Making all buildings solar-ready: 7 rules of thumb

Published on: 
Tuesday, May 1, 2018
Michelle Amt

VMDO is a Green Schools Conference and Expo sponsor.

Despite recent changes in U.S. trade policy, the cost of photovoltaic (PV) technology continues to fall dramatically, and in many places solar is cost-competitive—without subsidy—relative to conventional electricity. In addition to the explosion in local solar installers, there are now various investment and purchase vehicles that allow all client and project types to implement solar arrays, regardless of capital investment and nonprofit status.

On-site solar provides a host of benefits. It reduces project and community contributions to climate change, increases operational savings, improves project resiliency, and in the case of K–12 schools, provides a dynamic teaching tool. Given the compelling case for solar-ready design, at VMDO we now consider future photovoltaics in the design of every project.

Devin Welch, Vice President of Charlottesville-based solar provider Sun Tribe Solar, recently offered some helpful rules of thumb for incorporating solar-ready strategies in all buildings, many of which are reiterated in the Solar Ready Buildings Planning Guide published by NREL. They include:

1. Maximize unshaded, south-facing roof space.

Planning with a future solar array in mind can inform site placement, building massing, and roof shapes at the earliest stages of design. Optimally oriented PV arrays will have a shorter payback period because of their higher production compared to other orientations. The best tilt angle for photovoltaics is determined by latitude, ground vs. roof installation, array density and weather conditions.

For example, for a single row of ground-mounted panels, a good rule of thumb for tilt is .65 of latitude plus 7 degrees. (For Virginia, this means 31.7 degrees). For larger, roof-mounted arrays, we have found the best panel tilt for our photovoltaics to be 5–10 degrees to optimize for panel shading and desired density and wind loads.

2. Minimize and group roof obstructions.

Panels are wired together in a series, so if one panel is shaded, you can lose a significant portion of your array. Maximize the available open roof area and group all roof obstructions and penetrations to the north of the array. Ensure taller plantings that might shade the array are on the north, and keep nearby trees trimmed to minimize array shading.

3. Design extra roof load capacity and specify long-lasting roofing materials.

Assuming an installed array when performing structural calculations ensures adequate capacity for future photovoltaics. In addition, given that panels have a 25-year warranty, it’s important to specify roofing materials that will last. Some materials, like standing seam roofing, are ideal for photovoltaics because the panels can clip to seams and do not require additional penetrations. EPDM and TPO membranes, given their longer life span, are also good choices.

4. Pre-install roof conduit to facilitate easy installation.

In addition to providing a capped sleeve through the roof, provide metallic conduit, sized to accommodate system capacity, to the main electrical room. There should be no more than a 360-degree bend before the pullbox.

5. Plan space for inverters.

A good rule of thumb is to plan around 3 linear feet for every 30 kW of array capacity. Inverter performance is affected by temperature, so an interior, conditioned location is ideal. If needed, the inverters can be placed in a shaded exterior location.

6. Plan for a “PV Main Service” upstream of the main distribution breaker.

Anticipating a PV Main Service allows for a significant amount of renewable energy to be connected to the electrical service and ensures that costly modifications to the distribution system are not required in the future.

7. Consider locations for battery storage.

Although most battery storage technologies are currently not cost-effective, costs are dropping, and technology is improving—both at exponential rates. When anticipating an array, it is a good idea to also plan for battery locations, which are almost always pad-mounted exterior to the building near the main electrical service. For every 50 kWh, a good rule of thumb is a 3x3-foot space (plus clearances). At the 500 kWh scale, batteries are normally the size of a shipping container.

Not every project can achieve zero energy when it opens, but the falling cost of solar, and rising public expectations for implementing photovoltaics, means that there is no better time to deliver solar-ready buildings.