Does Solar Power make sense for my roof?
Residential Solar Power is big time business – 2,099 MW at about $3.60/W means $7.6B in sales in the United States in 2015. SolarCity, the largest national installer at about 34% of all residential installs, is one among thousands of installers. In my home state of Massachusetts alone, there are greater than 400 companies and 15,000 employees involved in the solar power supply chain. Across the US, there are more employees in the solar power industry (200,000+ as of right now) than oil. And of course, we’ve recently crossed 1,000,000 residential solar power installs in the US. You might be among many considering this investment. Let’s get into the devilish details…
Let’s get started!
While the National Renewable Energy Lab thinks the US can get 40% of its electricity from rooftops – they estimate only 22-27% of residential roofs have solar power potential. The questions you ought be considering are as follows:
- How complex is your roof?
- What direction does your roof face and what are the angles of the roof surfaces?
- What material is your roof?
- What’s the shade situation like? Can you affect this by cutting down trees, etc?
- How old is your roof?
- Can the structure support the additional weight?
- How much solar power can you fit on the roof?
You should become familiar with PVWatts – this tool is put out by the National Renewable Energy Lab (NREL). A PVWatts electricity production projection is likely to be required when applying for interconnection (connecting your solar power system to the larger power grid). If you want to get a little more into things – picking out modules and inverters – you can use a tool like Helioscope. The purpose of these two tools is to help you better understand the total amount of solar that you can fit on your roof and how much electricity those modules will produce.
An overly complex roof like above, while beautiful, is not always conducive to solar panels. The many peaks and valleys lower the amount of continuous usable space available. A solar module is about 5ft by 3ft and a perfect minimum space need hold be 10-20 modules (200-400sq ft). Complex roofs increase the amount of time that installers have to plan and increase the complexity of the conduit runs (conduit holds the copper wiring that connects the modules to your electric panel) – this added amount of time and hardware will affect your pricing.
In addition to the complexity of the roof – is the direction the roof faces and the angles of the roof. A roof facing due east or due west will produce up to 15% less electricity than a roof facing due south, as seen in the above image. Your first choice is always due south. If you have to choose between east and west – there is a current need for more west roofs due to there being an uptick in electricity usage in the evening. This west facing demand may change as incentives and battery systems become popular. There has been a lot written on the subject recently.
The angle of your roof plays a part as well – though a smaller one. A 5° roof versus a 30° is a 10% difference in production – however, going from 30° to 40°, or 30° to 20° is only about a 1% difference. A great technical walkthrough of this can be found on the well named website SolatPanelTilt.com. Of course, the angle of your roof cannot change – it is how your home is built. If you happen to have a 5° or flat roof – then definitely invest in a racking system that will angle your modules closer to 20%. A lifetime of 10% higher production will make up for the expected price increase of about $0.10-25 cents/W. If you roof is anywhere above 15-20°, then I’d suggest you stick with that angle as increasing the height also increases the effect that wind will have on your system – and that increase is not worth the 1-2° that you’ll get.
The image on the left shows a typical connection on an asphalt shingle roof – an L Foot sitting atop the flat piece of aluminum flashing with plenty of silicon sealant. These L Feet are the base upon which you build a fuller racking system to hold the solar modules. They are also where the hard work must be done to make sure there aren’t leaks. On the right you see a similar process with a Spanish tile roof. Spanish tile and shake are more brittle than asphalt shingles, and they must be removed and later reapplied. This brittleness means that during construction, in order not to damage the roofing material, your movement on the roof must be slower. Replacing material and slow movement can add 10% to 25% additional cost to the installation job.
Obviously, you want your solar modules to get as much sunlight as possible. If you have a wide open area – no trees, a simple roof and no nearby structures then you can skip this section. A great tool to start off with if you are in the right area is Google’s Project SunRoof which shows how much sunlight your roof will be getting from the vantage point of satellites. If not, get up on a ladder or nearby hill/tree on a sunny day and take a look.
If you do have some shadows – then good planning might make a solar system financially viable for you. A residential solar power system will have two or three ‘strings’ of solar modules – a string is a group of solar modules on the same wire. When using a standard solar inverter – a shadow on a single solar module will affect all of the solar modules in that same string. Now a days – you have options.
In 2014, around 55% of residential PV installations used some form of ‘module level electronics.’ These electronics – like EnPhase, SolarEdge and Tigo – offer solutions that mitigate the energy production losses in high shadow situations. Photon Magazine (a very respected solar analysis group) reviewed SolarEdge in shadow situations and found a benefit anywhere between 4.4% to just over 10% (in high shadow situations). If there is a large roof area that has a small shadow problem, ask your solar designer to give a production report with the shadow and a regular inverter, and then an analysis with a module level electronics type product. Compare the cost of the upgrade with the extra energy generated – and you’ll know whether your shadow issues can be overcome or ought simply be avoided.
Of all of the parts of a solar power installation – the roofing situation is the one that involves the most ‘feel.’ A solar power system could easily last 40 years. Generally, a roof that is out of warranty and having leak problems should not have solar power installed on it. It costs extra money to move panels to fix a leak, or remove and reinstall modules when replacing a roof. To add a level of complexity – solar modules will extend the life of the roof and lower the temperatures below them by 30-50°F. Your answer to the roofing situation will greatly depend upon your comfort levels with home repair.
Somewhere around 10-12 years into a 20-25 year roof is when I start to mention to the average homeowner to check out the roof. Get a professional to look around the attic for leaks, mold and rot and look over the tiles for damage. A 15 or 20 year old roof will sometimes last 15 or 20 more years – a talented roofer can make this happen. Some tile roofs will last 35 and more years, some metal roofs will last 50 years +.
Roof weight capacity
If you really want to get into the science of load capacity – dig in deep! Truly, this is purely an engineering question – and as part of the permitting process, you will be required to have a structural analysis done by a professional engineer (PE). Most modern residential rooftops will have the structural strength needed to handle the weight of a solar installation. Solar modules and the racking to hold them in place add about 3 lbs/sq ft to a roof. That weight is about equivalent to a layer of shingles. If you have access to the original architectural/engineering drawings – search out the documents for the term ‘load capacity.’ If your available load capacity is greater than 3 lb/sq ft – and you don’t plan on installing anything else, then solar works from a weight perspective.
Roof production capacity
The question of “How much solar power can fit on my roof?” has been made much easier with modern software tools. PVWatts and Helioscope are the two I’m most familiar with. If you really want to get into your solar power purchase create an account on Helioscope. The software is free for thirty days and you can finish your design in a lot less time than that. You will probably want to sign up for a one hour webinar – the introduction webinar is enough to let you do a design that will help you in contractor discussions.
A residential sized solar module is about 6′ by 2.5′ (commercial sized modules are 6′ by 3′). A rule of thumb I use is that a single solar module needs 20 sq ft of space. The space needed by these solar modules is also preferred to be bunched together – a minimum of four or six modules – to maximize the time spent on the roof (and minimize the number of holes put in your roof).
Once you get a feel for the number of modules you can fit – multiply that by 250 watts. 250W is about the smallest sized module you’ll get these days for a residential installation. The average electricity usage for a US residential customer is about 10,932kWh/year – and since one 250W solar module will produce between 280 and 450kWh/year that means you’ll want between 24 and 38 solar modules to bring your bill to zero. And 24 to 38 modules, at 20 sq ft each, means about 480 to 760 sq ft of space will be needed. Of course, this number is REALLY rough. Your electricity usage and geographical location will affect your needs and the output of the solar modules.
We will talk about in a later post how we decide the amount of solar you need, the hardware you should be considering and what the financial aspects of a solar installation are. And, please, in the comments ask questions, make comments and post pictures as this post is a growing, living tool that will be updated.
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