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Why Your Solar Installer Will Probably Oversize Your Array

By Kristy Hoare on in How Solar Power Works

Why Your Solar Installer Will Probably Oversize Your Array

On first thought, it may seem logical to have a solar array size similar to the inverter size, such as pairing a 6 kW array with a 6 kW inverter. After all, why generate excess power when the inverter will limit the power to its own rating? And yet, oversizing solar arrays is an extremely common practice. Most companies install a solar array that is 10-30% larger than the inverter rating. Why? Let’s find out.

Benefits of Oversizing a Solar Array

1. Actual generation from solar panels is lower than rated output

The manufacturer-specified power rating of a solar panel is seldom what the panel actually generates - just as the actual fuel efficiency of your car is almost always lower than the advertised efficiency. That doesn’t mean companies are openly lying to us, it’s just that the specified power ratings are only possible at Standard Test Conditions (STC).

The STC is a fixed, universally accepted set of laboratory conditions to measure the performance of solar panels. What’s important to know is that these conditions are the absolute ideal conditions, which can’t be achieved in the real world for a sustained period. 

Here are some factors that lead to the difference between advertised and actual power generation:

  • System Location: Your system may be located in an area with low sunlight intensity, limiting the power generation.
  • Roof direction and pitch: Your roof direction and pitch may not allow prolonged direct sunlight on your panels, reducing the maximum possible energy generation.
  • Panel cleanliness: Dirt accumulation on solar panels reduces the absorption of light and thus its conversion to electricity.
  • Panel degradation over time: The output from solar panels gradually reduces over time due to temperature and other factors.

Based on the above factors, a 6 kW solar array can generate close to 6 kW, or it can generate a measly 2 kW. It therefore does not make sense to have an equal-sized inverter, as it would be mostly running at a lower capacity, offering low efficiency and costing more.

That said, it is true that at times your solar panels will operate at full capacity, and a 6 kW system may actually offer 6 kW power. In such a case, if you have a 4 kW inverter, the excess 2 kW power is lost, and is called ‘clipped power’. 

But the important thing to note here is that there are very few occasions when solar panels operate at peak capacity. Even in places where conditions are ideal, the rated solar generation only occurs a few hours in the middle of the day. Of the total daily energy generation, this constitutes a relatively small portion, typically around 20%. Here is a graph that shows this:

2. Running the inverter close to its rated capacity offers high efficiency

Inverters are like workaholic humans - they work well when they are working at their maximum limit. Most modern inverters come with excellent efficiencies of 95% or more. However, that efficiency is achieved only when they operate at their rated capacity. The lower the DC input, the lower the inverter’s efficiency. Here is a chart by Fronius showing the changing efficiency of its Primo model based on input power:

Source: Fronius

You can see that based on the input power, the inverter’s efficiency can vary from 80.8% to 99.9%.

Let’s understand this with an example. Suppose our previously mentioned 6 kW inverter is paired with a 6 kW solar array, and it receives only 4 kW power. Since it is not running at full capacity, this inverter is now operating at a lower efficiency of around 90%. In other words, of the 4 kW DC power flowing into the inverter, somewhere around 3.5 kW comes out of it as usable, AC power. 

This means an hourly loss of about 0.5 kWh or a daily loss of about 2 kWh. This difference may not sound like a lot, but over the lifespan of the system, you could lose thousands of kWh of energy. Instead, spending slightly more on oversizing your solar array can offer high inverter efficiency, which offers excellent energy generation over the years, saving a lot more money in comparison.

3. Saves on inverter costs

How does changing the solar array size affect the inverter cost, you ask? Well, a system with an oversized array can also be called a system with an undersized inverter. If your roof is able to fit 6 kW of solar panels, you save money if you buy a 4 kW inverter instead of a 6 kW one.

4. You can avoid losing extra power by installing a DC-coupled battery

While it is true that the inverter clips any power higher than its rating, there is a way to capture that excess power generation too. In other words, for a 6 kW array and a 4 kW inverter, if the array does generate more than 4 kW power, this additional energy can be stored in a DC-coupled battery.

DC-coupled batteries, such as the BYD solar battery, are connected to the solar panels before the inverter, and are unaffected by the size of the inverter. This way, you can have the best of both worlds - high inverter efficiency as well as no lost power. Plus, you also enjoy the benefits of a solar battery, such as security from blackouts or electricity bill savings through load-shifting.

Are There Drawbacks to Oversizing Your Solar Array?

Aside from the occasional, nominal clipped (lost) power, oversizing a solar array generally does not cause any disadvantages. However, if the oversizing is done incorrectly, it may affect the system performance negatively. For example, you cannot use a small inverter with large, power-hungry appliances, especially in off-grid settings. So, if you are using a 2 kW inverter and you have a dryer that draws 3 kW power, you won’t be able to run the dryer even in full sunshine.

It is also important to check your inverter specifications for input and output voltage and current. Oversizing a solar array beyond the inverter’s input capability can damage the inverter. Most inverters on the market today are designed to work with at least 10-20% oversizing, which is the range most installers prefer.

Summing It Up

A customer buying a solar power system is spending thousands of dollars, and generally wants to extract maximum value from his system. As such, it may sound odd, perhaps even wasteful to install a solar array that’s significantly larger than the inverter’s output rating.

And yet, installers worldwide are installing systems with oversized arrays, thanks to obvious benefits such as better inverter efficiency and cost savings in the longer run. When you decide to explore solar power for your home, your installer will probably oversize your array. And if they don’t, it is a good idea to ask them for it.

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