Microinverters and Power Optimizers – How Much Do They Boost Your Solar Panel Efficiency?

How to Use the PVWatts Loss Calculator to Adjust Your System Losses

Who wants to get more power out of their solar panels? If you can, you could even reduce how many solar panels you need in the first place.

Who could say no to that? The question of course is, how do you do it?

A Brief Overview of Power Optimizers and Micro Inverters

No matter what hardware you use to move electric current, there are always small losses of power. It’s just physics. One goal of system designers is to find ways to minimize those losses.

With solar energy technology, one of the best tools to improve the efficiency of your panels is to install microinverters or power optimizers. These are also referred to as “distributed power electronics.” What are these?

An “under-optimized” solar panel is made of modules, and each module converts solar energy into DC electric current. A module can lose performance because of several factors, such as:

  • Temporary shading
  • Debris like leaves or snow on the panel
  • Excessive temperature or other weather factors

If you’re using just a central or string inverter, one underperforming module can affect the whole solar array. Power optimizers and microinverters prevent this from happening and boost the performance of each individual module. They increase design flexibility, improve system performance monitoring, and boost solar energy capture.

As a result, they lower the system losses. The Loss Calculator on the PVWatts calculator webpage helps you figure out about how much your losses will decrease, and how it affects your overall performance.

PV Watts Loss Calculator

For a full tutorial on how to use the PVWatts calculator, click here.  (Note: If you’ve never used the PVWatts calculator before, you should read that first so you can get your first set of calculations, without any adjustments from microinverters or optimizers).

Now, when you reach the PVWatts calculator main page, you’ll notice next to the fourth space, to the right, an icon for the “loss calculator.” It’s circled in red in the screenshot below.

Find the Loss Calculator on the PV Watts main calculator page

The loss calculator includes a whole bunch of parameters that can affect your solar energy system losses. The use of microinverters and power optimizers affects a few of these parameters for the better.

Use this article to learn how to adjust them. Then, when you use the complete PVWatts calculator, you can see how much it improves your estimated AC current and energy cost savings. (These directions come from the “Modeling Microinverters and DC Power Optimizers in PVWatts” NREL report, from Feb 2015).

Here’s a screenshot of what you see when you click on “loss calculator.”

Learn what to adjust in the loss calculator when using power optimizers and microinverters

With microinverters and power optimizers, you’re going to adjust two of these terms – mismatch and shading. All the rest of the terms will be unchanged. Then, back on the main calculator screen, in the “advanced parameters” section, you’ll also want to adjust inverter efficiency. We’ll cover all this here, one item at a time.

Shading

Especially if your solar power system uses a central inverter, you can recover lots of lost power with optimizers and microinverters added on to your system if shading is a problem. If shading is never a problem and has no chance of becoming one, then you can probably lower this term to zero either way.

But remember – even if just a tiny portion of your panels get covered with shade at certain parts of the day, like from another object on your roof or a nearby tree branch, it can affect your whole system. The technical term related to this is the Shade Mitigation Factor – the SMF. A higher SMF means higher performing solar panels.

SMF – the annual percentage of shading losses that can be recovered by using distributed electronics

Here’s how it works.

If your panels get hit with shade, you have a power loss. With power optimizers and microinverters, your power losses go down. The SMF helps you calculate your new “shade loss percentage,” which is what goes in the space in the Loss Calculator.

Here’s the formula:

                  Shade loss (new) = Shade loss % (original) * (1 – SMF).

So, the original shade loss default is 3% (see Loss Calculator screenshot). With distributed electronics, the National Renewable Energy Laboratory (NREL) recommends an SMF of 0.33. But take note – this value depends on your specific situation and it can range from 0.25 to 0.4. It depends on the amount of shade, how it’s distributed across your panels, and on your solar array configuration. Consult a professional for a precise figure.

If we use the recommended 0.33 for the SMF, then (1 – SMF) = 0.67. Multiply that by the 3% original, and you get 2.01%. This is your new shade loss percentage, and you can put this in the Loss Calculator in the “Shading %” space.

Again – you can only do this if you use power optimizers or microinverters. Without them, and with shading, leave this term at the default of 3%. Also – if your panels will receive excessive shade at the location you’re considering, you might want to find a new location or adjust the configuration of your array. Once again – a solar power professional consultant can help with this.

And this matters – a lot! You’re talking about a huge difference in power production when spread out over 30 years. So get this right before you install.

Mismatch

When different solar modules perform differently, they experience “mismatch”. All the DC current has to reach the central inverter, and mismatch causes a reduced power efficiency.

This is actually the main problem microinverters were invented to solve. So if you’re using distributed power electronics, you can reduce the mismatch term to zero.

The Other Terms

All the other terms in the Loss Calculator will remain unchanged. Power optimizers and microinverters don’t affect most of these.

Microinverters do affect the wiring – in fact they eliminate losses in DC power transfer. But because there are now more AC wire transfers than before, the AC losses increase. Thus, the overall losses in the wiring remain about the same.

There is some evidence that distributed power electronics should decrease downtime, and therefore increase system availability by 1-2%, but the evidence isn’t yet conclusive enough to adjust this term.

So once you’ve adjusted the mismatch and shading terms, click “save” and you’ll return to the main PVWatts calculator screen. You should now see your losses a bit lower than the 14% default that was there. And remember – just a couple percentage points means a lot more power production over the 30-40 year lifetime of your solar panels.

Inverter Efficiency

Now, back on the main screen, you want to click on “Advanced Parameters.” Power optimizers and microinverters also affect the inverter efficiency. This term is discussed in more detail in the PVWatts Calculator tutorial article found here.

The whole point of distributed power technology is to improve efficiency. But as with everything, the specifics depend on the brand, the type of inverter, the age of it, and other factors.

You can see huge lists of all kinds of inverters on this Solar Power World page.   Each inverter has a different rating.

The California Energy Commission (CEC) has rated every inverter on the market, including microinverters, and their ratings are what you should use in this part of the calculator.

If you’re using power optimizers, the CEC hasn’t rated these, but the manufacturer should have. If not, you can safely assume a 99% efficiency with most newer optimizers.

See Your Improved Numbers

Once you’ve adjusted for those three values, finish the PVWatts calculator and see your results.

Ideally, you would do this calculation twice – once without the distributed power electronics, using the default Loss Calculator values – and once with them.

Then you can see the energy cost savings you’ll get each month, and factor this in to how much you want to spend on optimizers or microinverters, if anything.

And as always, you should consult a solar power professional before making big decisions like this. But with the PVWatts calculator and the Loss Calculator adjustments, you will be much more informed when you talk to that person than you would otherwise.

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