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Solar Ratings Explained

Solar Ratings Explained

By David Rosner

Solar power is quickly emerging from the black smoke of the energy sector. It is becoming imperative that we see this growing technology less as a magic sun scoop and more as a realistic power production tool that meets real world expectations. The current disconnect between what we think solar does and what it actually does needs a little explaining. I’ll try to fill this gap by expanding our knowledge of solar panel ratings and how it relates to the power we need and use.


We at Goal Zero have made using solar power as easy as plugging in a cord. Lights flash, numbers scroll, icons blink, and the power of the sun sits at our fingertips. Unfortunately, our incredibly observant customers quickly realized that not everything adds up. We regularly get asked why “my new 30 Watt panel is producing 23 Watts, is something wrong?” The short answer is NO, but let’s look at why this happens across the solar industry.


Panels of the solar variety are rated according to STC, or Standard Testing Conditions. This is done in a lab, not even out where the sun hides! As solar panels are produced, they are all tested at the exact same conditions so we have comparable numbers to know what to expect.

Power is measured in Watts, and sunlight is measured in how many Watts are hitting the earth within one square meter (yes, we’ve gone metric for solar). STC has been designated at 1000 watts/meter^2, which is some pretty intense sunshine. This is done on a horizontal panel with the “sun” directly overhead for optimal angle of interaction. STC is also designated at 25 C (77 F). The important factor here is that the hotter it gets, the less efficient the solar panel becomes.

Finally, STC is calculated at 1 Atmosphere, or ATM. This is a standard accepted number (precisely equal to 1,013,250 dynes per square centimeter) representing how much all the air above us is pushing down on you. For solar, it is how much all the different air particles are bouncing and blocking the sun’s light from getting to us. It is important because when the sun is directly overhead, it goes through less atmosphere than when it rises and sets on the horizon.


So now we build these factors into a relationship of efficiencies. The sunny summer months are great because the sun is close to directly overhead, like in the lab. However, the increase in heat reduces panel efficiency. In the winter months the panels are cold, so more efficient, but the sun is lower on the horizon so it fights through more atmosphere to get to our panel. On top of this, most panels do not point directly at the sun except for a fraction of the day, reducing efficiency again. This compounds with voltage drop, the lost power as electricity moves through wires.

Essentially there is almost always something(s) that prohibits solar production at lab settings, a factor that pulls down the panel from its rated wattage.


It means we can expect to see between 67% to 75% of a panel’s rating when used. So if a 30 watt panel is in sunshine, you should expect 20-24 watts. This is the most you’ll see most of the time, less as the above factors add up to reduce power production. Even less as clouds and shadows block your panel. Less as dirt builds up on your panel. Each connection usually has a minuscule loss as well. All these individual nuances build into a visibly significant lower number than expected.


For this inconvenience, the entire solar industry would like to apologize for any confusion or perceived under performance. We are trying to adjust expectations through education. Some solar companies are even putting both rated watts and “true” watts on their panels to help solve this confusion. It’s like when auto companies started giving urban vs highway miles per gallon because no one was observing the potential efficiencies while commuting to work in stop and go traffic.

The “Bright” side: The opposite is true too. A solar panel placed at high altitude, in the cold, with the sun at mid-summer angles, while directly pointed at the sun, can produce above the rated watts for a panel! The other 99% of the time, however, you should expect less.