Our Take On The Tesla Rooftop Solar Panel

We’ve been hearing a lot of buzz in the industry surrounding Tesla’s recent announcement of the “Tesla Solar Roof” product. Being designers and installers of solar technologies we wanted to share some of our thoughts on this product announcement and hopefully clear up some of the questions and confusion left by the announcement event.

Previous Attempts at Integrated Solar

Firstly, the concept of a solar integrated roofing shingle is not entirely new. It has been attempted previously by several manufacturers in the residential market including some big industry players like Dow with their Powerhouse shingle, and Sunpower with their SunTile product. Both products underwent significant marketing efforts and the building out of dealer channel networks, but ultimately were met with limited success in the market and discontinued.

For those with a negative view of the aesthetics of solar panels, there is a clear advantage to using an integrated roofing shingle product. Others, however, find the look of roof mounted solar panels quite beautiful and see their addition as an upgrade to the home’s overall aesthetic.

Points to Consider:

• Power output: Tesla has not yet said much about the technology behind the “Solar Roof” product, but historically PV roofing shingles have utilitized less efficient “thin film” solar technologies which would require far more roof space to achieve the same system power when compared with standard, framed PV modules. Even if the end product uses a higher-efficiency PV cell, the fact that the cells will be spread across individual shingles will again cause a decrease in array efficiency since they cannot be packed as tightly onto the roof. Furthermore, these cells and shingles will be installed directly on the roof’s surface which will cause them to operate at a higher temperature. Standard PV panels utilize racking systems which allow for air to flow below the array, thereby cooling the cells. The power output of PV cells is reduced as cell temperature increases, so this cooling effect is very beneficial.

• Maintenance: For a PV array to operate efficiently it needs to be capable of moving electrons from all of the PV cells making up the array into the home’s electrical system where they are utilized. This requires thousands of electrical connections. In a standard system most of these connections exist within the solar panels themselves and are protected by the glass face, aluminum edges, and insulating backsheet of the panel. The panel-to-panel connections and those into the house are made with highly durable locking components and wire tested to stand up to harsh, outdoor conditions. In PV shingle products each shingle needs to be electrically connected to the shingle next to it, and for this to remain relatively flat will require those connections to be more fragile than those utilized to interconnect standard PV panels. Also, when one of these fragile connections breaks between two shingles, imagine the task of standing on a roof covered with thousands of shingles and trying to troubleshoot and isolate where that problem is located! The modular nature of a PV system built with standard modules makes it relatively easy to determine where a problem has occurred and, if need be, replace a panel or a section of wiring. Lastly, once this solar shingle roof is installed and functioning the smallest change to the roof to add a vent, chimney, or change the roofline for any reason becomes extremely complex and costly.

• Cost: If one were to ignore the maintenance issues with this type of product, it could be argued that cost-competitiveness could be achieved in a new home construction scenario or possibly a re-roof. Many homebuilders, however, will not want to risk the reliability of a new roofing product nor the need establish relationships with new sub-contractors in the conservative roofing installation market. Regardless the solar shingle will most likely cost more than standard roofing plus solar panels combined, will require a more expensive installation procedure, and as noted above, will incur a higher cost of maintenance across the system’s lifetime due to lower overall reliability. When working with roofing companies on Dow solar shingle projects we saw turnkey system costs that were over 400% higher than standard solar PV system costs.

Elon Musk has great vision, is a clever marketer, and certainly has done well to produce a high-quality electric car. With that being said, we will withhold final judgement on the Tesla Solar Roof until technical specifications and pricing are released, but certainly have concerns about Tesla’s ability to get this product to market and find a place for it to be competitive against the increasingly efficient, reliable, and cost-effective mainstream PV market.

Interested in going solar? Speak with our Austin solar panel installation experts at Lighthouse Solar to get started with a free consultation.

How Can Net Metering Benefit You?

Net Metering is a system that allows you to send the excess electricity your system produces back to the electricity grid and receive credits that can then be applied to your utility bill.

Without net metering, a homeowner must pay for the electricity they use at night, when the cloud coverage is too dense, or when snow is covering their panels. With net metering, a solar panel system can be installed that can cover the entirety of a home’s electrical needs because the system does not need to be producing the exact home usage in real time 24 hours a day, 365 days a year. Of course, that kind of real time production would be impossible due to weather and daylight hours.

HOW DO YOU COLLECT CREDITS?

When you install solar system on your home, your panels produce DC (direct current) power, which is sent through an electrical converter called an inverter. An inverter converts the DC power to AC (alternating current) power that can be used by your house. Your house uses the AC power and any excess power is sent out to the grid to power other neighboring homes.

As the excess power leaves your home, your power meter credits you for this power by spinning backwards instead of forwards. These credits are then applied in full retail value as you draw from the grid at night, or in the winter months, when your panels are not producing enough to cover your usage.

HOW IS NET METERING MEASURED?

Net metering is measured over a year. If you have a system that is designed to produce 100% or more of your annual usage, your installer sizes the system to produce an excess in the sunnier months to create a supply of credits to be used in the winter months. If you produce more than your usage during a month, your utility bill will show your credits that roll forward to the next month.

If your solar system produces less than your usage during a month, you pay the difference (after credits are applied) to the utility. At the end of a year, if you have produced more power than you have used, your utility will convert the credits into a refund check at the wholesale rate (which is much lower in value than the retail rate). This is called the Anniversary Month. You may change your Anniversary Month one time with the utility. It makes sense to have your Anniversary Month in the spring because you start collecting credits in the sunnier spring and summer months that can then be applied through the fall and winter. It is more valuable to use your credits for electricity (retail value) than to be refunded for them at the end of the year (wholesale value).

Your electricity bill will never be $0 because you must continue to pay a monthly connection fee to the utility in order to remain connected to the grid. This fee is under $25 and supports the utility infrastructure in your community, which you use for net metering, sending electricity out to the grid and pulling electricity from the grid.

Interested in going solar? Speak with our Austin solar panel installation experts at Lighthouse Solar to get started with a free consultation.

The Seasonality of Solar Energy Production

As an owner of a solar system or when considering going solar, it’s helpful to understand what your system will output over the course of the four seasons. Here in the northeastern United States we do see significant variation in daily energy output from our systems over the course of a calendar year.

SPRING AND FALL

Based on real data from the Lightgauge monitoring systems we install for our customers we can closely track each system’s energy output variation during the year. If we split the year into two equal parts at the Vernal and Autumnal Equinoxes (March 21st and September 21st) we can get a quantitative handle on this variation. It turns out that, on average, 65% of our local solar system’s annual energy output is generated between March 21st and September 21st of each year. The other half of the year, between September 21st and March 21st, accounts for the other 35% of annual output.

SUMMER AND WINTER

Furthermore, if we take a look at the two month windows surrounding both the Summer and Winter Solstices (June 21st and December 21st) by comparing system outputs for June and July vs December and January we can further accentuate the seasonal variation. On average our residential solar customers see a total energy output decrease of 40-60% during the months of December and January as compared to July and August.

The factors involved in this variation are threefold.

• Firstly, we know that in our area we have shorter days in the winter than we do in the summer. This means that the solar system will be running for less time each day and therefore produce less average energy per day.

• Compounding the effect of the shorter days is the fact that the sun angle changes dramatically in the winter as well. The sun, even at it’s peak around midday, is much lower in the sky during the winter months. For most residential rooftops this means that the sun’s rays will be hitting the solar panels less directly than during the summer months. This will cause the system’s power output to be lower which also has a direct impact on energy production.

• Lastly, atmospheric conditions need to be considered. Not only do the winter months provide plenty of stormy weather and cloud cover, but the effect of snow cover on the panels after a storm is significant as well. With a thin covering of snow the system will often still be able to turn on and output a small amount of energy. Larger snow accumulations on the panels, however, can keep the system from converting energy for up to a few days until the panels clear.

IMPACT ON UTILITY BILLS

So how does this work with your utility billing? Won’t this cause system owners to get high electricity bills all winter long when their systems are under-producing and their usage is increased due to more time in the house, higher lighting loads, etc.? Not necessarily, and this is where net metering comes into play. When we design solar systems for customers we always look at the total annual electricity usage when sizing the system. For customer’s with adequate roof space (or area for a ground mount) this allows us to design a system which overproduces enough during the spring, summer, and early fall to build up a bank of kilowatt hours with the utility which will carry the homeowner through the winter months. Thereby the effects of reduced energy production during our northeastern winters can in fact be mitigated through correct system design, sizing, and net metering (read more about net metering here).

This is also why, for our customers who get their systems interconnected in months other than March and April, we advise them to utilize their utility’s “Anniversary Date Change” process to make sure that they are optimizing the use of their net metered energy credits over the course of the year.

If you’d like to learn more about optimizing your anniversary date please call our office and speak with one of our Technical Sales Engineers.

Interested in going solar? Speak with our Austin solar panel installation experts at Lighthouse Solar to get started with a free consultation.