Good news: Most people these days have a pretty good idea of whether or not their roof works for solar.

Bad news: They often don’t know that what makes a good solar roof has changed a lot in the past few years.

For years, roofs that were fairly shady, or faced east or west, couldn’t produce enough electricity to pay for themselves in an acceptable timeframe. But solar system prices have come down so much that solar pays for itself faster than ever, and roofs that weren’t “economical” before can work today.

I’m talking about roofs with a TSRF* between 60% and 80%, because 80% or higher has historically been considered a good roof.

If you’ve been told your roof is in the 60-80% range, or simply doesn’t work for solar, or if you ruled it out yourself, please think again. Obviously roofs in complete or near-complete shade still won’t work. Your roof has to have some sun after all. But if you’re not sure about your so-so roof, let us decide.

And it can be tricky to tell, in some cases. For example, a partly shaded, east-facing roof with a steep pitch probably won’t work, but a partly shaded, east-facing roof with a low pitch might.

I ran these numbers to show that roofs performing at 60% of ideal can work. These are real payback times for a 20-panel system in National Grid’s MA service area. While the 80% roof pays back faster, the 60% roof pays for itself in a very respectable time frame that many homeowners would find acceptable.

It may be time to re-think if your roof works for solar. If you’d like us to help, give me a call at 978-56-SOLAR.

(Note that to qualify for the Rhode Island Renewable Energy Fund (REF) grant, you must have at least 80% TSRF, and for the Mass Solar Loan at least 70% TSRF.)

* TSRF stands for the Total Solar Resource Fraction, which analyzes roofs as a percentage of the ideal (which would be 100%) based on a combination of the roof’s direction (azimuth in solar terms), pitch or tilt, and shading.