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Advanced Photovoltaic Systems

AET 230-8204 Notes-Week 4

Our guest today is:

Brian Swanson of Fidelity Roof Company 

You can learn more about Brian's company from their website:

Brian has spent most of his life in the roofing business, where he specializes as a commercial roofing and solar salesperson. There is no roofing job to big for Brian to tackle.

Today we had a presentation by student Scott Cornell who shared with us his PG&E solar "bill". It is not really a bill because as you can see by the documents below, he is an overproducer. Thanks Scott!


Here is a link to the latest news related to the stimulus:

Here is a website where you can watch CO2 levels increase:


Next Monday April 20, NorCal Solar is having a networking Event. Read about events at their website on the right margin:

Next Friday April 24th at 6:30pm, Vote Solar is having a volunteer appreciation party at 2360 Polk Street in SF. Sign up to be a Vote Solar volunteer, so you can go at

Next month May 11 through May 16 is the ASES (American Solar Energy Society) Annual Convention/Trade Show/Training Event, etc. I will be there, but make it back on time for you final on May 16th. They call it "Solar 2009" in Buffalo NY and if you can, sign up to be a volunteer at "Solar 2010". You can find out more at

Next Month on May 20, 2009 John Wiles (the main person responsible for Article 690 of the NEC) will be speaking in Palo Alto. Deadline to sign up is May 1 and this class is expected to fill up fast. You can click on the NEC book below for a link to John Wiles presentation schedule on the right side of the page. There are also many other useful links on this page, such as the Code Corner articles, which you should look at. Also, if you sign up to be a member of NorCal Solar, you get $5 off at

This weeks topics are System Sizing & Mechanical Integration, which you can read about in the book, Photovoltaic Systems in chapters 9 & 10.

System Sizing

System Sizing in PG&E Territory-

System sizing in the textbook and system sizing in PG&E territory can be a different proposition with a unique set of rules.

Residential customers in PG&E territory have what are called tiered rate structures. What that means is if you conserve energy, your rates are not very high. If you use a lot of electricity, your rates are sometimes 4 times what the low level (baseline) rates are. In sizing a system here, we try to offset the more expensive electricity for a faster return on investment. This can vary on a case by case basis, since some people do want to offset even their cheap electricity, because they care about the environment. Also, another factor to consider is that if you oversize a system &/or cut back on your usage to a point where you would make more electricity than you use, you end up giving it for free to PG&E. There are lawmakers trying to change this, but that is the way it is now.

Some people also want to size their system for the future. They might want to add extra capacity for an electric car. They might want to just wire it and leave the space for the future extra modules. Others might think the future will be more efficient, with energy saving devices that would make them under-size their system.

Another thing to take into consideration when sizing a system is time of use (TOU) rates. When you have net metering, your net is for the price of electricity and not the net KWHs. This means that if you are on a TOU rate structure and when you are making electricity during the day in the summer when it is 3 times more expensive than in the night, that you are getting 3 times the credit. Theoretically, if you only produced energy at peak times and only used it during off peak times, you could offset your whole bill by only making one-third of the electricity that you use. This is an extreme example which probably never happens, but it makes a point.

System Sizing off the Grid-

Sizing an off-grid system can be an art form. There are many variables. One thing to consider is how much of an inconvenience it would be to be without electricity part of the time on a bad day in January.

Another thing to consider is if you are willing to have a hybrid system. Many people have back up fossil fueled generators. A lot of my off-grid friends in Alaska also have wind turbines hybridizing their systems.

Unless you are only using your PV in the summer, you will probably increase your tilt angle to latitude plus 15 degrees to maximize production for the short days when the sun is low. You could also have a way to seasonally tilt your modules, which means you change the tilt angle to go with the seasons.

The text book gets into more detail about how to estimate your load and size your system from there.

System Sizing with Monthly Net Metering-

Apparently in some states they have monthly net metering, which means that you zero out your bill every month. Probably in those places, you would want to increase your tilt to get a little more production in the winter, unless you were using a lot of air conditioning. If you overproduce in some of these places, you would be compensated very little to nothing for anything over what you use.

System Sizing with a Feed in Tariff-

What makes a feed in Tariff different is that your system makes money for what it produces and does not have to offset you usage. You could have a large field, with no house on it and produce as much as you can.

Feed in Tariffs would really simplify everything. You would get paid for what you produce and would not have to deal with rebates, offsetting your usage, etc. The rates are guaranteed for a long time, so it also takes the risk out of the investment.

Mechanical Integration

There are many ways to put PV on a roof, field, etc.

Here are some links to some of the most common ways to go with "mechanical integration" of PV:

"Flat" v. Sloped roofs:

In practice, if someone says commercial roof, you think flat and if they say residential, you think sloped.

A flat roof is actually a low-slope roof in practice, since some drainage is needed. Typical low-slope roofs start at a 1/8 (one eighth):12 slope and a steep slope roof starts at 2.5:12 slope according to the book "Roofing Handbook" by Robert Scharff and the editors of Roofer Magazine.

In general, you design for a sloped roof with racking going across the roof in an east to west direction. You usually use lag screws and screw them into rafters. Your attachments should be properly flashed if you want it to not leak.

There are many different types of flat roof racking systems. Some penetrate the roof, some don't and are ballasted with "pavers" (like bricks). Some are ballasted with a few penetrations. They can be in rows. They can be flat.

Thin film is not the most typical installation in the US, but its use is growing. There is the UniSolar peel and stick solar, which is like a sticker. This is typical for a standing seam metal roof. If it is put on a single-ply roof, the manufacturer should tell you that it is compatible. Single-ply roofs are usually white and rolled on in sheets. Some types of single-ply roofs are TPO (Thermoplastic Polyolefin) and PVC.

The standard low slope (flat) roof over the years is built up or hot mopped. You can smell when they are putting one of those roofs on (tar). Sometimes they top it off with gravel or a cap sheet. A cap sheet goes over the surface and can be different colors. The color you usually want in this area is white or a lighter color that reflects the solar energy back to space.

Another type of mounting system, that is a product in it's own category, is the Solyndra, "thin film" cylindrical product, which is made here in the Bay Area.

You should be concerned about voiding roof warranties. It is best to work with whoever installed the roof, or someone that is certified to work on the brand of roof you are working on by the manufacturer.

When a roofer installs a particular brand of roofing product, they are commonly inspected and signed off by the manufacturer.

Probably the most critical piece of the PV installation puzzle that is commonly overlooked by the inexperienced person is proper mechanical integration into the roof.

Post class follow up...

Here are the answers to the quiz:

quiz week 4 answers.pdf (PDF — 54 KB)

Also, Brian Swanson recommended roofer training and here is the National Roofing Contractors Association website, where you can find out about online and other training from the experts.

& they have a PV online learning module

If you can't understand the complexities of integrating PV to the roof, you can get sued years later for moisture problems.

Here is a video of a solar installation going on a spanish tile roof. Notice how they remove the spanish tiles...

Solar Install on old Spanish Tile Roof in Palos Verdes, CA

3.6 KW DC Solar Electric Installation on a Spanish Tile Roof in Palos Verdes Estates, California. Features Mitsubishi 180 watt solar panels and SMA America Sunnyboy 3000US inverter.