Ask Slashdot: Home Testing For Solar Roof Coverage?

DudeTheMath writes "Here in the Sunshine State (Florida), solar should be a no-brainer. However, large oaks that require permits to trim partially shade my roof. I'd like to (inexpensively) 'pre-qualify' my roof for effective panel area. Googling for 'home solar testing' gets me equipment for checking the efficiency of an existing PV installation. Do any makers know what I can do on my own in terms of placing a few individual cells and, over a year, measuring and recording their output, so I can get an idea whether solar would be cost-effective for me?"

Comments

Homebrew solar insolation analysis

[Hyperion X]

Before I put solar panels on my roof, I built a system with a camera placed vertically over a reflective sphere (one of those cheap garden decorations), and then took photos from each corner of my roof. I then manually aligned each photo to north based on a compass in the photo and trimmed it to a square centered on the sphere. A script computed the path of the sun transformed onto the surface of the sphere, and drew a line over the photo for each month, with crossing lines for hours in solar time, and a point plotted for the position of the sun at the time the picture was taken. The point lined up "close enough" with the sun in the photo for me to assume that the lines were accurate. Any segment of a month line that was across sky would signify time where the panels would be active. and line crossing trees would be time lost to shade, enough to get a rough estimate of how well the panels would work.

Then I called a solar installer, who came out for a free quote with a handled tool that took a single photo, autodetected the position, orientation, and where the photo was sky vs. trees, and spit out the percentage of total incoming solar energy that would be absorbed at that point. I recommend doing it that way.

Re:Use the telephone

[tftp]

why not just buy a cheap time-lapse camera, set it to record every half hour or so, and check the solar coverage on the images of some representative days?

My advice is based on the fact that I have a PV system at my house; I had it ordered and installed.

There are many reasons why your own measurements are pointless:

• Installers have instruments, specialized software, and the knowledge how to use it.
• Installers have access to local insolation levels - not from the US department of statistics but from the house next door.
• Installers are up to date on many laws, from local to federal, that you must be aware of. I got a huge tax credit, for example, that took two years to draw out. It considerably dropped the cost of the system.
• Installers know of all the typical problems. The trees in the way are probably the most common issue. They know how to deal with that and they can arrange for the permit for you if necessary.
• Installers also know the size of the system that is optimal for your house. Too small and you won't realize all the benefit. Too large and you will be selling the energy for peanuts (if you can get any money for it at all.)
• The PV system must be inspected by a building inspector; to pass, it must be done by the code. The cost of doing it right can vary from low to high depending on your specific circumstances. It's part of the equation, and your measurements won't help to estimate it. But the installer will give you a decent estimate.
• All of the above is free and you get your personal proposal, with spreadsheets, drawings and financials, within a few days.

If you don't trust one installer, bring another one in and compare the numbers. If the numbers match then perhaps they know what they are doing.

It would be a waste of time to do the analysis yourself. You won't be even aware of many potential problems that installers know by the heart. Why would anyone want to risk a large amount of money ($20-30K at least?)