The conventional wisdom is that you want your solar panels pitched to the angle of the sun. When selecting a roof for solar panels, the orientation and pitch are important factors. But how much does pitch really matter, and should you mount your solar panels parallel to the roof surface or pitch them?
In Southwest Florida, the majority of residential roofs are 5 in 12 or 6 in 12 pitch. Many older homes have more shallow 3 in 12 or 4 in 12 pitches. It is not common to see roofs pitched at more than 7 in 12 pitch. Up north, steep roof pitches are more popular in order to shed snow more easily or to add loft space. Steep roofs add unnecessary cost, and are don’t fit into the architectural look of most communities in our area.
We have amazing tools at out fingertips when it comes to predicting solar electricity energy production from photovoltaic (PV) panels. The National Renewable Energy Labs’ PVWATTS calculator has detailed weather data that models solar irradiance at any given location. This data is used to predict the energy production from solar panels based on a set of assumptions.
Most newer homes in Southwest Florida have a 5 in 12 or 6 in 12 pitched roof
In order to understand the effect of pitch on energy production, I modeled the energy output of a 1 kilowatt solar array in Fort Myers, FL at a variety of common roof pitches found in Southwest Florida. In the graphic below you will see the energy output grouped based on the true compass orientation, with bars representing energy at the five common roof pitches for each orientation. The findings are pretty conclusive.
South roofs will net you the greatest performance from a solar electric system regardless of roof pitch. More notably, the roof pitch does not make much of a difference in energy produced. Whether you have a relatively flat 3 in 12 roof or a steep (for Southwest Florida) 7 in 12 pitch roof, the energy production will be about the same.
East roofs are the second best performer in Southwest Florida, largely because despite long days in the summer, summer afternoons tend to be warm and cloudy meaning slightly more energy is produced in the mornings. The variation is still fairly small based on pitch, with the best pitch outperforming the worst by 2.0%.
West roofs look much like east roofs when it comes to the variation with pitch, but even more so. The steeper the roof, the less energy is produced. A 3 in 12 pitch will result in 5.5% more energy than a 7 in 12 pitch.
North roofs are taboo in the solar energy industry, and for good reason. Solar energy production drops off quickly, especially as the roof pitch increases. It is interesting to note, however, that a 7 in 12 west roof only outperforms a 3 in 12 north roof by 3.9%. It is not completely unreasonable to put solar panels on a north facing roof if the pitch is very shallow. However, a north facing panel will only perform at 73% the output of a south facing panel at a 5 in 12 pitch (most common).
Solar Energy Rose
I created another way to look at this data on what I call a “solar energy rose.” The distance from the center of the rose represents the relative amount of energy produced per year for each orientation at 10º increments. The red line is a 7 in 12 pitch and the purple line is a shallower 3 in 12 pitch. Where the lines are further apart, the difference is greater as pitch changes. Where they cross, the pitch makes no difference in annual energy production. The orange dot represents the point of optimum production, which is a 7 in 12 pitch at 170 degrees true. The intermediate roof pitches would fall somewhere in between the purple and red lines.
What this tells us is that at southeast and southwest orientations, the roof pitch makes almost no difference. The pitch makes the most difference with north orientations, but that is to be avoided anyway.
Just a few years ago, solar panels were very expensive. In fact, about 15 years ago, solar panels cost ten times what they cost now! Prices have dropped over 75% in just the last 5 years alone. This is from where the concept of pitching solar panels came, and why many people today ask if they should pitch their solar panels on a roof. Back in the day you really wanted to squeeze every drop of electricity out of an expensive solar panel. Nowadays, solar panels are relatively cheap. As long as you have the space available, simply installing more solar panels makes the best economic sense.
South is best when it comes to solar electricity production, which is common sense and obvious to most. The pitch on a south roof is negligible. As you deviate from south, the pitch becomes more important. However, for common roof pitches in Southwest Florida, there is not a drastic difference in energy production that would justify pitching solar panels on a roof. A parallel mounting method is recommended, and will result in the best return on investment.
Regardless of pitch, 1 kilowatt of rated solar power will generate over 1.5 megawatts of solar energy annually!
When you pitch solar panels on a roof, a few things happen:
- You lose roof space because you need to leave room between rows to avoid one row from shading another. This reduces the amount of total space available.
- You incur extra costs. The cost to mount solar panels with a pitch requires extra hardware, which increases the cost of materials and labor. This cost almost invariably outweighs the extra energy production you would gain.
- The wind loads increase, meaning you need more attachment points and hardware, which in turn further drives up cost.
The combination of the above three factors makes a parallel mounting system the best recommendation for roof mounted solar panels in Southwest Florida.
Solar Pool Heating and Solar Water Heating
Note that the above discussion is about solar electricity only. The data does not apply to solar thermal technologies, which operate with a different set of rules.
Solar pool heating panels are not as finicky as solar electric panels. While there is not much data on the subject, or at least not as much as we have for solar electricity, conventional wisdom is that you want a good south orientation. If a good south roof is not available, west, then east facing panels are ideal. The pitch of the panels is not as critical. West facing panels perform a little better than east because, generally speaking, the ambient air temperature is warmer in the afternoons when the sun hits the west side of your home and the solar panels can absorb heat from the ambient air.
More importantly, pitching flexible solar pool heating panels is not practical. Because a typical solar pool heating system mounts directly to the roof with no substructure, adding the hardware to pitch panels is completely impractical.
Solar water heating panels, because they are rigid, can be pitched relatively economically, but for the same reasons of cost with solar electric panels, pitching solar water heating panels is not advisable except perhaps north or flat roof mounted panels.