Our Solar System Takes Shape

In the past few months we have finalized the basic design of our solar power installation.

Our system will have two arrays, one over the garage and one on the main part of the house. Each array will have eight 410-watt solar panels from a local manufacturer called  TenK. These will feed 12 microinverters made by Altenergy Power Systems. The total nameplate capacity of the system is 6.56kw, but because the two arrays will face different directions it will never produce that much power at any given time. Instead, with a southwest and a southeast array, one will catch more morning sun, and the other will catch more afternoon sun.

The estimate is that this system will produce, on average, about 5,800 kWh per year. This is relatively low production for a system this size in this area, and the lower production is mostly because of partial shading on the arrays, especially in winter. The garage array, in particular, is estimated to produce almost no power in the month of December because the garage roof will be mostly shaded by the rest of the house. That's not such a great loss, though, since Minnesota gets relatively little solar energy in December anyway.

We chose this system because of a very generous incentive program Minnesota is offering for solar panels made in Minnesota. For the first ten years the system is in production, we will get an incentive payment of $0.29/kWh for all the power it produces. This is in addition to the net metering credit which is currently about $0.12/kWh and will increase as electric rates go up. The Made in Minnesota incentive is paid for through a conservation program established several years ago by the state which requires electric utilities to set aside a small percentage of their revenue towards energy conservation programs.

The Made in Minnesota incentive is so generous that we expect this system to pay for itself in under ten years, despite the shading on our site and the slightly more expensive panels from TenK. Our benchmark for making solar worthwhile is that the system pays for itself within its lifetime (25-30 years), so this system meets that threshold by a large margin.

TenK Modules

The TenK solar modules are a new and innovative product, which was another reason I liked this option. Some people might read "new and innovative" to mean "unproven and risky," especially for a major capital investment expected to last decades. For us, however, since one of our goals is to learn and explore solar energy, the chance to work with a product taking a new approach to solar power is definitely a bonus.

Traditionally, solar panels are very dumb devices. The basic solar module consists of a few dozen photovoltaic cells sealed in a weatherproof enclosure and wired together with a couple diodes. In many cases, the panel manufacturer doesn't even make the solar cells, they just buy the components and assemble them into the final package. That's part of the reason why there are so many solar panel manufacturers and it's such a low margin business. There's been fairly little technology in the module itself, and all the magic happens in manufacturing the photovoltaic cells and in the inverters and controllers.

TenK, on the other hand, takes a very different approach. They sell "smart" panels which incorporate the MPPT electronics (which maximizes the harvest of power from the solar cells) into the module itself, and do a DC-to-DC power conversion to control the output of the module.

This allows them to get more power from the system in situations where a traditional module performs poorly (such as when half the module is shaded and the other half is in the sun). It also allows them to use a power bus for connecting the modules to the inverters, which makes it practical to generate a lot of power but keep the DC voltage at or below 60V.

The low voltage DC bus is important because high voltage DC (traditional photovoltaic strings can operate at hundreds of volts) is dangerous and requires special equipment to manage. The TenK modules also have built in ground fault protection, so if there's a short circuit in the power bus the modules shut down automatically.

So (in theory) the TenK "smart" modules should allow us to get more power from our system (especially in December), and while the modules themselves are more expensive, the rest of the installation is simpler. The total system price quoted by our installer for the TenK system was about 10% higher per watt than what we were quoted for a more traditional system built around "dumb" panels, but it's possible we will actually get 10% more power from this system than from a similarly sized array from another manufacturer

The risk, of course, is that TenK goes out of business and our modules break earlier than expected. With a more complex module there's more risk something will go wrong and the system will need to be repaired; and the solar module business is notoriously brutal.

In the near term, TenK seems fairly stable since they very recently raised a substantial amount of money from investors. I spoke to some of the company's early customers and they were all pleased, so I'm comfortable that they will be around to fix any problems which develop in the first few years.

Next Steps

One downside to the TenK modules is that the product is currently in short supply. Our installer advised us that we can expect the modules to be available in June, which is 2-3 months from now. We're hoping that won't get further delayed, since we want to take advantage of the most productive solar months of the year.

In the meanwhile, we're starting on the paperwork for the utility approvals and the solar incentive program, and looking at what work we can get done in advance so that when the solar panels arrive we can get into production as fast as possible.