Before I started my solar project I wanted to get an accurate idea of how many hours of sun I would get on my solar panel. Since I was only planning on having one solar panel, I wanted to be sure it was positioned in the best possible spot to maximize the output. I rented a Solar Pathfinder from realgoods.com for about $25 for the week (I think it’s more now) and it was perfect for the task.
The device is a very simple low tech tool consisting of a compass and a reflective dome. It shows the average sun path for every month of the year, and allows you to plot where objects like trees or buildings will block the sun depending on time of day and month of the year. The sun doesn’t even need to be out to use the tool. Here is a general idea of what’s involved:
- Place the unit where your solar panel will be installed.
- Level the unit with the built in bubble level.
- Find the magnetic declination for your area, and with the built in compass (and declination helper), point it at true south (not magnetic south).
- Trace the shadows you see on the reflective dome onto the black paper below using the white chalk.
- For each month add up all of the numbers inside of the white line. Each number represents the percentage of daily sun for that half hour period.
click on the black sun path to see it full size:
My results:
Jan: 71% = 3+7+8+8+8+8+7+7+6+5+4
Feb: 96% = 3+4+5+6+7+7+7+8+8+7+7+7+6+5+4+3+2
Mar: 94% = 2+2+3+4+5+6+6+7+7+7+7+7+7+6+6+5+4+3
Apr: 95% = 1+2+2+3+4+5+6+6+7+7+7+7+7+7+6+6+5+4+3
May: 92% = 1+2+2+3+4+5+6+6+7+7+7+7+7+7+6+6+5+4
Jun: 91% = 1+1+2+2+3+4+5+6+6+6+7+7+7+7+6+6+6+5+4
Jul: 92% = 1+2+2+3+4+5+6+6+7+7+7+7+7+7+6+6+5+4
Aug: 95% = 1+2+2+3+4+5+6+6+7+7+7+7+7+7+6+6+5+4+3
Sep: 96% = 2+2+3+4+5+6+6+7+7+7+7+7+7+6+6+5+4+3+2
Oct: 92% = 3+4+5+6+6+7+7+8+8+7+7+6+6+5+4+3
Nov: 69% = 7+7+8+8+7+7+7+6+5+4+3
Dec: 68% = 7+8+8+8+8+7+7+6+5+4
So in a perfect world the panel could “see” the sun for the entire day which would be 100%. You can see from my results that a little bit of elevation makes a big difference ( compare Oct to Nov for example). The angle of the sun in the sky in the winter months is just low enough to cause the trees to block the sun in the morning hours. Also for most of the year I have other large trees next to the barn which block the sun past 4pm.
Overall I’m happy with the numbers I get. During the winter months I use much less energy in the barn so I can get away with less sun hours. Since all of the inverter power comes from the battery bank, I can still run all of the lights & tools I need in the winter. The difference is that it may take 2 or 3 days to fully charge the battery bank vs. 1 day in the summer months.
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