Peak Sun Hours for your geographic location.
The amount of annual solar energy available at your location is called “Insolation” and it's measured in Peak Sun Hours (PSH).
Peak Sun Hour (PSH) = 1 Kilowatt/per square meter/per hour.
On this chart from NREL.gov you can see that for northern Illinois the average Peak Sun Hours available each day is between 4 and 4.5 kilowatt hours per square meter per Day (kWh/m2 /Day).
This roofs location gets 4.3 Peak Sun Hours per Day.
And 4.3 X 365 Days a year = 1,596 Peak Sun Hours per year (kWh/m2/Year).
Please be aware that this measurement (1,596 PSH/Year) assumes a flat surface of one square meter with a due south azimuth and optimal fixed angle of tilt toward the southern sky. For northern Illinois this is 180 degree azimuth and around a 36 degree tilt. We’ll now show you how to determine how much if this energy your particular roof can capture.
IS YOUR ROOF A GOOD CANDIDATE?
Determine how much of the available solar energy your roof can capture.
We’re going to use the roof pictured here as an example for all the calculations on this website.
Of the available 1,596 annual Peak Sun Hours (PSH) available at this location, this particular roofs azimuth and angle allow it receive only 1,464 Peak Sun Hours.
Move down this section to see how we came up with this 1,464 number.
What makes these calculations easy is the energy unit called a Watt. From the sun to that 100 Watt light bulb next to you, to your monthly electric bill, it's all measured in Watts.
And there are a lot of Watts involved so to make the numbers smaller and easier to manage 1000 Watts is equal to 1 kilowatt.
Peak Sun Hour (PSH) = 1 Kilowatt/per square meter/per hour. So this roof receives 1,464 PSH/Year. (1,464,000 Watts)
Your roof tiles need to be either fairly new or should be replaced before panels are installed.
Our Example Roof has Architectural shingles and was recently replaced so it’s in good condition for solar panels.
Solar panels have a lifespan of at minimum 25 years so your underlying roof will need to be in good shape at the time of panel installation.
Standard asphalt shingles can have a lifespan as short as 20 years. For architectural asphalt shingles the lifespan is around 30 years.
Be sure your underlying roofing material is expected to last at least 25 years.
ROOF ORIENTATION (Azimuth)
A southerly facing exposure is best, but east and west facing roofs still work well.
Our Example Roof Surface Azimuth is 165 Degrees
While a due south facing roof surface is the best orientation for capturing solar energy, a roof surface that’s oriented due east or west can still capture 75% of the available solar energy.
Determining Roof Azimuth
The first step to determining how much solar energy your roof’s surface will capture is to find your roof’s Azimuth. True north (not magnetic north) is described as zero degrees azimuth, East is 90 degrees, and south is 180 degrees azimuth.
By due south we mean geographic south or true south which points to the south pole. Not Magnetic south.
ROOF TILT (Pitch)
Determining Roof Angle
Our example roof surface is at a 45 degree Angle.
To determine roof tilt/angle you measure degrees from Horizontal. Here you can see this roof has a 45 degree tilt toward the southern sky.
Roofers use a ratio of inches of rise to inches of run to describe a roof's angle which they call Pitch. For this roof there is a 12 inches rise for every 12 inches of run, so a roofer would describe this roof as having a 12:12 Pitch.
Percent of Energy Available for different Roof Orientations and Tilt.
The National Renewable Energy Laboratory (NREL) has been tracking the amount of Peak Sun Hours available for different roof orientations at locations throughout the United States.
By referring to the graph at a location nearest your installation, you can determine what percent of the total annual available solar energy your roof can capture.
ROOF ASSESSMENT RESULTS
Peak Sun Hours available for this roof surface.
Our example roof is located nearest the NREL Aurora Municipal, IL monitoring station. So we’ll refer to that Annual Insolation Chart to determine our roof’s surface available peak sun hours.
Annual Insolation is a function of panel orientation.
As you can see from the chart, the optimal orientation would be 182 Degree Azimuth and a 34 Degree tilt. This would capture the maximum amount of Peak Sun Hours for this location (1,596 PSH/Year)
Our roof has a 165 Degree Azimuth and a 45 Degree tilt, so per the chart, we can expect to capture 97.6% of the available solar energy which is a 2.4% reduction.
From our roof shading analysis we know we’ll then lose another 6% of the annual insolation which will take us down to 1,464 PSH/Year.
Shading Analysis Tool
One of the best tools for accurately determining how much solar energy you’re going to lose to shading throughout the year is a Solar Pathfinder.
The Solar Pathfinder uses a clear plastic dome to reflect and overlay the southern sky on an underlying paper grid. This grid shows the percent of time the sun occupies each part of the sky throughout the year.
By looking straight down at the reflective dome you trace on to the paper grid the parts of the sky that will be blocked by objects at certain times of the year.
Shading will block 6% of available annual solar energy for our example roof.
This panorama view from the base of the roof area gives you an idea of the surrounding trees and structures.
The results show that most of the shading will occur in the early morning winter months, and late afternoon throughout most of the year.