What Solar Power Inverter Should You Get For Your PV System?

solar inverter types

Solar power inverters and micro inverters are an integral part of the residential solar system.

They are responsible for changing our solar system’s electricity production from direct current (DC) to the more useful alternating current (AC).

Without inverters, we wouldn’t be able to put electricity back into the grid! Currently, there are 3 main options for inverters and when I was a project manager for a nationwide US solar company, homeowners often asked if they could have micro inverters, a high-efficiency inverter, installed in their system. With a desire to squeeze as much energy as possible from their system, many homeowners were interested in this technology.

Micro inverters do produce more power than the standard ‘string’ inverter, but at a higher cost. So really the question is, “With my specific roof, does it make sense to install micro inverters?”

To answer this question, let’s look into the different types of inverters, the pros and cons of each, and then compare and decide which one is best.

First, a Brief Introduction to Solar Power Inverters

As mentioned, solar creates DC electricity. The grid, however, uses AC. Inverters’ job is fairly simple: they change the electricity your solar system produces from DC to AC.

The vast majority of homes with solar are still connected to the utility grid, so inverters are a necessary component of the system – they allow you to use the electricity you produce in your home as well as pump your excess electricity back into the grid so others can use it! (For more information on the grid and net metering, see our article What Is Net Metering? Breaking Down the Controversy.)

If your home is not connected to the grid (let’s say you live in an off grid cabin high in the mountains), you might not need an inverter, as special appliances are available that run off DC. But for those of us on the grid, inverters are required!

String Inverters aka “Old Faithful”

Most solar systems on homes today use string inverters due to their low cost and simplicity. With string inverters, all the panels in the solar system are plugged into each other (like a daisy chain), then connected to a single inverter. If you live in an area with a lot of solar, you’ve probably seen these inverters before – they’re usually installed on the exterior of the house, next to the existing electrical panel.

A typical string inverter installed on the exterior of the house, with safety disconnect underneath.
Source: Glen Ivey via Flickr, licensed for commercial use

 

String inverters are prevalent because they are relatively simple and low cost. Outlined below is a cost comparison of 5,000 watt inverters (a size that would cover a typical residential installation of about 20 solar panels) from three leading inverter manufacturers. As you can see, average cost hovers around $1900 – a good deal when compared to micro inverters.

However, with this low cost, there are drawbacks – mostly around string inverters’ limited functionality.

Since all the panels go to a single inverter and therefore function as a single entity, string inverters work best when the solar face the same direction and are placed at the same angle. Because your solar panels function as a single entity and aren’t optimized individually, your system can only produce as much energy as your worst producing panel.

For instance, let’s say it’s a beautiful day and your solar system is producing like you’ve never seen before. A few minutes go by and you see a little cloud come in and just hover over your house, but only covering a few panels.

You wouldn’t think this would make a big difference right? Most of your panels are still bathed in sunlight! But with a string inverter, this is a big deal. Even if just a couple of your solar panels are shaded, your production drops significantly, sometimes up to 60%!

A good candidate for a string inverter. All the solar panels are located on one section of the roof, with all the panels facing the same direction and the same angle.

Source: Alan Light via Flickr, Creative Commons license

In addition, because your solar panels work as a group and the inverter provides no data on the performance of individual panels, if your system production drops and you suspect a faulty panel is the culprit, troubleshooting can be more time consuming, as each panel must be physically tested for proper functionality.

Keep in mind though that even with these constraints, for the cost, it’s hard to beat a string inverter!

Micro Inverters – Great When You Need Them

Micro inverters, instead of having a single main inverter, are small inverters placed under each solar panel that optimize production for that individual panel. So instead of your entire solar system working as a single entity, micro inverters work at the panel-level.

An Enphase Energy micro inverter bolted to the railing and ready to be hidden by the solar panel on top.

Source: Maury Markowitz via Wikimedia Commons, Creative Commons license

Because micro inverters optimize each panel individually and can handle complex roofs better than string inverters, they are best suited to roofs with regular shading from trees or nearby buildings, or roofs with numerous angles or directions, or in areas that are especially cloudy. Micro inverters also track each panel’s performance, which allows homeowners and solar companies to watch for issues at the panel-level, something string inverters simply can’t do.

You might be thinking “Well, why not just put micro inverters up on every system? It certainly can’t hurt!” From the perspective of production, that’s certainly true. Even for a simple system with no shading and great weather, you’d probably see a little increase in production with micro inverters over string inverters. However, micro inverters are more expensive and that additional cost might not make sense for all situations.

Enphase Energy, the most well-known manufacturer of micro inverters, offers their residential M215 micro inverter for about $125 each. Going back to our previous example of a 20 panel system, the total cost of all the micro inverters would amount to $2500 – about $600 more than typical string inverters.

Due to the increased cost, micro inverters are mainly used when its simply not feasible to use string inverters, either due to shading issues or the structure of the roof.

Power Optimizers – Bridging the Gap

So, have you decided between a string inverter or micro inverters yet? Before making your decision, let’s look at one more inverter type that is currently on the market.

In the last few years, a new technology has bridged the gap between the cost-effectiveness of string inverters and increased performance of micro inverters. This technology, known as power optimizers, uses a single inverter on the side of the house (similar to string

Put simply, power optimizers allow each solar panel to produce as much energy as possible by adjusting the power output of the solar panel to match the current needs of your home as well as the operating environment of the panels.

Power optimizers, like microinverters, monitor each panel’s performance individually, which allows for faster (and therefore cheaper) troubleshooting and maintenance.

Typically power optimizers are equal to or slightly less expensive than micro inverters. SolarEdge, a leading manufacturer of power optimizers, manufacturers the P300, which goes for about $65 each. For our example of a 5000 watt, 20 panel system, this equals $1300 for the power optimizers. Tacking on $1200 for the necessary inverter (SolarEdge SE5000), the total cost for the inverter system is $2500 – about even with a micro inverter system.

You might be asking, “What’s the practical difference between power optimizers and micro inverters, they seem very similar!” That’s a good question. While they function very differently, their end results are very similar – squeezing more power from your system than you could with a string inverter. Both are great options to discuss with your installer!

So Which Inverter Should I Get?

Which inverter to get? Today we’ve outlined some basic guidelines, but in the end, it’s really a decision you and your installer need to make together after studying your roof. If your solar system is very simple – your panels all face the same direction and same angle with no real shade issues – keep it simple and cheap and go with a string inverter.

However, if your roof has multiple roof planes or if you have shading issues, micro inverters or power optimizers will likely fit your needs better. Both are excellent options that can produce energy under less than perfect roof conditions.

Really, it comes down to cost. The solar industry progresses fast and cost changes among different installers, so when you are ready to install and are getting quotes from a few different companies, be sure to discuss concerns about your roof, its structure, and any shading issues so you can find the inverter that best suits your needs.

 

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