If you’re a homeowner who is thinking of going solar, or you’re already working with a solar installer and have a technical proposal in front of you, you’ll know that one of the key components in a home solar system is the inverter.
Solar panels generate direct current (DC) electricity, which is the kind of electricity you get from a battery. Grid electricity and your home, however, use alternating current (AC). It’s the job of a solar inverter to convert DC electricity to AC electricty that your home can use.
This is a pretty indepth topic, so I recommend reading our guide to solar inverters if this is all new to you. If you’re further along in the shopping process - maybe you’re looking at a solar installer proposal and scratching your head at the inverter line item - there’s a good chance you’ll come across module-level power electronics (MLPE).
The conventional type of inverter is a central unit that does the DC-to-AC conversion for a group of solar panels. For a small installation, like a home solar system, you’ll usually need just one central inverter. This type is called a string inverter because the solar panels are wired together in one or more strings, like Christmas lights. This is usually the least expensive inverter option. A string inverter is installed near your electrical panel and the solar system cutoff switch. This might be in your basement, on the side of your house, or in a garage.
For more complicated solar installations, an installer will often recommend module-level electronics. There are two types: microinverters and power optimizers, which are also called DC optimizers.
With both types, each solar panel is paired with one electronics package that is bolted to the rack adjacent each panel. That means if your system has 20 solar panels on the roof, you will have 20 MLPE units on the roof. Depending on the manufacturer, there is usually some type of electronics package that is also installed near your electrical panel.
A microinverter is a fully self-contained solar inverter. It’s about the size of a paperback book, and is bolted onto the rack underneath a solar panel. Each microinverter fully handles the DC-to-AC electricity conversion for one solar panel.
The largest player in the microinverter market, by far, is Enphase Energy. They have a large product lineup that is currently on its 7th generation, which has helped the company iron out buggy electronics that plagued early iterations of their products. Because it has such a significant presence in the residential inverter market, we wrote an indepth overview of the Enphase product lineup.
But Enphase isn’t the only microinverter company out there. For example, APsystems and Chilicon Power are US-based companies that offer microinverters comparable to those from Enphase.
Inside a string inverter, there are usually one or more units called maximum power point trackers (MPPT). An MPPT helps the inverter to better deal with shading that falls across only part of a solar array, a situation that can impair the electricity production of the whole array. This is especially useful if your system has multiple strings, which is common on many homes. For example, you might have one string of panels on part of your house, and the another string installed on your garage. Multiple MPPTs in an inverter would allow it to optimize the power collection from each string.
Power optimizer or DC optimizer inverter systems take this approach a step further and move the MPPT units outside the inverter and put them alongside each solar panel. They resemble microinverters in this respect, even though they don’t handle the actual DC-to-AC conversion. Like microinverters, a power optimizer is a small electronics package that is bolted alongside each solar panel in the array. Because each solar panel has its own optimizer, partial shading across the system has much less of a negative impact.
Both microinverters and power optimizers have the following advantages over cheaper string inverters:
Microinverters, the most advanced inverter option, have some additional advantages:
If you do a little internet searching, you might notice that different solar inverters are popular in Europe, Australia, and North America. While each solar installer company will have different preferences and supplier relationships, here are some of the most popular solar inverter companies.
Enphase offers a wide variety of products, including their IQ7 series of microinverters. Enphase offers only microinverters - there are no string inverters in their lineup. The microinverters come in different capacities that are suitable for solar panels from 195 watts up to 460 watts or more. (For the lowest wattage panels, Enphase still offers the earlier IQ6 series.)
With Enphase, system monitoring is provided by a separate unit called the IQ Envoy. It’s a WiFi-enabled unit that sends real time data to Enphase cloud servers, and lets you see monitor your system from a web page or smartphone app.
SolarEdge offers multiple inverters, but the homeowner will be looking at the SolarEdge single-phase HD-Wave inverter. The HD-Wave inverter looks like a conventional string inverter, but it needs to be paired with the SolarEdge power optimizers.
SMA is a German company that has been around since 1981, but has only recently started offering their Power+ optimizer product. It’s a DC optimizer comparable to the SolarEdge product. The Power+ optimizers are paired with an SMA Sunny Boy inverter.
APsystems is based in Seattle and manufactures microinverters. For the home market, you’ll be looking at their APsystems QS1 and YC600 microinverters. These microinverters take a unique approach: instead of one microinverter per panel, each microinverter will handle two panels (in the case of the YC600) or four panels (QS1). This has the advantage of cost savings from lower hardware and installation costs. As with Enphase, you can get a warranty up to 25 years, although the base warranty is only 10 years.
Chilicon Power is another US-based company (California) that is making microinverters for the residential market. Their CP-250E microinverter is aimed at the residential market and handles panels up to 345W DC. It also comes with a 25 year warranty.
Should you go with one of these lesser-known manufacturers? Both offer solid warranties, although with APsystems their 25 year warranty is an upgrade. One of the hazards of early microinverters, especially the second generation of Enphase products that suffered from a lot of failures.
This was a big knock on Enphase’s reputation, and they’ve been putting a lot of focus on product reliability since then. From what I’ve heard from installers, their reliability since the second generation microinverters is a lot better. It’s probably safe to say that other companies have treated Enphase’s experience as a cautionary tale, especially with the use of poor quality capacitors that were at the root of those Enphase failures.
The upshot is that it’s probably a safe bet to go with one of these manufacturers if your solar installer is recommending it, especially if you have the 25 year warranty.
I’ll again point to you our article on solar inverters to better understand that pros and cons of different types of inverters, but here’s a couple other things to think about.
There’s a relatively recent addition to the National Electrical Code (NEC) that requires the ability to close down the electricity in the wiring near a solar panel to 30 volts or less in an emergency. This regulation is there to reduce the electrocution hazard for first responders in an emergency. Here’s the actual language of the code:
690.12 Rapid Shutdown of PV Systems on Buildings.
PV system circuits installed on or in buildings shall include a rapid shutdown function to reduce shock hazard for emergency responders in accordance with 690.12(A) through (D).
(A) Controlled Conductors. Requirements for controlled conductors shall apply to PV circuits supplied by the PV system.
(B) Controlled Limits. The use of the term, array boundary, in this section is defined as 305 mm (1 ft) from the array in all directions. Controlled conductors outside the array boundary shall comply with 690.12(B)(1) and inside the array boundary shall comply with 690.12(B)(2).
(1) Outside the Array Boundary. Controlled conductors located outside the boundary or more than 1 m (3 ft) from the point of entry inside a building shall be limited to not more than 30 volts within 30 seconds of rapid shutdown initiation. Voltage shall be measured between any two conductors and between any conductor and ground.
While the risk is extremely low, solar panels do have a small risk of electrical fires when components fail or the system is installed improperly. This risk is higher with string inverters because high voltage DC electricity is carried across your rooftop and converted into lower voltage AC power only at the inverter. You can read our article on solar panel fires and watch the included video to see the difference between DC and AC power when it comes to arcing.
Both microinverters and power optimizers have software that automatically does a rapid shutdown at the panel when a fault is detected. Microinverters have an added safety benefit because the DC-to-AC conversion happens immediately, right at the panel. This means that lower voltage AC power will be flowing across your rooftop with a microinverter system.
The NEC regulation is effective January 1, 2019, but only in some jurisdictions. See the map on this page to find out if it applies to you: https://www.nfpa.org/NEC/NEC-adoption-and-use/NEC-adoption-maps
It can be confusing to understand the equipment proposed by your solar installer. When it comes to module-level electronics, there’s one detail that’s worth explaining: inverter clipping.
Let’s say that your installer has specified a really high power solar panel for system - something like a 380 Watt REC Alpha panel. But then you notice that they’re also proposing to use the Enphase IQ 6, a previous generation microinverter with a maximum input power of only 330 Watts. This means that the maximum output per-panel will be 330 Watts, and not the 380 Watts that the panel is rated at. Does that make any sense?
It definitely can. First of all, you should know that the nameplate rating of a panel is only its theoretical power output. Mainly because of operating temperatures, you shouldn’t expect that power level in any real-world setting. There are different testing standards (such as PTC and NOCT) that try to better model real-world output, but as a rough rule of thumb, you can expect the maximum output on a cool sunny day to be 80% of the nameplate rating at most.
Read our article on solar panel specifications to learn more about real-world performance and these different testing standards.
Because your solar panels may only hit maximum performance for part of the year - usually, these will be sunny days that are also cool - it often doesn’t make financial sense to pay extra money to upgrade to a higher capacity inverter to squeeze out a few extra watts from your system, when you can instead save money on the inverter by going with a lower capacity and letting your power output get “clipped” on those maximum output days.
There are a lot of advantages to MLPE systems, and the main disadvantage - higher cost - is becoming less of an issue as their market share increases, driving down costs. With 85% of solar homeowners going this route, it’s clear that the MLPEs are here to stay.