Dow Solar

Federal and state incentives have lowered the cost of residential photovoltaic (PV) systems, piquing the interest of homeowners hoping to slash their energy bills. But when choosing among the growing stable of solar technologies available, designers must consider other factors beyond economics.

Before specifying a PV system, an architect or consultant must calculate how many kilowatt-hours of energy a project will require. Austin, Texas–based architect Peter Pfeiffer, FAIA, uses an energy-modeling program that combines information about the space’s use and design metrics to estimate energy loads. Considerations include the roof’s lifespan, occupants’ water consumption and thermal comfort needs, and phantom energy sinks, such as outlets loaded with charged devices.

The local climate and the project’s orientation also will affect the estimated energy load. Experts recommend installing solar panel systems on expansive, south-facing roofs (for projects in the Northern Hemisphere) that are free of vents and obstructions from the sun. Panels tend to function more efficiently in cooler climates, like all electronics, although year-round sunlight is key. Permit fees and installation costs are other considerations, and designers should check whether the local utility company will tie in the residential system to the city grid.

For residential construction, PV systems generally fall into two categories: rack-mounted and building-integrated photovoltaics (BIPVs), both of which are usually installed on rooftops. Rack-mounted panels comprise 86 percent of the market, according to a 2013 survey by the Lawrence Berkeley National Laboratory (LBL) and the U.S. Department of Energy. The remaining share is split among rooftop BIPVs (4 percent), ground-mounted panels with a fixed tilt (8 percent), and ground-mounted panels fitted with solar tracking (2 percent). Most solar cells are made of crystalline silicon.

On a per-area basis, rack-mounted systems are about one-third more efficient than most BIPVs, which lack multiple surfaces for conductive cooling. PV systems on the market today can generate up to 1.4 kilowatt-hours per roofing square each year depending on their geography, says Mark Stancroff, director of building-product manufacturer CertainTeed’s solar division. Typical PV systems in new construction projects can generate 2 kilowatt-hours to 4 kilowatt-hours in perfect conditions, while the average retrofit system can generate up to to 5 kilowatt-hours, according to LBL’s study.

Rack-mounted systems on new construction averaged $5.30 per watt in 2012, down from $7.40 per watt in 2009, LBL reports. New-construction BIPV systems averaged $7.60 per watt in 2012, down from $8.70 per watt in 2009. Developments in microinverter technology—which allow solar panels to function individually, rather than forcing them to be part of an array—could also lower costs and help designers maximize the roof’s solar potential, experts say.

Schooled in Solar
Learn about improving energy efficiency and integrating photovoltaic systems into your projects during these sessions at the Greenbuild Conference and Expo in Philadelphia on Nov. 20–22:

TM08: Making it Modular: Innovations in Building Design & Construction
Nov. 18, 2:00 p.m.
Study cases of three modular projects’ energy-efficient designs that include solar systems.

TM13: Getting to Scale in the Suburbs: The Eagleview Master Plan
Nov. 18, 2:00 p.m.
Learn how one mixed-use suburban development gets its edge from sustainable design.

A12: Energy-Producing Landscape: Harvesting the Power of Urban Locales
Nov. 20, 9:00 a.m.
Learn about parks, campuses, and mixed-used projects that have achieved net-zero or net metering through the use of renewable energy.

C10: A LEED School: Renovation, Addition, Community, & Curriculum
Nov. 20, 3:30 p.m.
See how one school, built in 1923, was renovated through energy-efficient upgrades.

E05: Paseo Verde: Case Study of a LEED ND & Homes Mixed-Use TOD
Nov. 21, 9:30 a.m.
Study how a mixed-use complex incorporates solar to achieve two LEED Platinum certifications.

E07: Introducing Passive House Standard—A Valuable LEED Partner
Nov. 21, 9:30 a.m.
Understand approaches to Passive House design, including the use of solar arrays.

F07: Net Zero Energy Buildings—From Design to Occupancy to Certification
Nov. 21, 3:30 p.m.
Compare case studies on three buildings that attained net-zero energy performance.