(1) Actual 2017 harvest was 9,835 kWh as measured from the PV production meter, which is 83.5 percent of the standard, baseline configuration.
By John Gardner, TXSES
April, 2019
Solar panels or photovoltaic (PV) arrays are most efficient when aligned perpendicular to the sun’s rays. However, while rooftop PV systems are fixed in place, the sun is a moving target that changes position throughout the day and throughout the seasons. So how should PV systems be oriented to achieve maximum electricity yield over the course of a year?
When designing solar PV systems it is common practice to face the array due south (180 degrees) and tilt the panels at an angle equal to the local latitude, about 30 degrees in central Texas, to maximize production. The article will refer to this orientation as the standard, baseline configuration. The reality, however, is that few homes have roofing structures that conform to this ideal orientation.
In recent years, the public utilities and individual homeowners have learned that a PV array facing southwest (180 to 225 degrees) provides greatest production in the afternoon when power demands are high, such as when air conditioners are hard at work during hot summer months.
This article will illustrate that methods of installation outside of these typical design parameters can still provide productive results. For comparison, we will use three years of solar production history from my own home, with the following, atypical installation characteristics:
- Array size: 7,250 watts (7.25 kW) of total nameplate capacity, with two different orientations (see table above)
- Mono crystalline solar panels rated at 290 watts
- Roof mount at 20-degree tilt
West facing array composed of 15 panels rated at 4.35 kW.
Northeast facing array composed of 10 panels rated at 2.90 kW.
My residential installation utilized a split array with 4.35 kW facing 240 degrees west and 2.9 kW facing 60 degrees northeast, both with a 20-degree tilt. The array is mounted to both sides of the roof of an existing metal barn.
We will review and compare my system configuration with four alternate array configurations that conform more closely to the norm: two with 30-degree tilt and two with 20-degree tilt (like mine).
The comparisons will utilize PVWatts, a free PV calculator developed by the National Renewable Energy Laboratory. PVWatts allows homeowners, small-building owners, installers, and manufacturers to easily develop estimates of the performance of potential PV installations.
The following table provides a summary of different mounting schemes and their resulting energy production. All configurations assume a solar system of 7,250 watts (7.25 kW) of nameplate capacity, in order to compare to my own system.
The above design comparisons show that in central Texas, shifting an array to the west with an array tilt of 20 degrees or more (Design Variations #1 and #3), will provide an annual production of 93 percent or greater of the standard, baseline configuration. It can be seen that a variety of installation schemes can be utilized resulting in a system that meets the requirements of the customer.
The residential example further indicates that a west/northeast split array scheme will provide an excellent system when it is not feasible to mount all panels on the same side of the roof. Furthermore, the array shift from 180 degrees south to 240 degrees west moved peak production about two hours later in the day during the summer. This results in good correlation with late afternoon power demands such as air conditioning.
A homeowner who is planning to install a solar power system should provide a full year of utility consumption data (monthly bills with kWh) in order to enable a design for an appropriate system size. Weather conditions can have a significant impact on both PV productivity and customer demand, so it would be helpful to gather two years of consumption data so that weather variables are included in the final calculation.
Homeowners and designers should consider scenarios other than standard, baseline configurations. Breaking the rules can still lead to a productive and useful PV system.
John Gardner added solar to the first of three of his homes in 2004. He worked for a residential/commercial solar design and installation company, a solar distributor, and as a solar engineer on utility scale projects for a major engineering company. John has been a member of the Texas Solar Energy Society since 1995 and served as Board Chair in 2010. Now retired, he provides PE services to solar design and installation companies and is an Adjunct Professor at Tomball College, teaching online courses in the Electrical Technology Department.