Photovoltaic Design, Inverter Size, PV String Considerations, Roof Layout

Pick Your Products and Vendors

Find a reputable vendor and select products from reputable manufacturers.  Get recommendations from others.  Double check references.  You're spending a lot of money and you should have confidence when you write those big checks.

dmsolar.com was recommended to me by several people, their references checked out, and David at dmsolar.com has been great.  Their products were exactly as represented, and the prices are excellent.  As expected when shipping from the mainland to Hawaii there were a couple of problems and their customer service was always quick to help me.  My system is built around

• SMA SB4000US inverter
• SF190-27-M modules made by Solar Fun http://www.aeesolar.com/PDFs/SF190-27-M.pdf

Evaluating PV Sizing

The first step in the electrical design is to verify your inverter can handle the number of modules you want to use, and how to configure the modules.  Modules are configured in series to create a high DC voltage into the inverter.  Although 1000V is considered the maximum allowable voltage in Europe, in the USA UL and code limits the maximum voltage to 600V. 

Most inverter companies have online calculators that help you size configurations.  The SMA calculator tells me the inverter I choose can handle up to 17 modules in series, or 24 modules in two parallel strings of 12 series-wired modules.

Why Temperature Maters

The sizing programs will ask you about the coldest and warmest temperatures expected.   PV output goes way up when the modules are cold, and down when very hot.  If you were putting PV on your cabin in Alaska or Minnesota, you couldn't put as many modules in series as I can in Hawaii without exceeding safe voltage limits.

Final Electrical Configuration Results

Layout on Your Roof

The electrical design may be done, but we're not done until we know that the modules fit on my roof in some reasonable layout.  As simple as it looks, a LOT OF TIME went into getting an optimal roof layout.  Roofs have skylights and plumbing stacks, and in my case solar thermal water heating panels.  And they are all probably right in the middle of where you would have put your PV modules if you could.  And shade is bad, really bad.  In addition to avoiding shaded parts of the roof, I fully expect to do some tree trimming as I monitor shadows on the roof throughout the year.

Permitting (Especially Electrical Permits)

You will need building permits for this kind of work, and even if you normally like to avoid the permitting process, probably your utility won't connect your NEM without a final building inspection.  I needed both a building permit and an electrical permit.  Most places let you take out your own building permit as a self-builder-contractor; expect them to want to see a diagram of your roof showing where the modules will be installed, like the diagram above.

You will also need an electrical permit since you are installing a new circuit and that is a little more of a problem.  First, you will need a circuit diagram - building inspectors call these electrical one-line diagrams and the one for my system looks like the one at the right.

Here's the more complicated problem to obtain an electrical permit: in most states you have to be a licensed electrical contractor to get an electrical permit.  If your creative, and if you have friends in the building trades, you can probably find a licensed electrical contractor willing to supervise your work as a consultant for a reasonable fee.  If you go this route, remember, it is his signature on the permit so it is perfectly reasonable for him to insist on inspecting your work, and you'll have to pay him for that.  And before he agrees at all, expect him to quiz you thoroughly on code and wiring issues.

Try reading the articles by John Wiles (at NM State University by clicking on agree and then click on to Code Corner) before approaching any electrical contractor so you'll have the background to answer his questions.  Remember, even though you are paying a consulting fee,  your electrical contractor is doing you a favor - this is not his normal service, and you're the one savings thousands of dollars.
 

Search Our Site

Page lasted updated April 06, 2008