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

Use the telephone

[tftp]

You don't need to do it yourself. Call a solar installer, and they will come for free and measure everything. They don't need to wait for the whole year because there is only one Sun for all on Earth.

Re:Use the telephone

[Moryath]

If you want a good back-of-the-envelope measurement and don't trust the solar guys, 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?

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?)

Re:Use the telephone

[dwywit]

My off-grid system was audited (part of the subsidy conditions) for safety and performance. The guy who did the audit very kindly brought out his specialist device for calculating theoretical performance based on environmental conditions. It was a very fancy data logger/field computer, equipped with GPS receiver, and a camera with a 180-degree lens, i.e. it could "see" from the eastern horizon to the western horizon. He placed it dead centre on my PV panels and pressed the "go" button. This was very close to midday.
 
Then he plugged it into his laptop and it produced a report on theoretical performance. It was able to extrapolate the sun's angle over the course of the year, account for shading by trees (a lot less than I thought - there's shading, but it happens so late in the afternoon that the incident angle of the sunlight is very low anyway), insert the projected position of the sun over the year into the images of the trees that it could see, and so on. It came up with a theoretical performance value of over 90% - and he said it was quite rare for domestic systems to exceed 90%. Anyway, the device costs about AUD$3000.00 - a bit too much for a one-off use. There's quite a bit of Android software available for this purpose - search on "PV performance" or similar.

Re:Use the telephone

[AlienIntelligence]

Actually it only takes 6 months to get all the representative days since the solar cycle is effectively symmetrical.

Not on Earth.

http://scienceblogs.com/startswithabang/2009/08/a_question_of_why_the_analemma.php

-AI

Re:Use the telephone

[Anonymous Coward]

Because it is fun to learn this stuff...

Re:Use the telephone

[ShakaUVM]

>You don't need to do it yourself. Call a solar installer, and they will come for free and measure everything. They don't need to wait for the whole year because there is only one Sun for all on Earth.

Yep, they're required by law (at least here in CA) to do it before they do an installation, in fact. They need to run all the math to find out what the rated capability of the system actually is. They can't just use the nominal rating of the PV panels.

It's called a solar site survey or obstacle survey, something like that.

It's really important for them to get it right, too, since if your system underperforms (I think less than 80% of the rated capacity), you can get your money back and they have to uninstall the system on their own nickel.

Start here...

[bobbied]

http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/atlas/

This will get you maps that will tell you the expected power, accounting for panel angle, cloud cover etc for your area. Then it's just a matter of subtracting your unique situation, shade from trees, angles of collectors, type of collectors etc.

DIY or call a pro?

[sylvandb]

Well you could do it on your own just as you described. Make it as fancy or as simple as you like. Hook up some cells in various places on your roof to an arduino's analog inputs, write some arduino code to read the output and send it up to the PC, write some code on the PC to read and store the readings. Write some more PC code to analyze the readings, or simply pull them directly into a spreadsheet. Then wait a year or more to get comprehensive readings, and hope your sample is truly representative of what a full install will experience.

Or you could it on your own in a much shorter time using (expensive) tools like the Solar Pathfinder or cheaper (more cumbersome) home-made equivalents to evaluate your solar potential.

Or you could call a local installer (or multiple) to come out and do a site survey using similar tools, and to give you a bid. The site survey should include all the info re. solar potential and the bid will let you know the capital cost, both of which you will need to know to make a decision.

As part of your analysis, consider whether you can take advantage of net metering and time of day billing to minimize the amount of solar needed to offset your bill. Consider whether you can carry over excess solar credit from month to month (annual zeroing) or if your utility does monthly zeroing, or if (very rare) you can get paid cash at retail prices for your excess production. Consider whether you are doing this for economic, environmental or "just for fun" reasons (or a combination).

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.

Solar PathFinder

[AaronLS]

You can use on of these and spot check in a grid like pattern. When you place it on a particular spot, the reflection will show you trees overlayed with a grid which indicates the times of the year which that tree would shade that location. This can help guide your panel placement trimming of trees, and tell you how much of the year that each spot would be unshaded.

If you went about in a grid like pattern you could enter the results into a spreadsheet and do a weighted average.

http://www.solarpathfinder.com/

Disclaimer: I do not work for nor am affiliated with this company.

That's pretty much my job

[BlackPignouf]

That's pretty much my job.
You want something like this:
http://www.solmetric.com/buy210.html
If you don't have access to it :
Do you have any picture? Did you model your roof+trees it in Sketchup? Could you sketch an elevation profile of your horizon with (azimuth,elevation) coordinates?
I usually ask $$$ for design/monitoring of big projects, but I have many scripts to get a complete report with rough estimates in a few minutes for smaller projects.

Solar coverage

[JWSmythe]

Some people posted good links. Here's some more information.

I live in Florida too (Tampa/St. Pete area), and have looked into it. We're finally making starting ourselves. I've lived in this general area of Florida most of my life. I assume you have too, but in case you haven't, I'll mention some obvious weather patterns.

One of the things you'll want to consider is, how much of the roof is shadowed and when. Look around online for information on peak solar exposure. There's a 5 to 6 hour window of the day that you get enough sunlight to make the panels practical. Ideally, you want the panels on the south and west facing side of your roof. Although we are farther South than most of the country, the North side of your roof is useless, unless you mount the panels very high and angled to the SSW. You don't really want to mount them high because of winds.

Morning fog and haze partially obscures the sun at sunrise, but typically burns off by 10am to 11am. That happens more in the winter, but we also have shorter days, so it has more of an effect.

Summer thunderstorms start building up cloud cover around 1pm to 5pm, so even though we have longer days, they'll be cut short many days of the summer.

Look at how your roof is shaded, Just check out your roof in the morning, afternoon, and evening. If that part of the roof is shaded for any part of the mid-day, from about 10am to 5pm, you probably don't want to use it. If you don't have a good candidate area in that period, you may want to consider panels mounted in your back yard. Check your local zoning to see if that is acceptable.

Thunderstorms are accompanied by heavy winds, rain, and hail. The winds can frequently have gusts that are the same as Category 1 hurricanes, and tornadoes spawned by them that are harsher than any hurricane. You may have a plan in place to pull the panels down for a hurricane, but a summer thunderstorm can build up and hit with very little notice.

Then we have tropical depressions, storms, and hurricanes. Your panels will experience heavy winds at some point in their lives. Make *very* sure they are well secured. If the wind hits just right, they can rip right off the roof. A friend of mine lost his solar pool panels a few years ago during a Category 2 hurricane. He found their remains about a half mile away.

Make *very* sure that you have the panels grounded properly, and the power system set up for lightning surge suppression. Something similar to what the power companies provide is a good idea.

Have the appropriate plans in place to provide for electricity in days without enough sun. I'm sure you've seen the skies stay black for days during a hurricane, or even just a very stormy summer. You'll probably want to reinforce your solar with a generator and tie it to the power grid. The boxes to do that can be pricy, but you'll want it.

I believe Florida law currently states that the power company will pay you wholesale rates for feeding back into the power grid. They charge you retail rates when you draw back from it. Depending on who your local power provider is, they may charge differently for day and night. Wholesale is usually about 10% of the retail rate. You do have to request a special power meter to make it work properly. They will work with you and your electrician to get that in for you. In some areas, it's free. Others charge a nominal fee.

And finally, just about everywhere in Florida that I've been, zoning is strict, and will be arbitrarily enforced. Make sure you have the proper permits, and licensed people doing the work as applicable (i.e., a licensed electrician does at least the final connections). Some HOA's have specific restrictions. For example, where my mom lives, she isn't allowed to make any external changes to the house. She can't put

Re:Retrofits aren't usually cost effective

[drinkypoo]

I took a solar installer class, and learned that it's usually not cost effective to retrofit a house.

I took a solar industry class and in my area of Northern California (pretty far north, but not by the border or anything) the estimate is maybe 15% of roofs in the entire county are suitable between the issues of facing, occlusion, access, and physical integrity. In Texas it's more. In Oregon it's less. In Florida, fucking move.