Microinverter Clipping with High Wattage Solar Panels

Solar Panel vs. Microinverter Rating

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We recommend and install systems frequently that use solar panels rated much higher than the rating of the microinverter (or inverter) to which they are connected. Microinverters take DC current generated by the solar panels and convert it to usable AC current for your house. Clients are often confused about this rating discrepancy, thinking it might waste electricity, or they are not getting what they are paying for. I have even had building inspectors challenge me on this point. It seems strange to do this. It sounds like I’m putting 93 octane gasoline in a 1986 Yugo.

Why is a higher wattage solar panel paired with a lower wattage microinverter?

The reason is really pretty simple. Solar electric panels almost never put out the amount of power for which they are rated (GASP!) Before you get too upset, let me explain…

Solar panels are rated using a figure called DC-STC, or Direct Current Power at Standard Test Conditions. The standard test conditions are a set level of solar irradiance at a given temperature, with the source of the energy perpendicular to the panel. The important things to understand are that the irradiance is virtually never greater than standard test conditions, the temperature is usually lower or higher, and most importantly that the sun is almost never perfectly perpendicular with the solar panels (for fixed mounted systems). In fact, there are exactly zero, one, or two days per year at a specific time when the sun is exactly perpendicular to any panel’s surface.

Let’s say you have a 270W solar panel. It can and will put out 270W, and even more if the temperature is low (solar photovoltaic panels produce more power at lower temperatures). The vast majority of each day is spent with the sun at some angle to the solar panel, and the angle affects the amount of solar energy absorbed. Clouds, haze, and smog all affect the power produced by solar panels. The better the conditions and the better the angle, the more power will be produced. This is why a 270W solar panel is perfectly fine to use with a 215W rated microinverter.

There are some other factors that make this relationship work. Microinverters are not 100% efficient, so no matter what, a 215W microinverter will need about 224W of power to generate it’s maximum output. Microinverters are also more efficient as they approach maximum power, so it is important to produce the most power possible at steep angles to the sun and in sub-optimal conditions.

What happens with the excess power if the solar panel is producing more than the microinverter can handle?

This is what’s known as “clipping.” Clipping simply results in the microinverter “wasting” the excess energy. It can’t turn the extra DC power into AC current. There is no danger of overloading. The panel simply delivers less power to the microinverter (more accurately, the microinverter simply draws less power from the solar panel).

I’m losing money if my solar panel output is clipped!

That is true, but a properly designed system will have minimal clipping. It comes down to the investment analysis. A small amount of clipping may result in pennies of lost energy in a given year. If the next larger microinverter costs several dollars more to purchase, there is no financial case for increasing the rating. The clipping is acceptable.

How much is too much power on a microinverter?

It has been my experience that 270W is about as high as you want to go to avoid clipping for south oriented solar panels on a pitched roof in Southwest Florida. Very little clipping will occur with this combination. We are starting to recommend 280W panels now, and we are pairing them with 250W microinverters if the orientation is ideal.

Below is a graph of  power produced by a recently installed 215W microinverter with a 270W solar panel. As you can see from this one week window, there were almost no points where the inverter reached maximum output. Florida doesn’t suffer as much from the clipping phenomenon because the temperatures here are generally warm, which reduces solar electric panel output. That’s why it makes sense, especially in this climate, to oversize solar panels relative to microinverters. Enphase, the leading microinverter manufacturer, has done studies that show 215W microinverters in warm climates can benefit from solar modules rated at 300W or more!

Microinverter Clipping with High Wattage Solar Panels

This microinverter shows almost no signs of clipping.

As we get further along in spring the solar energy resource will increase, but temperatures will also rise. These effects will offset each other, and we won’t see much clipping on this microinverter at any time during the year. For certain, the cost of installing a higher rated microinverter to offset clipping would not be justified. The amount of “extra” energy generated during low light conditions will greatly offset the clipped energy.

There is one possible argument for purchasing a microinverter with a higher rating that isn’t based solely on the investment analysis in today’s terms. It is plausible that solar panels will continue to increase in rating and come down in price. If that happens, you might be able to economically swap your panels for higher rated ones, and use the existing microinverter. I wouldn’t expect this to happen, but technically it is possible.

When you’re ready to become your own power company and use microinverters with high wattage panels, contact us!

 

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