Solar Panel Output – Jo-Ann Perez

What is Solar Panel Output?

Solar panel output is kind of a mystery. You’ve might’ve read up on solar panels and seen solar panels advertising 270 watts or 60 solar cells, but what does it all mean?

These numbers refer to the size and output of solar panels and it’s definitely something to keep in mind when you are first looking into your solar system. Let’s look at what we mean when we say solar output, what a solar panels wattage and cell count mean, and how these affect the output of the solar panels.

Cell Count vs Wattage

When we discuss output of the solar panel, we usually use its wattage. For residential applications, a typical solar panel is about 260 – 270 watts, meaning that in perfect conditions that solar panel could produce 260 watts of power in a given instant (for reference, an LED light bulb uses about 10 watts).

The number of cells (a solar cell is actually what creates the electricity) in a solar panel determine its size and wattage. Most residential solar panels are composed of 60 solar cells, each producing 5 watts each, and is about 3 feet by 5 feet. Some commercial solar panels have 72 cells, allowing a single panel to produce more electricity, but they are much taller.

However, just because a solar panel has more solar cells and produces more energy doesn’t make it more efficient. When comparing solar panels, also look at the efficiency of the panel – the amount of sunlight that hits the panel which is turned into electricity. Most residential solar panels are about 18% efficient – though they can typically range from 15% to about 18%.

Nominal Output of your Solar Panels

Solar panels are tested using STC – Standard Test Conditions. STC is, in essence, the perfect situation for the solar panel, in that it would be a bright, sunny, cloudless day with high irradiance levels – ie a lot of strong sunlight.

The wattage printed on the backs of solar panels, referred to as the “nominal wattage” is the output of the solar panel in these perfect conditions. In the real world, it’s possible that the production could be much lower, due to shading or weather.

A lot of homeowners are confused between nominal and real world output. You would think that the wattage printed on the solar panel is what it would produce. That’s certainly possible if your roof had the perfect situation like in the test conditions, but it’s rarely the case. However, having a standard test condition is fairly important. This allows installers and homeowners to compare the efficiency of different solar panels using common testing procedures.

Real World Output

All solar panels are installed in real world setting – not a lab – where the sun shines randomly and irradiance isn’t always super strong. In real world settings therefore, production won’t be as high as during testing. The age of the solar panels, dirt over the panel, and environmental considerations can play into this, including temperature, weather, and haziness (and how much sunlight it blocks).

Nominal Installation Size

We discussed nominal output for an individual solar panel. Now let’s look at nominal output for a solar installation.

A typical solar installation residential is about 5 kilowatts and is based on the nominal output of the individual solar panels. So, a 5 kilowatt system could be composed of 20 solar panels each at 250 watts a piece.

However, just like a solar panel, you can’t assume your solar system will be working at 100% efficiency at all times. As usual, environmental conditions, especially temperature, play a large role in the efficiency of your system. During test using STC, the temperature of the solar panels is at 25 degrees Celsius (about 76 degrees Fahrenheit) – a fairly low estimate. It’s a fact that as the temperature of a solar panel increases one degree Celsius, it loses about .4% efficiency. So, if a 270 watt solar panel increased from 25 degrees to 45 degrees (113 degrees F, which many in the southwest US see frequently), it would lose about 8% efficiency!

To account for these real world conditions, the National Renewable Energy Lab developed the PVUSA Test Conditions (PTC). PTC mostly accounts for this higher temperature of solar panels in the real world by increasing the temperature of the solar panel to 113 degrees Fahrenheit. Using PTC, a solar panel’s estimated output can be over 10% less than with STC.

Real World System Output

When you decide to move forward with your solar installation, solar companies will account for these “real world” conditions in the quote they provide you during the sales process. The quote will always include the nominal output of the solar installation in kilowatts, but also provide an estimate for how much electricity the system will produce over its lifetime (in kilowatt-hours), using data for typical weather and irradiance levels for the local area.

Solar equipment like inverters and wires are also not 100% efficient. Inverters are typically about 97 or 98% and there’s also ‘line loss’ – electricity lost during transmission through the wires connecting the solar panels to the inverter. Solar companies usually account for these losses in production as well, with what’s known as a ‘derate factor’. The derate factor is just a percentage, usually around 80%, that they multiply your production by which accounts for those losses.

So if you’re a homeowner looking to install solar, you need to discuss with your installer what efficiency losses their estimates take into account. Does it include efficiency loss due to hot summer weather? How about line loss and the inverter’s efficiency? What about dirt on the solar panels?

Now that you know how solar output is calculated, you have the knowledge to be sure that your solar installation will cover as much of your energy use as possible!