What are inverter clipping losses in a solar array?
One of the more technical details in the design of a home solar system is called inverter clipping. There are some misconceptions, so this article explains what you need to know.
If you’re a more detail-oriented (or perhaps nerdy) person, you might spend a lot of time studying the technical specifications of your home solar system.
Maybe you want to better understand a system you already own, or you’re comparing quotes from installers and trying to make the best choice. Once you get beyond the basics like the system output and inverter type, one of the concepts you might encounter is called inverter clipping, which is closely related to the DC/AC ratio of the system.
Inverter clipping in a system occurs when the output of the solar panels exceeds the capacity of the inverters. When this happens, the inverter limits - or clips - the power output to what the inverter can handle. In your solar array’s monitoring system, this will be visible as a flat-top graph. You can see an example at the top of this article.
When clipping happens, you are “losing” electricity that your solar panels would otherwise generate. That sounds bad and something you’d want to avoid, right?
Not necessarily. In fact, a well-designed system that experiences some inverter clipping will often be financially smart because using a cheaper, lower capacity inverter will save you more money than the value of electricity you would gain by eliminating clipping with a more expensive inverter.
To understand this better, there’s a few concepts you’ll need to know that this article will explain.
A quick introduction to solar system specifications
Before we get into that, you need to know a few basic technical concepts about a solar panel system.
The first is the output power of the panels themselves. There are two numbers to know: the nameplate output rating, and the realistic output you can expect.
The nameplate rating is determined by testing the panel under standard test conditions (STC). This is called the STC rating. For example, if you have a solar panel advertised as 350 watts, that’s the STC rating.
In the real world your solar panels will rarely, if ever, work at their STC rating. Solar panels work less efficiently in the heat, so unless it’s a really cold and sunny day, they will generate less than the STC number. There are a couple alternative ratings that try to measure realistic solar panel performance, and you can find them on a panel’s specification sheet as NOCT, PTC, or CEC.
You can read my article on understanding solar panel specifications for a more detailed explanation.
The other relevant specification is the output rating of the inverter. Look up the datasheet for your inverter and find its AC output power in watts. If there are separate specifications for peak and continuous output, we’re interested in continuous output.
You can ask your installer for copies of the equipment datasheets, or look them up at the manufacturer’s website.
What is DC/AC ratio in a solar array?
The DC/AC ratio of a solar system is the total output of the solar panels in DC watts divided by the total output of the inverters in AC watts. This gives you an idea of how much inverter clipping you can expect your system to experience under ideal weather conditions.
A common DC/AC ratio is 1.2, but this isn’t the “right” number for all systems. There isn’t a single, universally correct DC/AC ratio that you should ask your solar installer to meet. Instead, the number for your system will depend on your specific roof conditions, your local climate, and the pricing of the equipment.
Calculating DC/AC ratio
As mentioned above, you’ll need to know the rated output of the solar panels and the inverter (or inverters, if you have microinverters or more than one string inverter) to calculate the DC/AC ratio. The solar panel output is easy: it’s just the advertised wattage.
The output power of the inverter might not be obvious, so you may need to look up the datasheet for your inverter model. You can find this on the manufacturer’s website. For example, here are the specs for my inverter, which is an Enphase microinverter:
You can see that the output is 215 watts. Because this is a microinverter, I have 18 of them on my system - one for each solar panel. This means that the total inverter capacity for my system is 18 x 215 watts, or 3,870 watts in total.
My solar panels are 260 watts each, for a total of 4,680 watts.
To calculate the DC/AC ratio from these numbers, just divide them like this:
This means my DC/AC ratio is 1.21, which is close to the typical number. But is this the “right” number? The answer is a little complicated.
By the way, the example above uses microinverters, but the equation is the same if you have a single string inverter too.
Why does inverter clipping happen?
If it’s sunny all day and you don’t have any shading on your solar panels, your power output will look something like this:
The graph of your power production will be a nice, smooth curve from sunrise to sunset. On a day like this, your solar panels are generating lots of electricity and your inverter system is turning all of it (minus a little lost in the DC-AC conversion) into clean AC power.
As mentioned above, solar panels are less efficient when they’re hot. You can know exactly what this efficiency loss is by looking up the temperature coefficient on the datasheet. For every 1°C increase in temperature, the efficiency of your panel might drop by 0.25-0.50%. This also means that your panels will become more efficient in cold weather.
Because of the effect of temperature, solar panel panels will rarely hit their STC rating, and will instead be closer to their “realistic” rating that might be listed as NOTC, PTC, or CEC on the specification sheet.
Your best days for solar production might be in the spring and fall. Even though the days are shorter, the cooler temperatures in those months will help the panels perform better. This is also when you might experience the most clipping.
Here’s what that looks like:
You can see that the curve is smooth until it hits a peak, and then the graph abruptly flattens. That’s the point at which my inverters are maxed out: even though the solar panels can generate more electricity, the inverters don’t have enough capacity to handle it.
Here’s why inverter clipping might not matter
On this particular day, there are about 3 hours in which I “lost” some electricity due to clipping. Isn’t this like losing money? That seems bad, right?
Not exactly. As mentioned earlier, clipping is a normal occurrence in most solar systems. Here are some cases when you can ignore inverter clipping:
- It only happens a few days a year.
- Upgrading to a larger capacity inverter would cost more than the value of the electricity you would gain by eliminating clipping.
- Solar panel degradation will mean that clipping will be less of a problem in a few years.
As you can see, it’s not a cut-and-dried answer like picking a “correct” DC/AC ratio. There are other variables to consider.
One factor is inverter pricing. Let’s take a typical microinverter as an example: the Enphase IQ 7. This is the current base model in the Enphase 7 series, and it has an output capacity of 240 watts. On the Enphase website, it retails for $140.
But what if you have a 290 watt solar panel? On a cold and clear day, your solar panels could generate close to their STC rating. If you had the IQ 7 microinverters, the output would be clipped to 240 watts. You could avoid this by upgrading to a higher capacity inverter, such as the IQ 7+, which has an output of capacity of 290 watts - enough to avoid clipping your 290 watt panels, even under ideal weather conditions.
The IQ 7+ retails for $187, or 33% more than the cheaper model.
Would an upgrade like that be worth it to you? Finding out the answer is a little complicated because you need to know the estimated value of the electricity you would generate, which is affected by climate and your utility rates, which in turn could be a flat rate or have a time-of-use adjustment.
Using PVWatts to estimate the effect of DC/AC ratio on your inverter clipping
Fortunately, there’s a tool you can use to estimate the effect of DC/AC ratio on the value of electricity that a system will generate. It’s a free tool developed by the National Renewable Energy Laboratory called PVWatts.
It’s available online. When you go to the website, just enter your address and provide a few details about the solar array. On that screen, you’ll see an option called “Advanced parameters”. Click on it, and you’ll see some extra fields like this:
You can enter your DC/AC ratio and the other parameters for your array, and then click to go to the next page. You’ll see your estimated production in a table like this:
PVWatts uses climate data and the details of your system to estimate how much electricity you’ll generate in an average year and how much that will be worth in dollars.
You can use this to understand how your system will be affected by inverter clipping. Simply run it again with a different DC/AC ratio, and you’ll see how your electricity production will change.
While this tool is a good to estimate inverter clipping, it has limitations. First of all, it’s based on historical climate data, so there’s no guarantee that you won’t experience abnormal weather in the years ahead.
It can also only estimate the value of your electricity based on a fixed price. If you have a TOU plan, this tool won’t be able to factor that in.
Still, it’s a quick way to see how you might be affected by inverter clipping. In many cases, I think you’ll find that the effect is minor and not worth an inverter upgrade to avoid.
Inverter clipping might be a problem only when the system is new
Even if inverter clipping is noticeable when the system is new, the problem might go away as the system ages. This is because solar panels slowly degrade and generate less electricity as they get older.
The amount that your panels will degrade is small: usually it amounts to a loss of less than 0.5% efficiency every year. The worst you can expect with a solar panel you purchase today is that it will generate around 80% of its original output after 25 years, while the best panels will still be generating >90%.
Depending on how much degradation you experience, inverter clipping might go away after a few years as your solar panels no longer reach the high output they did when new. That’s the case for me: while I still get very minor clipping on bright and cold days, I had to go back to 2013 to find a graph with really noticeable clipping. Now that my panels are 8 years old and have degraded somewhat, clipping is less of an issue.
How can you determine if this will be a problem for you? Solar panel degradation is predictable and described in the manufacturer’s power warranty. You’ll need to read the fine print, but the 25 year power warranty will state how much efficiency you might lose each year and what the lowest output should be after 25 years. If you have a premium panel such as LG or SunPower, the degradation will be small, and clipping will be a problem for a longer time.
When should you worry about inverter clipping?
There are many reasons why you probably shouldn’t worry about inverter clipping, but this assumption is based on your installer putting together a good system design for you. Two installers can look at the same roof and propose completely different layouts, so there is some subjectivity in the process. If their design assumptions are off, you could end up with more clipping than anticipated.
To sanity check this, take a look at the DC/AC ratio of the proposed design, and test the number in PVWatts. While
solar installers have more advanced software, PVWatts is still a good way to do a rough test of any system design. At the very least, it will help you have a more informed discussion with your solar installer.