Mistake = Good news

Taking the advice of some postings, I purchased a Kill A Watt to get a more accurate measurement of the watts my computer system pulls. When using my system, the following components draw power.

• Mac mini (1.5 GHz Intel Core Duo)
  
• Mac 20" display
  
• Lacie 500GB external hard drive
  
• JBL Duet 2.0 speakers

These components pull 75 watts of power. 

• Airport
  
• Cable modem

These components pull 5 watts of power.

So instead 197 watts like I previously thought, I'm only looking at 80 watts. This is great news. I only need to collect 320 watt hours of solar power to run my computer system for 4 hours over the weekend.

Research - Part 3

In part 2 of my research I learned I need to collect 65.6 amp hours (788 watt hours) of electricity over 5 days if I'm going to power my current computer setup. I currently can't place a solar panel on my roof. The only other place is on the sill of my front window. It's 4.5 feet wide and 5.5 feet tall and has 8 inches of window sill to place solar panels.

There are a few options for this situation. 

Global Solar makes some folding solar panels that might work. Only problem is that they are too wide (the most narrow is 9 inches wide) to fit on my sill. I also want to tilt these toward the sun. These don't seem ridged enough to do that.

They do make a 9 inch wide, ridged panel called the GSE Module 6W. This is the panel I've chosen to go with.

My sofa backs up against the window with the back extending a few inches above the sill. This panel can be placed on the sill and lean against the sofa back for the tilt. As a smaller panel, I have some more options if this setup doesn't quite work out. It's also not too expensive and an easy way to jump in and figure this stuff out.

So I picked up this panel ($67.50) at the Alternative Energy Store. It arrived a few days ago and is now sitting in my window. I also went to Radio Shack and picked up a 29-Range Digital Multimeter ($29.99) to measure how much electricity I'm picking up from the panel.

This panel is rated at 6 watts. It's open circuit voltage is rated at 23 volts. It's short circuit current is .45 amps. I went up to my roof and took a measurement and sure enough, it's hitting close to these levels.

Sitting inside in my window at 12 noon on this sunny day, it's measuring 18.48 volts and .17 amps. This means the window is knocking down its efficiency quite a bit. I'm betting it will only collect 50% of its rated power inside my apartment.

I'm going to take measurements every hour for the rest of the day.

11 am (19v - .19 A)

12 noon (18.48v - .17 A)

1 pm (18.48v - .19 A)

2 pm (18.3v - .22 A)

3 pm (17.8v - .16A)

4 pm (8.4v - .01A)

I didn't measure at 10 am but will assume I had basically the same sun as 11 am. This means with one panel I can collect good sun for 6 hours a day and operate at about 50% of the panel's rating...meaning on a sunny day I can generate 3 watts for 6 hours or 18 watt hours per day.

Let's say we get an average of 3 sunny days during the week. That means with one panel I'll collect an average of 54 watt hours by Friday night. 

On my sill I can fit 3 panels. So with an investment of 2 more panels I'll be at 162 watt hours.

Remember I needed to generate 788 watt hours to run my current computer setup over the weekend. Looks like I have a problem. I either need to get more solar panels or reconsider what I'll power with the 162 watt hours I'll store.

Research - Part 2

I learned from Part 1 of my research that I need to generate and store 788 watt hours of electricity to power my current computer setup for four hours. Part 2 of my research concerns how to store that electricity. The answer: batteries. Big batteries.

Batteries most commonly used in solar setups are either "Flood Lead Acid" or "Sealed AGM" batteries. Because I'm living in an apartment (ie. no garage, hardwood floors) and am not around much during the week, Sealed AGM batteries are the way to go. Because they are sealed, they won't spill acid which could eat through my floor and drip on my downstairs neighbor. They also don't require much maintenance. They just sit there and do what they are supposed to do.

A battery's storage capacity is measured in "amp hours". To figure out how many amp hours I need to power my setup, I follow this formula.

Watt hours / Battery volts = Amp hours

In my case this means....

788 watt hours / 12 volts = 65.6 Amp hours

The batteries we're talking about are 12V DC batteries...much like a car battery. If you've ever picked up a car battery you know it's heavy. I don't like carrying car batteries. And I certainly don't want to pay to ship these things. So I found the closest battery distributor to my apartment ( Interstate Batteries ) and checked out their inventory. 

There are plenty of AGMs to choose from. Which one depends on how many I can stand to carry to my apartment. It also depends on how much time I've got to fill them up. Remember I want to collect electricity during the week to use on the weekend. That means I've got five days to fill up 65.6 Amp hours.

How I'll fill up the batteries will of course have some influence on exactly the batteries I purchase. I live in an apartment. There's a chance my landlord would let me place a big solar panel on the roof...but that's doubtful. So my solar panel choices are limited...meaning I'm limited on how fast I can collect solar energy....let's hope Research - Part 3 will hold the answers on the solar panels I need thus helping me decide on batteries.