Many thanks to my friend Michael Levesque for his very valuable editing of this article.
Experience dictates that although a solar roof would be ‘nice’, few homeowners will install it if the economics aren’t there. So this article is focused on identifying the factors you need to consider before deciding if a solar roof makes economic sense. And within that thought process, do you install solar panels or a solar tile roof?
For clarification, in this article we will equate solar ‘shingle’ with solar ‘tile’. Both names are used in various settings and mean essentially the same thing.
Factors to Uncover
- Your existing roof.
- Roof condition?
- Estimating a Price for a Solar Roof
- Solar Panels
- Solar Tiles
- Design Considerations
- Energy Generation
- Solar Panel
- Solar Tile
- Estimating Tools
- How to Value a Solar Design
- A Simple proforma cashflow
List of Figures
Your Existing Roof
Solar Tiles would replace whatever existing roof you have at present. Solar panels can be installed on an existing roof, depending on the utility of that existing roof. So the question here is: Does the requirement for a new roof impact my decision point?
If you do not need a new roof then the replacement of the existing roof for solar tiles adds an element of expense that is, shall we say, prohibitive?
www.918construction.com identifies 17 types of roof shingles, including solar tiles. Clearly the most important function of a roof is as a waterproof barrier for your home; but they also affect the cooling and heating profile of your home – and this will impact your energy requirements. If you plan to replace your roof then you can more equitably compare a solar panel installation to a solar tile one.
Importantly, the life of a roof will depend on its materials. Building a metal roof that is designed to last 100 years is not the same as an asphalt shingle roof that is designed to last 10 – 15 years. Nor will the cost be the same. Installing physically robust solar tiles would be comparable to a higher quality roof. These factors will impact your financial analysis of the project.
Solar panels are frames that contain solar cells connected in series and parallel configuration. They make use of the photovoltaic effect to produce voltages and this is how solar power is harnessed.
Solar panels are usually placed on the roof, but they can also be installed in any other place that receives an ample amount of sunlight. Mostly, the solar panel versions that are available in the market come in blue or black color and are available in different shapes and sizes. Typically, a solar panel that is meant to be used for your home contains 60 photovoltaic cells and is rated for around 320 watts of power.
Solar Tiles / Shingles
Solar shingles are a type of Building-integrated photovoltaics (BIPV). While solar panels are installed above the shingles on your roof, solar shingles are a new technology that has the solar cells already embedded in the roof shingles. Therefore, it serves two purposes: one to protect the roof as any ordinary shingle would, and the other to produce electricity as a solar panel would. As regards electricity production, solar shingles are functionally the same as solar panels.
Typically, solar shingles come in a standard size of 300mm x 2180mm. They can also be thin sheets that cover your roof and additionally produce electricity. Companies that sell them are also keen to promote the aesthetics of solar shingles as well while working on their efficiency. For example, Tesla produces solar shingles in four different designs, including textured, Tuscan, slate, and smooth, making them useful for a wide variety of architectural designs. There are other brands too, notably the manufacturer Dow Corning, CertainTeed (a roofing company), and SunTegra who also typically mount a junction box on the underside of the roof which is then connected to a combiner and inverter located elsewhere according to the local Electrical Code.
Solar shingles are not very commonly known outside the solar world. The main reason for that is that solar shingles are a lot newer technology than solar panels. While solar panels have existed for more than 40 years, with some of the earliest ones even installed in the 1950s, the first solar shingle project dates back to 2011. One of the earliest solar shingle projects that had some viable results was installed in Colorado, USA in 2011.
Assuming your roof does need replacing, or you are in a new build situation, you can compare the installation cost of a solar tile system with the installation cost of a solar panel system that is placed on a concurrently installed new roof.
Estimating a Price for a Solar Roof
Assumptions: 2,000 ft2 roof with 10 kW solar
|Figure 3 Source: Sunstyle||Figure 4 Source: Bluewater Energy|
|Solar Tiles||New Roof & Solar Panels|
|$17.00 / ft2||$4.50 / ft2|
|$33,950||$1.99 / watt|
|$9,000 + $19,900|
|Caveat Emptor: This is an example; Check your own prices|
In this example we’ve used standard asphalt shingle installation cost and a generic solar tile cost estimate. Your quotes can vary significantly from these.
I hesitate to mention specific $ since the price on both of these technologies is a moving target. So….on average, solar shingles cost a lot more than solar panels, for any given amount of rated power. Some argue that the cost of a solar shingle installation is about the same as a solar panel installation combined with a roof replacement. However, additional factors like labor, installation material, permits, and inspection fees will eventually determine which option will be cheaper as a whole.
Choosing an Installer
Your choice of installer, their experience, reputation, qualification, ability to finance a warranty and their cost to install a solar panel system will all need to be considered. A comparison of quotes and a little research into various companies will be time well spent.
Qualified? Your installation company needs a qualified electrician; in most places a Master Electrician. The other trades required will revolve around this one. EnergySage.com has identified the best-qualified installers in 38 States and can connect you with multiple installers for a cost comparison. Connect with them here.
Which installation will generate the most electricity for the lowest capital and operating cost? How highly do you value the aesthetic appeal? We’ll cover aesthetics shortly but for now let’s deal with practical considerations. Solar panels take the prize for real estate efficiency. The latest record for the highest efficiency of a solar panel is about 23%, while solar shingles have achieved a maximum of 20% efficiency up until now. This power density dichotomy is accentuated by the additional real estate required for solar shingle installation. A typical solar shingle can produce up to 114 watts per shingle, and, doing the math for a household demand of 5kW, about 46 solar shingles will be needed. They will approximately cover an area of 328 square feet. As a comparison, around 270 square feet of roof space will be needed by solar panels to produce the same amount of power. Your efficiency calculation will also be affected by the layout plan of your house. If your roof angle and orientation are such that it does not receive the sunshine it needs then your electricity generation will suffer. How much it will suffer “depends”, as they say, on how much sunshine is harvested.
Here is a 5kW example:
Figure 6 – Real Estate Needed
Solar Panels are mounted above the roof; how far above the roof is a design consideration that includes wind lift, wind cooling, and the temperature coefficient. It is not unusual for a solar panel to have a temperature coefficient of around -0.39% above the cell temperature of 450 C. This means degradation of production if the panels get too hot. So a designer will consider the wind cooling effect along with wind lift (not a good thing) to reduce the cell temperatures as much as possible.
Almost everyone we meet will say that the solar tiles look better than solar panels on a roof. But how much any one person values that ‘look’ varies greatly. This one you need to answer yourself!
String Inverter Versus Micro-Inverters
A string inverter an inverter that is typically located near your fuse box or somewhere like a garage. If there is a performance breakdown in the future, chances are that it is an inverter issue, so a string inverter is better from an access point of view. Optimizers are neat little additions that can mitigate the problems that can arise from panels being installed in series. They will force a ‘go around’ for direct current if an individual panel has output issues so the string is not impacted along with the individual panel. Optimizers are also great at measurement and subsequent monitoring of performance.
Microinverters are attached to solar panels, or directly beneath a solar tile installation. These invert the electricity to AC from DC directly at the panel. One primary advantage of this arrangement is that panels or tiles that face different directions and therefore have different voltage do not impact each other. Similar to optimizers, microinverters are also great at panel measurement and subsequent monitoring of performance.
How to decide? If you have your solar panels/tiles facing multiple directions then we would recommend microinverters. Otherwise, a string inverter is less expensive and therefore cost-efficient.
Fire Risk Mitigation
Your qualified solar installation company will need to be conversant with the local fire code requirements. To oversimplify, a firefighter will not want to touch the PV system or the wires attached to it. So it is better, from a firefighter p.o.v. if there is an area of the roof that is free from PV. This is for walking and also for ventilating the roof of a building that is on fire. This is an incredibly short version of fire considerations meant ONLY to let you know that your local fire department and building codes will have detailed requirements.
The main function of any roof is to keep the elements out. And since anyone wishing to be your installer will claim a watertight roof, your best option is to a) choose your installer carefully and b) get a roof warranty that is just as watertight as your roof.
A qualified solar installer will be able to provide an electrical design that will estimate the electricity generation ability of the proposed system. The details matter.
In general we can state that a roof-based solar plant will be a fixed-tilt system. This means that the system stays in one position only. Using a tracking system of some description will almost certainly involve a ground mount scenario. Fixed tilt systems are lighter, less expensive, and produce less energy.
We can’t declare every possible solar panel configuration, so we will provide selected details from an example; these details are from the Solaria PowerXT – 400R-PM. www.solaria.com
Performance at STC (standard test conditions) (1,000 W/m2, 250C, AM 1.5)
|Max Power (Pmax)||[W]||400|
|Max Power Voltage (Vmp)||[V]||42.4|
|Max Power Current (Imp)||[A]||9.41|
Now we do the same for solar tiles. Our example is the SunTegra Tile STT 70 (as opposed to the SunTegra Shingle, another product). www.suntegrasolar.com
|Peak Power Voltage (Vmp)||[V]||8.30|
|Peak Power Current (Imp)||[A]||8.43|
The electrical parameters, along with all other parameters are the concern of your system designer but the efficiency of the chosen panel/tile would be important for you to note. Higher efficiency means less real estate would be needed for the same size system. And your roof has limited real estate.
The angle from the horizontal that your PV will lay upon. If the position is vertical (an extreme example not very practical, but illuminating) then the electricity production in the northern hemisphere when the sun is highest in the sky, suffers dramatically. The optimal tilt is the one that gets you perpendicular to the sun’s rays. Your roof will have a ‘pitch’; the slope from peak to trough and this is the gradient you are dealing with in the case of PV that lies flat on the roof. Lesson: your location on the earth matters; roof pitch matters.
If your roof points directly at solar noon (that is, where the sun is highest in the sky) then you are optimized. Otherwise, your electricity calculation will need to be adjusted.
Solar panels can also offer the added advantage of being mobile and more versatile in their application. While solar shingles are very limited in terms of installation i.e. they can only be mounted like roof shingles, solar panels need not be bound to one option. The solar panels can be adjusted and optimized for the best angle to gather the maximum amount of sunlight – maybe even a ground mount? Some systems can automate the solar panel mounting such that it tracks the movement of the sun to be able to generate the maximum amount of power throughout the day. Trackers though are a topic for another day.
Solar shingles are very limited in this sense. Their feasibility must be analyzed beforehand for the best results. (See our video on the use of RetScreen, a planning tool) Some homeowners will find that solar shingles are of no use to them as their house design does not allow for the required sun exposure on the shingles. In the northern hemisphere, if the house is north-facing, for example, the solar shingles at the front of the house will not get a good amount of sunlight for a longer part of the day, especially in winter. The roof angle will also determine the feasibility of the roof shingles. If the roof is flat or is at a very inclined angle, installing solar shingles can be less economical since they will not be able to generate the rated power.
There are a number of software tools that can help you estimate the production of your theoretical solar plant. http://www.retscreen.net/ will redirect to Natural Resources Canada where you can learn about this estimating tool. http://pvwatts.nrel.gov will take you to an online free calculator where you can get a rough idea of the production you can expect from a given system. https://www.homerenergy.com/ is a for sale energy prediction software, as is https://www.pvsyst.com/. PVSyst is commonly used by solar energy professionals.
How To Value A Solar Design
What is the money in and the money out and when do the money flows happen? Cashflow for a solar project is generally charted on a monthly basis and over a 20 or 25-year timeframe. You will need these estimates:
Figure 11 Excerpt from Financial Spreadsheet
In this example chart, we show the cost of the system and the first month of electricity production. We have an excel spreadsheet that shows numbers that you can adjust for your circumstances and that includes the time value of money calculation. The net result is a Cumulative Net Present Value for your plant. Register HERE to receive a free copy of this spreadsheet.
The point of this calculation is to estimate, as accurately as possible, your money in and money out. Beyond this estimation are other expenses such as tax and operating risk that you can also put into your valuation. Whether you sell the electricity or use it yourself you will need to put a value on the price of a kWh.
Some American states have a Renewable Portfolio Standard (RPS) that specifies the amount of electricity that must be generated by renewable sources. The result is Renewable Energy Certificates (REC) and specifically Solar Renewable Energy Certificates (SREC). The sale of SRECs can increase your cash inflows but this RPS will vary from State to State.
Another variable is the time over which you can be certain of your price per kWh. Can you be guaranteed a price for 25 years through a Power Purchase Agreement (PPA)? Are you going to use the electricity yourself to offset your utility bill? Is the Utility price going to increase over time? The answer to these questions will determine the price you expect for your kWh over time.
Operating & Maintenance
There is very little maintenance required to keep a solar installation operational. Cleaning the PV is a good practice since snow, leaves and even dust can have a dramatic effect on the electricity production of your system. The aforementioned optimizers or microinverters can provide the measurements you need to effectively monitor your system for trouble.
All of the major components of any solar PV system will come with manufacturers’ warranties. Whether a manufacturer can actually back up their warranty or even pays a financial firm to offtake the risk of non-performance is a question worth investigating.
Your installer will also need to have warranty protection, not least because of the issue of roof waterproofing. It would not be out of place for them to have insurance or a reputable warranty system in place. Otherwise, a roof problem can bounce back at you.
Permitting and other Regulatory Hurdles
As with other major topics in this article, permitting and the approvals processes required are worthy topics for their own discussion. It is a local issue involving electrical codes, building codes, Health and Safety, and local policies. To cut to the chase: make sure your installer gets the required permits and installs your system in a safe manner.
Before choosing between these technologies it is important to understand the details of installation cost, electricity production, and the weight you put on aesthetic design. Regardless of choice, solar panels, or solar roof shingles, the best way to go solar is to compare all of the options in one place. Inspection services and experts can better evaluate the conditions for you and settle the issue from a much more professional point of view.
All in all, the race between solar panels and solar shingles is becoming tighter and tighter, and both will share equal popularity in the coming days and years.