Inverter clipping: Design flaw or smart investment?

Inverter clipping: Design flaw or smart investment?

August 9, 2021 Uncategorized 0
Bell curve | Wolf Track Energy

New solar users often wonder how they can avoid inverter clipping. Luckily for them, they don’t need to. 

Inverter clipping is, simply put, a situation in which the output of a solar system is limited by the size of the inverter, which is a device that converts direct current (DC) produced by panels into alternating current (AC) for use in your home or business. 

The phenomenon is commonly represented by a bell curve with a flat top. That shows the output of a solar array gradually increasing throughout the day until the inverter is maxed out. This is mostly likely to happen during the peak of a sunny day, when your solar array is soaking up a lot of sunlight.

Clipping may seem like a design flaw or sloppy planning on the part of the solar contractor. But in reality, solar professionals often plan for some inverter clipping because it makes more financial sense to install a smaller inverter rather than one that can accommodate more than enough power produced by the panels.

In fact, most installation projects have an inverter that has a smaller wattage rating than the solar panel array it’s connected to, according to EnergySage

Let’s dive into the ins and outs of inverter clipping. 

Inverter ABCs

As I mentioned before, inverters turn DC power into AC. That’s important because the outlets at your home or business offer AC. Batteries, on the other hand, provide DC power. We covered some electricity basics in a previous blog.

Inverters generally fall into two categories: string and micro. 

“String” inverters take the power produced by a group of panels and converts it into AC. “Microinverters,” meanwhile, are attached to individual panels rather than an entire array. That helps solar arrays produce power even when it’s partially shaded by trees or other objects, and microinverters allow for better system monitoring. But they are generally more expensive. 

Enphase Energy IQ7 microinverter. Image provided by Enphase.

Like solar panels, inverters are rated in watts, which helps installers pick the right one for the size of a solar array. 

Manufacturers warn against exceeding the DC-to-AC ratio, also known as the inverter loading ratio. That dictates how much larger your solar array can be compared to your inverter. If your solar array is 10 kW and your inverter is rated at 8.3 kW, you have a ratio of 1.2. 

EnergySage says the “majority of installations will have a ratio between 1.15 and 1.25.” That means solar arrays are commonly larger than their inverters. The Energy Information Administration says the ratio is usually between 1.13 and 1.3 for individual systems.

But why would you want your inverter to be smaller than your panels?

Inverter investment

I’m going to use a hypothetical situation to help illustrate the financials of inverter clipping.  

Imagine you are in the market for a water pump. You find one that you like that’ll give you five gallons of water per day. Next, you’ll need a bucket to carry that water home. 

The only problem is there’s no five gallon bucket available. There’s a 4.5-gallon bucket for $100, and a six-gallon bucket for $300.

Now, your instinct might be to pony up for the more expensive six-gallon bucket because it’ll fit all of the water you’ll pump each day. But when you pencil out the math, you’ll find that it makes more financial sense to leave behind that half gallon of water every day and instead buy the smaller bucket. 

That, in a nutshell, is how solar professionals often think about inverter clipping.

Residential rooftop solar panels | Wolf Track Energy
Residential solar array in Duluth, Minn.

Peak conditions

According to AC Solar Warehouse, inverters “spend a very small proportion of their life operating at ‘peak conditions’” and achieving the best return on investment may involve buying a smaller device.

And according to Aurora Solar, having a DC-to-AC ratio that’s greater than one “allows for a greater energy harvest when production is below the inverter’s rating.” Moreover, as the EIA points out, solar panels degrade over time and lose power. After 20 years, they may only produce 90% of the power they did when they were new, so DC-to-AC ratio will decline as they years go on.

“PV system designers also take these considerations into account and size the inverter to be large enough to capture most of the output of the system over its lifetime, but not so large that the incremental increase in inverter capacity becomes uneconomic,” the EIA writes.

In other words, buying a large enough inverter to fill out the flat part of your power bell curve may not be worth the extra cost. And if you’re determined to avoid clipping but you buy an inverter that’s too large, you may lose some efficiency, according to altenergy.org

“On sunny days, the PV system might clip output by 1% or 2%, but very little energy is wasted when it’s designed with a 1.2:1 to 1.3:1 DC/AC ratio,” Solar Power World writes. 

A single-phase string inverter costs around 15 cents per AC watt, while microinverters cost 34 cents per AC watt, according to the National Renewable Energy Laboratory. While that might not seem like a lot of money, the costs add up quickly when you consider inverters are rated for thousands of watts.

Bottom line

Hopefully this explanation helps you understand why, on certain sunny days, you’re not seeing as much power from your solar system as you might have expected. In all likelihood, the company designing your solar system took into account the size of the inverter and decided installing a larger device wasn’t worth it.

If you want to learn more about solar energy, check out our other blogs here. And if you’re thinking about going solar, give us a call at (218) 302-5601.

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