How much power you can get out of a given panel? We think lots of people think you can get what the manufacture rates the power at but unfortunately your highly unlikely to ever get that amount of power from the panel.
We found that in the process of selling solar panels. We regularly get asked why “my new 100 Watt panel is producing 70 Watts, is something wrong?” The short answer is NO, but let’s look at why this happens across the solar industry.
Standard Testing Conditions (STC)
A solar panel is first tested right in the factory. As the panel comes off the production line, a worker (or robot) places the panel on a “flash table” and hooks up the positive and negative leads to a measuring device. The panel is then “flashed” with fake sunlight. The connected electronics record a number of performance values including the panel’s voltage (volts), current (amps) and power (watts).
These testing conditions are called “Standard Test Conditions” or STC.
The electric output performance of crystalline silicon and thin film PV modules are generally measured under standard test conditions (STC), ensuring a relatively independent comparison and output evaluation of different solar PV modules.
The following is testing of a Lensun 200w solar blanket.
But what’s standard about them?
- 1.Temperature of the cell – 25°C(77F). The temperature of the solar cell itself, not the temperature of the surrounding.
The important factor here is that the hotter it gets, the less efficient the solar panel becomes.
Solar Irradiance – 1000 Watts per square meter. This number refers to the amount of light energy falling on a given area at a given time.
1000 watts/meter^2, this is some pretty intense sunshine. This is done on a horizontal panel with the “sun” directly overhead for optimal angle of interaction.
Mass of the air – 1.5. This number is somewhat misleading as it refers to the amount of light that has to pass through Earth’s atmosphere before it can hit Earth’s surface, and has to do mostly with the angle of the sun relative to a reference point on the earth. This number is minimized when the sun is directly above as the light has to travel a minimum distance straight down, and increases as the sun goes farther from the reference point and has to go at an angle to hit the same spot.”
This condition approximately represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun. However, these conditions are rarely encountered in the real-world.
The manufacturer uses STC testing to sort panels by power and ensure that similar panels are sold and used together.
“STC stands for “Standard Test Conditions” and are the industry standard for the conditions under which a solar panel are tested. By using a fixed set of conditions, all solar panels can be more accurately compared and rated against each other.
Relationship of efficiencies
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.
What does this mean in the REAL WORLD for you?
The solar panels won’t perform the way they do in lab-controlled STC in the real world.
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.
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.
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