Solar Contractors have three main options to comply with the rapid shutdown requirements of the National Electrical Code (NEC). These requirements can be found in section 690.12 of the NEC. The best known products are micro-inverters, power optimizers, and AC solar panels. They all have the ability to control the DC output circuits to a safe voltage. Let’s check out the basics of each one to compare the pros and cons.
Micro-inverters are exactly what they sound like, a small inverter. These install under the solar array and attach to the solar panel frame or the racking system. They convert the DC from the solar panels directly to AC before the conductors leave the array. They come standard with Max Power Point Tracking, to maximize the DC input of the solar panel to get the most power out of it. This is especially helpful where more shade might occur on the array. They also offer the advantage of being connected to sub-arrays that do not share the same tilt or azimuth. This is a really nice feature which gives more flexibility in the design and placement of the solar panels on the roof. Most micro-inverter brands come with a range of products that work with 60 cell and 72 cell solar panels. They also come in different models that can be connected from one to four solar panels. The wiring used for micro-inverters is simplified using the manufacturer’s proprietary AC cable, sometimes called trunk cable. Another nice thing about running AC straight from the array, is the flexibility of using Romex through the house. DC circuits need to be protected in metallic conduit throughout a building, but AC circuits can use any code compliant wiring method. This is especially useful, and can save on labor, when roughing in the solar AC run in new home construction.
AC solar panels are a solar panel that comes pre-manufactured with a micro-inverter integrated into the panel. Most major solar panel manufacturers are partnering with micro-inverter companies to offer AC solar panels. This simplifies the installation process and reduces the amount of parts that it takes to install a system. The wiring is the same as micro-inverters, where you have to use the manufacturer’s proprietary AC cable. AC solar panels provides all the benefits of a traditional micro-inverter in one easy package. There is an increased cost associated with AC solar panels, however, and they are usually sold to customers looking for the premium product. I haven’t personally dealt with maintenance on AC solar panels, but I would assume that you would have to replace the entire AC solar panel if the micro-inverter on the panel was defective. This is unlike traditional micro-inverter systems, where you would only have to replace the micro-inverter under the solar panel if it went bad.
Power optimizers have gained lots of popularity over the last couple years. They look like a micro-inverter and either mount to the racking system or to the solar panel frame. The difference is that they still output DC. The DC output runs down to a central inverter which then converts the DC to AC. The inverter and power optimizer work together to control the DC output voltage. When the system is shutdown it decreases the DC voltage in order to comply with rapid shutdown. The optimizers are wired in series at the array and use your typical PV Wire for home runs. Most optimizers are able to maximize the power output of each solar panel that they are connected to. They also share the advantages of micro-inverters with regards to design flexibility with sub-arrays at different tilts and azimuths. Unlike micro-inverters, you will have to abide by DC wiring methods in buildings. There are newer rapid shutdown only optimizers on the market. These don’t have the capability of maximizing the power on each solar panel, but do control the DC voltage for rapid shutdown. These are typically paired with standard string inverters that are UL listed to work together. I have used power optimizers the most in recent years, since the adoption of rapid shutdown. I have seen lots of maintenance issues with power optimizers and their associated central inverters, but it could be a coincidence since I install them the most. I do like the design flexibility with power optimizers, and PV wire is a little less expensive than micro-inverters’ AC cable.
One thing I am hesitant to compare in these three products is cost. That is because it differs so much between vendors, distributors and manufacturers, that I don’t want to give the wrong impression for either product. Cost is a big driver though, so it would be prudent to do some initial research before making your decision. In my experience, I like installing and commissioning micro-inverters over power optimizers. I really haven’t installed more than a couple AC solar panel systems, so I can’t give a strong opinion on them. However, from a maintenance point of view, I do like that traditional micro-inverters are separate from the solar panel. Solar Panels will definitely last longer than their module level electronic counterparts, so it makes sense to me to keep the two separate. The rapid shutdown only optimizers are gaining lots of popularity, and are usually at a comparable price point compared to traditional power optimizers. If your site does not need to combine sub-arrays at different tilts and azimuths and is relatively shade free, then I would suggest taking a look at these products. In short, the best product you pick is one that you and your customer can have confidence in. Solar is probably the fastest changing, and fastest growing industry right now. So maybe the best product is one that shows reliability and resiliency in this market. Maybe go ahead and try all three out to decide for yourself.