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Community Solar – Better on the Roof?

| Written by John Farrell | 3 Comments | Updated on Oct 22, 2010 The content that follows was originally published on the Institute for Local Self-Reliance website at

When we released our report on community solar power last month, we expected a few comments on the grades we gave to the nine featured community solar projects. We also generated a really robust conversation about the location (on buildings or on the ground) of community solar PV projects and made a disheartening discovery about the cost of roof repairs when a solar PV array is present.

In the report, our criteria for solar PV location gave high marks for rooftop solar PV systems because of their use of existing infrastructure, lower marks for ground-mounted systems in brownfields, and the lowest grades for greenfield systems. In one particular case, we gave a ‘C’ grade to the Clean Energy Collective’s project because while it used otherwise unusable land near a sewage treatment plant, it was still ground-mounted.

The response to their ‘C’ grade made us re-evaluate our grading system. On reflection, there are three major considerations for the location of a community solar (or any distributed renewable energy) project.

Location Criteria for Community Solar

  1. Preservation of Open Space 
  2. Use of Existing Grid Infrastructure 
  3. Lifetime Cost for Participants 

Open Space

The open space issue cannot be ignored, as demonstrated by the opposition to centralized concentrating solar thermal power and solar PV power plants in the Mojave Desert and San Luis Valley in Colorado. Projects that use rooftops will rarely encounter resistance on environmental grounds (although there can be issues with historic districts). From the perspective of open space, there is still a higher value in a rooftop project than a ground-mounted one.

Existing Grid Infrastructure

The issue of existing grid infrastructure is not as clear cut. In general, distributed solar PV projects minimize the need for new grid infrastructure by plugging into the grid at low voltages and in a variety of places.

Rooftop solar would seem to have an advantage in this.  With few exceptions, a rooftop solar PV system can easily interconnect through the building’s grid connection. A rooftop solar PV system doesn’t change the capacity required by the local grid connection because net metering limits typically mean that no one installs a system that produces more than the building consumers.

But our error was to assume that ground-mounted systems would not take advantage of existing infrastructure, as well. In fact, the Clean Energy Collective solar project connects to existing infrastructure at an adjacent sewage treatment plant. Several other community solar projects in the report were constructed by utilities and presumably built next to existing substations where the new generation could easily be absorbed into the local grid. In other words, we should have graded this location criteria separately from the open space issue.

Lifetime Costs

The third issue – and one we’d never considered – is that rooftop PV systems may have to be removed and reinstalled if the roof needs replacement or repairs. While PV systems typically lose a small portion of their potential output (< 1%) each year, the systems can operate for decades, far longer than the typical residential or commercial roof (20-25 years in Minnesota). In other words, there’s likely to be one roof replacement during the life of a PV system.

Reinstalling a residential rooftop PV system could cost $6,250 or 25% of the installed cost of the system

In our investigation, we found that moving residential PV systems to accommodate a roof replacement could cost as much as 25% of the initial system cost (and over 35% of the net cost after the application of the 30% federal tax credit).  Moving systems on a commercial roof was less expensive, on the order of 15% of initial installed cost (around 25% of the system cost after the tax credit).   

The following chart illustrates the range of costs we found relative to an initial installed cost of $5.00 per Watt for commercial and residential PV systems.  

But this chart is somewhat disingenuous, because solar PV owners never pay the full installed cost.  Instead, there are a slew of tax credits and rebates that reduce this initial price.  The next chart shows these roof repair reinstallation costs relative to the net cost after the 30% federal tax credit.  

The cost issue is also complicated by various ownership arrangements. If the building owner also owns the array, the cost of moving the PV system is their responsibility. But what if they lease the solar array? Does the leasing company bear the cost of system safety when the roof is repaired or replaced or is it still the responsibility of the building owner? Will that cost be assessed when the roof is repaired or escrowed from the start of the project?

A CEC representative noted, “I guarantee you that a building owner (lessee) will never sign a long term lease that requires them to pay the costs of reinstalling a system after roof repairs, etc.” If CEC’s recently completed 77 kW community solar array had been built on a rooftop and required a move, the cost to its individual investors would likely be around $2,000, increasing the upfront cost for those individuals by nearly 30%. In addition, CEC couldn’t have offered the utility or its customers a 50-year service level agreement.

Conclusion: Location is Complicated

Obviously, there’s much more to the ground v. rooftop issue than meets the eye, from interconnections to roof repairs. Look for a transformation in our Community Solar Report in the next few weeks reflecting on this complex issue.

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About John Farrell

John Farrell directs the Energy Self-Reliant States and Communities program at the Institute for Local Self-Reliance and he focuses on energy policy developments that best expand the benefits of local ownership and dispersed generation of renewable energy. More

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  • Bob Wallace

    One solution is to use a metal roof under the panel racks.

    Painted steel roofs are very long lasting. A fifty year life expectancy would be a bit on the conservative side, especially since the roof itself would be protected by the overlaying panels.

  • John Farrell

    I’d be curious about the cost of the metal roof. Would it be cheap enough to still be a cost savings over moving the panels?

  • davea0511

    Back east there are plenty of superfund sites that would probably be better off with ground-mounted solar, as the obscured light then is one more deterent from local animals which could otherwise be exposed to poisons, let alone that they spread the poisons elsewhere. Most of these sites also have grid power onsite, and overall should be considered overall a better use of that space than otherwise possible.

    I also would like to see references of impact studies of the Mojave solar installations. Could they be made to be more Bio compatible? Take the Heliostat arrays for example … could they be lifted another 10′ off the ground with mechanisms to keep life from becomeing problematic, and then intersperesed with light diffusers that would allow enough light in the shadows to support life?

    Lastly, If the installation is large enough (say 100 MW), what is the added $/W required to sequester that power if produced in say Death Valley where really pretty much nothing lives. I suspect there’s still some life there, but are we talking about wiping out something unique in the area that can’t be somehow preserved?

    I just have to agree that we have to be very careful when criticizing ground-based systems as a rule, and look at them on a case by case basis. What’s more deadly … burning fossil fuels and killing our atmosphere, or filling portions of the southwest with solar arrays, with some areas left untouched as “natural desert”? Community Owned Renewable Energy (CORE) is about providing energy for people without roof-top … and in fact it seems that we should be docking points from those which use roof-tops because CORE shouldn’t sabotoge the market created for independent solar installers and people who want to put their own solar on their homes.