Micro-inverters

Now that I had identified the best panel locations:

I carefully measured the available surfaces.  Then I started researching panel/inverter combinations with an eye toward which panels would conveniently fit in the available areas.  I'll spare you all the details of the research (many, many panels and all the various inverters) and simply say that I settled on the Sanyo panels because they seemed to offer the highest efficiency for a decent price with good reviews.

The Sanyo panels I originally looked at were rated at 205 watts.  Later, I settled on the 210 watt panels, known as HIT Power 210N or HIP-210NKHA5.  Each panel is 62.2" by 31.4" (1580mm by 798mm).  That's just about 1.26 square meters.  The individual cells are approximately 18.9% efficient and the entire panel hits 16.7% efficiency.

Playing around with various panel configurations resulted in the following two primary options for the house:

The 'portrait' orientation of the panels on the house would allow 26 panels, but the two panels closest to the left edge would become shaded a bit too early in the afternoon.  24 panels looks just right and would produce approximately 5,040 watts.  And the number '24' is also good for another reason we'll explore in a bit.

The 'landscape' orientation would allow a maximum of 27 panels but three of those panels would be marginal so we'll once again settle for 24 panels.  This orientation has a minor advantage of keeping the panels a bit further away from the left edge of the roof, postponing the effect of the shading of the adjacent roof line a bit.

Ultimately, I decided to go with the 'portrait' orientation primarily because of the mounting racks.  The rails upon which the panels are attached work best when they run perpendicular to the long dimension of the panel.  It is also easier to attach the rails to the roof trusses if the rails are aligned horizontally across the roof rather than up-and-down.

The garage configuration only works in the 'portrait' configuration:

The garage provides room for 16 panels, supplying about 3,360 watts.  '16' is a good number when combined with '24' from the house configuration.

The inverter decision has been somewhat difficult to make due to recent developments of the 'micro-inverter'.  Micro-inverters are small, single-panel inverters which convert the ~40 VDC output from one panel into 110 VAC.  Use of micro-inverters is considerably simpler than the use of a more traditional multi-panel inverter because one may simply combine the outputs of all of the small micro-converters and wire that directly into a breaker on a standard electrical panel.  If the utility power shuts down for any reason then each micro-inverter immediately shuts down providing safety for the utility and utility personnel.

Before the micro-inverter was invented the only choice was a more traditional inverter to which a series of solar panels are wired.  There is a wide variety of 'traditional' inverters on the market and careful calculations must be performed to match up the numbers of panels in a series and the number of series with the inverter.

In my case, 8-panel series works quite well.  The 24-panel house configuration will provide 3 series of 8 panels and the 16-panel garage configuration will provide 2 series of 8 panels.  It is critical for traditional inverters that each series be balanced, and the number '8' works well with '24' and '16'.

Carefully balancing the number of panels in each series would also be a great advantage in the future should I decide to switch over to a battery backed system.

I would like to use micro-inverters.  They are supplied by Enphase Energy.  One major advantage of the Enphase micro-inverter is that Enphase has a very nice monitoring system that allows you to see how each individual panel is performing.  This could save you a lot of time when trying to figure out why your system may not be performing as expected.

Unfortunately, the Enphase micro-inverters are not yet available for the 210 watt Sanyo HIT panels and it could be next year before they are shipping.  I'm not willing to wait and the price differential between the 205 and 210 watt panels is not compelling enough to step down to the lower wattage panel.

So I've decide to go with a traditional inverter.  More on the inverter later.

Panel Potentiality

The next step in my project was to identify where I could put PV panels.  Ground-mounted panels would only work in the front yard and I didn't think that the neighbors would be very happy with that solution.  Fortunately, there were several roof-mounted possibilities, on both the house and the small garage to the south of the house.

Each of the surfaces on the house is nicely oriented to the south, and that on the detached garage is only a bit off of south.  The ideal orientation is directly south so all four of the marked potential sites are a win in that regard.

A second win came in the angle afforded by the roof surfaces.  The ideal inclination of solar panels is the angle of latitude of the site.  Our latitude is 39.5° and the roof surface is at an angle of 40°.  It could hardly be better!

'A' is by far the best location and offers several advantages:

• No shading from trees.
• Presents a large contiguous space reducing wiring requirements.
• Can support 8 to 10 kW.
• Very convenient to the service panel.
• Very easy to access for cleaning and maintenance.

Unfortunately, 'A' is also exposed to the street and would distress the neighbors.  We have an architectural council and CC&Rs in our subdivision and I was pretty sure it would be difficult to get approval for this area.  I might consider 'A' for a future expansion should the laws be changed to bypass local restrictions but I would still be reluctant to do something that would cause friction with the neighbors. Eliminated.

'B' is a nice location, too.  There is a minor shading issue from some tall trees to the east in the early morning, and from the roof line to the west in the evening.  The tree shading will be minimal during the winter.  Though not as convenient to the service panel, it's still workable.  And being on the back of the house eliminates any neighbor concern.  It can support about 5 kW and is also very accessible for cleaning and maintenance. To be considered.

'C' is not a nice location.  It's a long distance from the service panel, supports only about 2 kW, and is not accessible for cleaning and maintenance.  Eliminated.

'D' is a decent location, easily accessed for cleaning and maintenance, convenient to a service panel in the garage, and nicely rectangular.  There is a tree just to the south-east which would need to be removed in order to reduce shading.  It can support about 2.5 kW of panels.  Unfortunately, it's exposed enough to the street that I would not feel comfortable committing to this site without first talking with the neighbors.  To be considered.

'B' and 'D' together could provide about 7.5 kW which would chop the most expensive half of my annual usage under average weather conditions.

So 'B' and 'D' are under consideration.

Solar Education

Determined now to figure out what it would take to tap into the sun's power for residential electrical generation.  What I discovered is that it is no longer 'rocket science' and that there are commercially available solutions.  I also learned that it's still an immature industry and that there are only a handful of installers in our area.  And finally, I learned that the Federal Government's 30% renewable energy tax credit is indeed ramping up interest and causing some interesting responses by the installers.

Not Rocket Science

• Everything is basic electronics.
• Solar panels
• Inverters
• Wiring
• Safety components
• High-voltage DC is usually involved.
• The concepts are relatively easy to understand for someone with an engineering background.

Commercial Solutions

• I would only ever use commercially available, off-the-shelf components.
• Solar panels come in a variety of types, configurations, prices and efficiencies.  They all output DC.
• There are many types of inverters available.  Some connect to your utility and some charge up batteries.  (I will only discuss the former, also known as grid-tied inverters.)

Installers

• People are anxious to get into the solar installation business.  There's good money to be made, particularly from those who are proud 'early adopters'.
• The three installers in my area from whom I got a bid unanimously marked up the system components by 30% (in addition to specifying a labor fee).
• There are some important concepts to understand before embarking on a solar project and these installers generally understand those concepts.

Where can you go to learn about residential solar?

• Subscribe to Home Power Magazine.  Get the on-line subscription and read the archived articles.
• Purchase Photovoltaics: Design and Installation Manual from SEI (Solar Energy International).  Make sure to get the most recent edition.  Do the exercises.
• Now go back and re-read some more articles in Home Power Magazine.
• See if your local community college has any courses on solar power.
• Talk to people who have already invested in solar power.
• Rinse and repeat.

I'm an engineer and have fooled around with electrical systems since I was a kid and I'm comfortable doing most of the work associated with this solar project, but that's only after studying and researching for about two years.  Even with that training, however, I'm not a licensed electrician and so will hire and supervise an electrician for the installation.  It's unlikely the electrician with whom I work will have any solar PV system experience nor will he have any high-voltage DC experience.  But the principles are straightforward and after reviewing the code and NEC requirements I am confident in a safe and reliable outcome.

If you are thinking about doing a solar PV system on your own then you certainly should be very comfortable with:

• Solar panel and inverter sizing calculations.
• 110/220 VAC systems.
• Interconnecting with the utility.
• Handling high-voltage DC (~400-600 VDC).
• Truss load bearing for a roof-mounted system.
• Local code and permit requirements.
• Utility interconnect requirements.
• Subdivision architectural/CCR restrictions.

Then Google Happened

I joined Google as a software engineer in early 2007.  One of the most enjoyable benefits (in addition to the fabulous food) is the vast array of 'Tech Talks' presented by Googlers and their friends.  There were a series of Tech Talks dealing with taking advantage of solar energy, including solar PV systems, which I attended.

One such Tech Talk, presented by a couple of Googlers, discussed their own residential solar PV investment and covered the cost, the effects of shading, equipment, and much more.  Google itself had started an installation of solar PV at their Mountain View facility which would ultimately have the capacity to produce up to 14.5 mW of electricity.

Another Tech Talk presented new technologies being developed that looked very exciting.  One particular technology in which Google has invested uses a printing technique for making sheets of PV material with a per-watt cost approaching $1.

Cost is one critical factor when deciding if a solar PV investment is reasonable.  Historically, PV panels have been very expensive, affordable only by NASA and a few remote facilities where shipping fuel in would be impossible.  Current prices for panels are hovering around $4 per watt.  But wait, there's a lot more involved:

• Inverters
• Protective devices
• Racking
• Batteries
• Synchronization devices
• Cabling
• Installation

Add all of this up and the final per watt cost will range from $7 to $13 per watt.

At those prices you can estimate that a 5 kW system would cost from $35,000 to $65,000.

Not cheap, eh!

Enter Uncle Sam with their 30% tax credit for renewable energy systems.  Get it while you can!  That would reduce the costs of a 5 kW system to something in the range of $25,000 to $46,000.

I'll do a little cost-benefit analysis in a future installment.

Early Ideas

"There's just got to be a way to tap into the energy of the sun!"  This idea drove me to consider many outlandish ideas.  How about a 'solar trough' — a long, parabolic reflector with a steam pipe at it's focal point?  This design would simplify the tracking while maximizing the captured energy.  I even designed a simple tracker with only two photodetectors.  Heat the liquid in the pipe, drive a turbine, and voilá!  Electricity!

If only it were so simple.

So back to more traditional approaches, like passive solar collectors for hot water and solar PV panels for electricity.

I've ignored the passive hot water system for now primarily because we just don't use that much hot water, natural gas is still cheap, and installation would not be trivial.

At this point in my life (about 12 years ago), solar PV systems were still pretty much in the 'emerging' category of development.  And while I don't mind being on the 'bleading' edge of technology, there were some significant challenges to overcome:

• Grid-tied systems were in their infancy.
• A battery-based system would not meet our total needs.
• The cost was prohibitive.
• PV efficiencies still left much to be desired.

And there was plenty else to keep me busy.  So I took a "let's wait and see what happens in the technology" attitude.

Introduction

Tapping the energy of the sun has been of great interest to me since I was a young boy burning up paper with my first magnifying glass.  Technology has come a long way since those days and is now such that the proposition of tapping into that big yellow disc in the daytime sky is within reach, perhaps with a bit of stretching, but possible nonetheless.

Google has inspired me!  I work for Google and am proud of Google's investment in Solar PV (photovoltaic) electricity production.  After attending several Tech Talks and watching numerous videos about residential solar I decided to get serious about my own system.

I don't have a system yet—I'm in the middle of the project.  So it's a great time to start recording my journey.

This blog will first go back in time and describe the path of my awakening to the solar potential.  Then it will bring us up-to-date with my investigations into different solar solutions, technologies, battery-based and/or grid-tied, utility interconnect, city and state hurdles, installers, tax credits, layout, and so much more.

So welcome and I hope my experiences will help you as you consider residential solar of your own.