The pitch (or slope) of a roof matters—first to the installer trying to mount modules, and second to the system’s energy production. The azimuth, or compass direction the roof faces, factors in as well. For most sites, the more closely a roof’s azimuth aligns with true south, and the closer the pitch is to the site’s latitude, the higher annual energy production will be.
When estimating the solar potential of a roof, slope and azimuth are used along with historical weather data and shading percentage to figure out how close to optimal the site is. The National Renewable Energy Laboratory’s PVWatts calculator (www.nrel.gov/rredc/pvwatts) is a popular and accurate tool for estimating production. When user error is avoided with correct data input, the estimates prove to be high-quality when checked against real-world figures.
The graph (above) shows the production and percent of maximum for a 2 kW array at different pitches and azimuths, including the optimal tilt and true south orientation, in Pittsburgh, Pennsylvania.
While a general rule is that tilting equal to latitude maximizes annual production in fixed grid-tied systems, there are many locations where the maximum occurs at a tilt angle lower than latitude. For example, the latitude of Pittsburgh is 40.5°, but highest annual production is at a tilt closer to 30°. Local climates can affect the general rule—in Pittsburgh, short winter days can be much cloudier than long summer days, so maximum annual output occurs at a lower tilt that is more perpendicular to the summer sun, high in the sky. Even at a 20° pitch, with other variables held constant, a system will still produce 98% of that highest production value.
Local weather conditions also affect the ideal orientation for an array. For example, coastal communities may have morning fog, in which case orienting the array toward the west may result in better production.