Tuesday, January 8, 2013

Solar Powered Shed - Determining the Equipment Needs

This is my third post regarding the project to install a PV system on my shed. The previous posts were Solar Powered Shed - The Idea and Solar Powered Shed - Sizing the System.

Once I had calculated the theoretical maximum power I would need for my system to generate I was able to start looking at equipment. This is where having input from someone with experience was a big help, because I would certainly have otherwise over-sized the system, at a much greater expense. I presented my plan on Eathineer, where I received some very helpful advice. My stated goal was to do the install for less than $1,000. To my surprise, the suggested system could be put together for just over $500, although I've since decided to build in a bit more leeway, so the final cost will be more than that, likely around $850.

I've known all along that I would have to rely on batteries for this system to work. Powering power tools directly from a solar array without batteries would require many more panels that I want to use, would be very expensive, and would be wasting a lot of power generation capability during the times I was not at the shed. Determining the battery requirements can get quite complicated in situations such as mine, where the actual usage patterns are unknown and will vary a lot. It is fairly easy, though, to add more batteries if needed, so I'm not going to stress over this point too much.

The advice I received was to start with a single 12 volt 55ah deep cycle battery. After doing some research I decided that I would really like to have have two batteries, so the load is spread across them both. My current plan is to go with two 6 volt 115ah golf cart batteries. When wired in series the two batteries will provide 12 volts, and the 115ah is more than double the suggested 55ah, although of course the cost is also double. I could have just as easily gone with two 12 volt 55 ah batteries, wired in parallel, which would provide 12 volts and 110 ah.

Once I decided on my batteries, I needed to decide on panels that would be able to keep them charged. The original suggestion was a single 100 or 120 watt panel to charge the single battery. Since I decided to double my battery capacity I also decided to double the panels, so plan to purchase two 100 watt solar panels, which, when wired in parallel, will provide 200 watts in full sun. The average solar hours/day in my area is approximately 3, which means, on average, the panels should generate roughly 600 watts per day. Of course this will vary a great deal, and will be much higher in the summer, which is when I'll likely be outside in the most anyway. At an average, however, of 600 watts per day, I can expect 4200 watts per week, which is less than the 5810 theoretical max I determined previously. However, since I don't expect to ever actually hit that max, and expect my usage in the winter to be far less, I'm ok with this. In most situations I should be able to recharge my batteries in two to three days, which meets the goal I had for the system. I have found a 200 watt pv kit that comes with two 100 watt panels a charge controller that I will likely purchase.

The remaining piece of the puzzle is the inverter. The suggestion I was given was to go with a 2000 watt standard inverter. After doing some research I think I've decided on an inverter that produces 2000 continuous watts, with up to 4000 watts surge capacity. While this might be overkill, it will make me feel better knowing I have capacity beyond what any of my tools should require.

The efficiency and power use of the inverter can, as I understand it, have a significant impact on the sizing of a PV system. I have decided to do a separate post to discuss this, where I can share the information I have gathered thus far about inverters and ways of dealing with the losses due inefficiencies of the conversion process.

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