If you own or are planning to own your own solar power system, you’ve probably heard of an Inverter. Although a vital part of any solar electric system, inverter choices can easily be overlooked in the shadow of seemingly more important decisions such as installer and solar panel options. I mean, doesn’t an inverter just change the solar energy (DC) to house energy (AC)? How complicated can it be? Contrary to what your salesman or inverter manufacturers may have you believe, there is no “one size fits all” inverter. As with many electronic devices; design, cost, warranty, time-testedness and monitoring capabilities are variables designed to offer as many choices as the different homes, businesses and climates they inhabit. For example, you wouldn’t want to pay a premium for an inverter designed to deal with partial shading issues – if you have zero shading. At a minimum, you’d at least appreciate knowing how the “other” inverter would affect the contract price. So, in this spirit – let’s take a preliminary tour of the Inverter (or optional, microinverters). Inverters have three primary functions:
- Inversion of the incoming DC energy (direct current) into home and appliance-friendly compatible AC energy (alternating current). This is its main function.
- An MPPT (maximum power point tracking) function ensures that the inverter receives the maximum amount of power from the solar panel by adjusting the voltage (load) to match what the inverter wants; managing the complex relationship between sunlight and other varying conditions.
- Various electronic monitoring sensors that will, for example, detect when there is a fault in the grid-tied solar panel or on the grid and shut down power so the solar system cannot injure power-line workers during a grid outage.
The most traditional form of Inverter is a String Inverter, so-called because it is connected to a line or “string” of solar PV panels. One solar power system, depending on the size/number of strings in the system, may have one or several string inverters which sit in parallel and convert the arriving DC current into an AC output.
These six string inverters service a large commercial solar electric (PV) system. Each inverter services forty solar panels, on four strings of 10 for a total of 240 panels.
Micro-inverters and Power Optimizers
Another type of inverter is the Micro-Inverter. As the name suggests, this inverter is a smaller version of a string inverter, but rated to handle the output of each individual panel. Therefore an inverter is located on the back of every solar panel unlike a string inverter that is often located on the side of the building/ground level. Micro-inverters have been around almost as long as string inverters. Historically, the manufacturing costs involved with making a dozen “mini” inverters couldn’t compare to the cost of one large string inverter. However, as the solar industry has grown – so has the demand for the micro-inverter; driving up demand while reducing production and distribution costs.
In response to the popularity of the micro inverter, Tigo, followed by String inverter giants SMA, developed a device called the Power Optimizer that works in conjunction with a string inverter. As with micro-inverters, power optimizers are placed on the back of each panel and provide an MPPT function to each panel – limiting individual shading and orientation effects to the panel they’re attached to as opposed to the whole string. However, because they are designed to operate alongside a string inverter they do not require individual transformers and are therefore much smaller, lighter and most importantly, cheaper than micro-inverters.
Solar design expert, Rich Schroeher says there is a time and a place for all three of these inverters:
Q: Aren’t micro inverters better when the solar arrays are facing different directions or have different roof pitches?
Rich: With micro-inverters, in essence, each solar panel is it’s own solar system, independent of the other panel/ inverter combinations, so various orientations of panels are easily accomodated.
With string inverters, a different inverter must be used for an array that is facing a different direction, or at a different pitch than the other arrays.
One way around this is string inverters that contain 2 MPPT inputs. These are, essentially, 2 inverters in one. They also have the added benefit of being able to handle more of the array than their rating. For example, let’s say you have two arrays facing different directions: one South facing and one East facing. The entire system would use a 5000 watt inverter divided into two channels of 4000 watts for a total of 8000 watts. The inverter will still only supply 5000 watts but when the south roof starts getting sun, the east roof is losing it. If at anytime more than 5000watts is being fed to the inverter, it just limits it to 5K.
Q: Some homes have partial shading on the roof. How do you know when to use a string inverter – possibly with dual MPPT, or a power optimizer – or stick with a microinverters that will ensure a shaded panel won’t negatively affect the whole array’s output?
Rich: It depends on the situation. Microinverters and optimizers can help with small areas of shading but are not a cure-all. They work best for an application where a small % of the roof is shaded at various times during the day, such as a chimney shading a few collectors where the shadow moves during the day.
Q: Are there any longevity differences between string and micro inverters?
Rich: Opinions vary. A solar system with 24 micro inverters, as opposed to 1 string inverter, has 23 more points of failure. Plus the extreme temperatures that microinverters are subjected to can be an issue; electronics generally don’t like heat. Microinverters are also located on the roof behind each panel, as opposed to the string inverter which is usually conveniently located close to the main electrical supply on the ground level.
Q: Enphase microinverters claim to get greater efficiency, up to 16%, than their string counterparts due to the fact that the maximum power point is matched to the solar panel as opposed to the inverter, which limits the available output.
Rich: In a Partial shading situation this could be true. If there is no shading at all or very early or late in the day when production is very low anyway it wouldn’t be near that high.
There are only two notable benefits that microinverters offer in a zero shade situation: they eliminate the string inverter losses from module mismatch (very small differences between modules), around 2 – 3%. They also help with DC wiring losses. Combined those two losses may be 5% overall. This all assumes that the module attached is not too large (powerful) as to have any appreciable amount of clipping – meaning the full power of the module is limited by the inverter.
Q: Microinverters, are more expensive than string inverters, at $.70/watt with a twenty-five year warranty. Although String inverters are as much as 50% less, they often come with only a ten year warranty. Doesn’t the microinverter’s longer warranty cover the inflated cost?
Rich: In the long run probably no. Most string inverter companies’ offer extended warranties up to 20 years. Some include the cost of labor to replace. The labor cost to replace is about equal for a string inverter and a microinverter or perhaps a bit lower for string inverters depending on where in the array a failed micro is located. Say you have a system with 20 panels and the labor cost for inverter replacement is $150.00
String inverter cost over lifetime $150.00
Microinverter cost over lifetime as much as $3,000.00 Plus 19 more scheduled trips and possible inconvenience to the owner.
Q: SMA’s new inverters include a “Secure Power Supply” capability. What is this?
Rich: This is a small (15amp) receptacle that is powered up when the grid goes down. As long as the sun is shining it will provide 120 volts of AC electricity. My first thought was to power a refrigerator or freezer to help carry a system owner through the outage without food spoiling. It can’t be used at night but it will let you use some of the system during outages.
As is often the case, there are no cut and dry answers here. Variables such as system size, shading, orientation, panel layout and budget will determine whether a String, MPPT optimizer or micro-inverter may or may not be the best choice for your roof. Most installers/designers worth their salt will consider all of these options and come up with the best solution for your house.