If you have been hunting around for a solar power system, a few of the solar installers might have given you a brochure/sheet of paper that has a picture of the what the solar panel looks like, followed by a few key bullet points and on the back are some numbers and terms that I (working in the solar power industry for 3 years) still I am trying to figure out. Unfortunately this is all the solar panel manufacturers give the solar installers to sell the panels so don’t blame them.
It’s hard to believe, but this is the manufacturers core piece of marketing that aims to convince you that you should buy their panels. Apart from comparing which data sheet has the prettiest picture, for someone like me without an electrical background it’s meaningless.
Recently, I worked out what some of it means which I would like to share with you.
When the solar power installers talk about what panels you could go with, they commonly talk about a 190W or 250W panel or somewhere in between. This number refers to the maximum power rating of the panels under Standard Test Conditions (STC) which is the equivalent of a strong sun (measured as Irradiance of 1000W/m2), while the solar panels are at a temperature of 25°C.
STC is very unlikely to ever occur because solar panels on your roof are generally going to be 20°C hotter than the ambient temperature. When do you ever get a blaring sun when it’s only 5°C outside?
Therefore the manufacturers have been kind to us and given us a Nominal Operating Cell Temperature (NOCT) rating, which means Irradiance: 800W/m2, ambient temperature: 20°C and a wind speed of 1 m/s, which is a much more realistic scenario.
Below is a screen shot of a solar panel data sheet showing the STC max power rating (250W) and NOCT rating (183W).
Most data sheets now have the maximum power at NOCT given to you. Some of the older ones don’t. Interestingly two different brands of panels might have the same STC max. power rating, but then one will perform better under the NOCT max power rating.
So when comparing solar panels with solar panels this is one key stat you can pick up on a potentially make a better choice when buying solar panels.