Monday, January 7, 2013

Solar Powered Shed - Sizing the System

As mentioned previously in Solar Powered Shed - The Idea, I am planning to install a small PV system in the shed, to provide power for lights, etc and the occasional power tool. Since my last post on the topic I've done a bit of research to determine my needs. I have also talked with someone who has experience with such systems to get some much needed advice.

The first step in determining my needs was to list the items that I want to be able to power from the system. Since I do not currently own all of the items that I may wish to use in the shed, I did some research to get an idea of the average power requirements for the tools I think I will most likely add in the near future.

Power Requirements
Lights - 200 watts or less, depending on the type of light I go with
Radio - 10 watts, again depending on the type I use
Fan - 50 watts or less
Circular Saw - 1430 watts
Jig Saw - 550 watts
Small Table Saw - 1650 watts
Small Chop Saw - 1650 watts
Air Compressor - 1540 watts

Once I had completed the list I needed to determine how I would use these items together. The first three items on the list will be used more often, and will be used together. This means that I may need up to 260 watts of continuous power for extended periods of time. The remaining items will not be used in conjunction with anything else, so the maximum power requirement would be 1650 watts for either the chop saw or table saw.

Knowing the maximum power requirement, however, is only part of the equation. I also needed to estimate how often I would use each item, and for how long. This is where a lot of guess work comes into play, but I made my best guess.

Usage Patterns
Lights - Max of 3-4 hours a day, a few times per week, less often during the summer months
Radio - Max of 3-4 hours a day, a few times per week
Fan - Max of 3-4 hours a day, a few times per week, with no use during the winter months
Power Tools - A few minutes at a time, for a total of maybe 30 minutes total use spread out over 3-4 hours, maybe as often as twice per week

After reviewing the numbers I was able to come up with what I expect to be the maximum power requirement over a period of time. I'm going to assume, for the purpose of this calculation, that the lights, radio and fan are used 4 hours a day, 4 times per week and either the table saw or chop saw is used for 30 minutes twice per week. This means that, based on a 100% efficient system, I should need to supply 5,810 watts of power over a seven day period. Of course no system is 100% efficient, so I will need to account for that when it comes time to actually pick out equipment.

Something else I should point out is that for these calculations I'm using the worst case scenario. The lights, for example, will most likely require much less power than I'm estimating. This estimate is based on a couple of halogen shop lights along with a couple of additional light fixtures. If I use more efficient lighting, however, that number should be able to reduced significantly. Also, I do not expect to ever use the lights, radio, and fan all for 16 hours each in a given week, but I wanted to be on the safe side with my numbers.

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