When you buy an 8 oz bag of potato chips, you generally expect the bag to actually contain 8 oz of chips.
Following the same logic, if you have 6 kW of solar panels installed on your roof - about an average size system in the United States - it would seem reasonable to expect that system to actually generate 6 kW of electricity on a clear sunny day. But it works a little differently with solar panels.
In practice that system will rarely, if ever, generate 6 kW of electricity even on the sunniest day. Before you call your solar installer and angrily demand a refund, you should know that this is normal, and even part of the system design.
A good solar installer will take the time to explain this, but in case they didn’t or you’re doing some research before purchasing a system, here are the reasons why solar panels will usually underperform their rated output.
If you ever take a look at the specification sheet for a solar panel that you plan to buy, you will find acronyms. Lots of acronyms.
An important one to understand is STC, which stands for Standard Test Conditions. The nameplate wattage rating (ie. the advertised wattage) of a solar panel is based on this test, which measures the panel’s output at a solar cell temperature of 25°C and a light intensity of 1,000 W/m² (watts per square meter).
25°C is a little higher than room temperature, and the critical thing to know is that this is a measurement of the solar cell temperature, not the air temperature.
Why is this important? When a dark solar cell is sitting in the sun for a few hours, it will get hot - a lot hotter than the surrounding air temperature. If a solar cell is operating at 25°C, the air temperature is likely going to be quite a lot cooler.
A bright sunny day with cold air is the ideal operating condition for a solar panel, because solar cell efficiency drops as it gets hotter. In the real world, perfect conditions like this are rare - think of those gloriously crisp, sunny days in late winter or early spring. But solar panels cooking in the hot sun are the norm, and it’s a key reason why your 6 kW system will rarely produce 6 kW.
Instead, you should look at the PTC, NOTC, or CEC test ratings listed on the specification sheet. These tests try to reproduce more realistic conditions by measuring the air temperature instead of the cell temperature. The NOTC test also lowers the light intensity to 800 W/m².
Different manufacturers will choose different test standards (whether PTC, NOTC, or CEC), but in every case these power ratings will be lower than the STC ratings, and are a more realistic estimation of how you should expect the panel to perform on your rooftop.
Bottom line: as a rough guide, the best you can expect your solar panel to perform is about 75-80% of its nameplate rating. That means if you have a 350 watt panel, you can expect it to generate up to about 260 watts on a good sunny day.
If all this seems a little complicated, it really isn’t. Once you understand these few concepts, you’ll know how to decode the specification sheet for any solar panel you plan to buy. For an indepth guide, check out our article on solar panel specifications.
Have you noticed that your knees don’t work as well as they did when they were young? Solar panels are like that too. As your solar panels age, they will generate a little less power every year. Older technology solar panels (such as those deployed before 2000) might lose 1% of their output every year, but current solar panels on the market generally perform twice as well, or even better.
For example, a new SunPower solar panel is warrantied to lose only 0.25% in efficiency per year, and Panasonic panels are warrantied for 0.26% of loss per year.
Another thing to know is that degradation is higher in the first year, and slows down after that. For SunPower, the first year degradation is warrantied to be 2%.
This means that if you buy a 360 watt SunPower panel, after one year it may have a maximum output of 352.8 watts. The next year, the output may drop to 352 watts, and 351.1 watts the year after.
SunPower is a premium brand, and you can expect budget panels to have higher degradation, but generally not much worse than 0.5% annually.
Why do panels lose power over time? Continuous exposure to light and temperature fluctuations can cause wiring to weaken or corrode, or panel discoloration that reduces the amount of light hitting the cells, or the silicon to degrade. The fact that panels can sit in the harsh elements from winter to summer for decades and still work almost as well as they did when new is a testament to how awesome solar technology is.
Let’s say that your solar installer proposes a system with enough panels to generate 6,000 real-world watts (ie. PTC or NOTC rated output). You might expect them to specify an inverter with the capacity to match - something like the SMA Sunny Boy 6.0, which can handle 6,000 watts.
However, in most cases, the installer will recommend an inverter with a lower capacity. For example, the installer might recommend the Sunny Boy 5.0, which can handle only 5,150 watts.
Wait a minute! Doesn’t that seem like a bad thing? After all, if you have 6,000 watts of solar panels available, doesn’t it make sense that you would to be able to use all of that output?
Not necessarily. The reason is that you might hit the maximum output of your panels for a few minutes on any day, or a few weeks out of the year. Again, this is because performance is limited by heat, and the ideal climate conditions require clear skies, cold temperatures, having the sun high in the sky, and even low pollution. (In many big cities, the summer skies are hampered by haze from ozone and nitrogen dioxide pollution).
A lower output inverter is cheaper than a higher output one, so you can save money by sacrificing a little solar panel output for the few weeks out of the year.
This is known as inverter clipping or the DC-to-AC ratio, and it’s a normal part of solar system design. In fact, the example given above - using a 5 kW inverter with 6 kW of solar panels is a 1.2 DC-to-AC ratio, which is a pretty common design. Here’s an example from my system:
This happened on a bright, clear day on April 25th when the high temperature was 32°F - perfect conditions. As you can see, for a couple hours in the middle of the day, the production curve is flat, indicating that the microinverters in my system hit their maximum output, even though the solar panels could have generated more electricity. And I’m completely fine with that, because perfect days like this happen only a few times a year. Most of the time, on hot summer days, my system output stays just below the clipping limit.
Your solar installer will calculate a DC-to-AC ratio according to your local climate, and take into the account the cost-benefit of upgrading to a more powerful inverter. If you’re curious about their design proposal, feel free to ask. You could have them price out a larger or smaller inverter to help demonstrate why inverter clipping is a normal part of the design that can help save you money.
If your solar panels were never cleaned dust, pollen, and even bird poop would slowly accumulate on them, reducing the amount of light that reaches the solar cells and eventually significantly impacting the amount of electricity they produce.
Fortunately, for most locations in the United States, there’s enough rain or wind to keep solar panels clean enough that you never have to bother cleaning them. However, droughts are happening in the US with increasing frequency, and many dry climates throughout the world, such as the Middle East, experience dust and sand storms on a regular basis. If that’s your situation, periodic solar panel cleaning is a necessity.
If you notice that your solar panels don’t hit the same daily peak power that they used to, you might want to take a look to see if they look a little dusty. If it hasn’t rained in some time, you might need to think about getting some cleaning materials or even paying a service to do it for you, especially if you’re not comfortable on a ladder.
All it takes to clean solar panels is plain water, a bucket, and an appropriate tool. Read our article on solar panel cleaning for tips on equipment and how to do this yourself.
If you have 350 watt solar panels, don’t expect them to produce 350 watts of electricity, even on the best sunny day. This might seem like a ripoff, like getting fewer potato chips than you paid for, but it’s actually a perfectly normal thing.
When you’re reviewing a solar proposal, don’t be afraid to dive into the specifications and ask your solar installer about the technical details. A good company will be happy to answer your questions, and it’ll help you better understand what you’re paying for.