batteries later. I purchased them from Chrome Battery here. Next I needed to choose the solar panels themselves. I chose the 2 panel Renogy kit with charge controller. They are 100 watts each at 12 volts. You get everything you need except the wire and connectors. I chose to use the MC4 connectors that are the new standard for solar panels. This allows me to easily connect and disconnect the wires going to the panels. On a sunny day the panels will charge the batteries at 12 to 16 amps. So in 5 hours which is the average sunlight for our area, I would get 60 to 80 amp hours. Because batteries are not 100% efficient, we need to put more amps into the batteries than we can take out. If I had 3 solar panels I could get about 125 amps a day out of the 3, which would be just right.
I chose to put the components minus the batteries and solar panels in a plastic tote box from Wal-mart. I cut a piece of plywood for the front and mounted the charge controller, fuses and meters. I had to make a cut out for the inverter to stick out of the panel about 1/2 an inch. The diagram at the left shows how I connected everything together. I used #6 awg for the battery to the inverter and to make jumpers for the batteries, and #12 awg wire for the rest of connections except to the meters. I used underground #12 gauge landscape wire to make the connections to the solar panels. It is very flexible and has good insulation however it does not have the UV protections required for rooftop installations.
I now had to make a portable mount for the solar panels which I chose to make from PVC.  I used 3/4 in., but afterward I thought it not quite sturdy enough.  It does the job, but I would use 1" next time.  Some of the joints are not glued so that it comes apart for storage and travel.  The support legs in the rear also are not glued so that they may be set to different angles depending on where the sun is the brightest.
In summary I am very happy with the system.  I have tested it at 500 watts for about 2 hours and all seemed fine.
Here is the cost breakdown.
600 watt inverter    $160
6 batteries              $396
2 solar panels         $362
charge cont. inc.     $0
Misc. wire etc        $91
PVC + fittings        $15
Total                   $1024