As discussed in our earlier article about solar panels and their costs, we need to understand another device called the Solar Inverter which is something you would need to evaluate as part of your solar energy system. As soon as someone thinks about an inverter, one of the first questions is what is an inverter and how does it work? Is it something we need to absolutely purchase or not? What other options do we have etc?
In general, Solar panels convert sunlight into direct current (DC) with a 12-17 volts low voltage. In North America, the lights and appliances use alternating current (AC) electricity at a higher voltage; 130 volts for North America, 220 volts for Europe, Australia, etc. An inverter comes into play as the inverter steps up the voltage and converts the power from DC to AC.
Off-grid or grid-tie inverters?
The first step in the inverter selection process is to decide whether you’re planning to have an off-grid or grid-tie system [1]. There are significant differences between off-grid inverters and grid-tie inverters. Grid-tie inverters synchronize the phase and frequency of the current to fit the utility grid (nominally 60 Hz). The output voltage is adjusted slightly higher than the grid voltage for the excess electricity that flows outwards to the grid.
For off-grid systems, you may need to ask yourself if you need an inverter at all! There is no need for an inverter when you’re using the solar panels for your boats, RV, or any other device that runs on DC. For any other electrical appliances, you will need an inverter to convert DC to AC. Electrical current flows from the solar panels through the solar charge controller and the battery bank before finally being converted into AC by the off-grid-inverter. Compared to grid-tie inverters, the off-grid inverters do not have to match phase with the utility sine wave
Central vs. Micro?
After deciding to go with either off-grid and grid-tie, you next have to decide whether to go with either central inverters or micro-inverters. There are two types of solar inverters in the market:
- Central or String inverters
- Micro-Inverters
Central inverters, also called string inverters, connects solar (PV) panels in a string and convert the DC electricity into AC. A solar panel string is a group of wired panels in series connected to a single inverter. In general, an inverter can handle 7-12 panels in series on a particular string. A solar energy system may be composed of more than one string, and hence these strings are connected in parallel and connected to an inverter
Micro-inverters, on the other hand, represent a very different take as compared to Central Inverters. One microinverter is attached to each solar panel and is much smaller in size. The microinverter converts the DC output from each panel immediately into AC. The resulting output of each panel’s AC is combined and sent back to the grid or the battery bank.
The technology behind the Micro-inverter is not something of a recent technological breakthrough. Instead, these microinverters were invented in the 1980s and have been commercially available since 1993. It was not until 2008, Enphase Energy released Enphase M175, one of the first micro-inverters that achieved large-scale commercial success.
After Enphase made the microinverter technology a commercial success, today’s installers provide a quote with a micro-inverter option, which is generally presented as a premium product. When someone decides to go solar, it’s always a good idea to explore the advantages and disadvantages of micro-inverters.
Advantages
Weakest link effect
One of the most common things you’re probably familiar with is the Christmas lights. In general, Christmas lights are lights connected in series where if one bulb fails, it takes the entire string down alongside it. And that’s the same vulnerability that Solar panels face when attached to a central inverter. A defective panel in the string can bring down the whole string of solar panels and stops working. This results in a significant or complete loss of electrical supply.
Shading also affects the Central Inverter. One shaded panel can cause the system to produce electricity at a low level since the central inverter optimizes the output of a solar energy system based on the weakest link in the string. The worst-case scenario for a homeowner is that the system performs at suboptimal levels since a minor output reduction, say 10-20%, may result from cloudy conditions or a defective solar panel. In the case of the central inverter, if a string is producing at a sub-optimal level in the case of the central inverter, then every solar panel will need to be tested by a qualified solar technician to find the root cause of the problem and check if it’s a shade or defective panel issue. On the other hand, microinverters will only have a minor impact on your overall system if any micro-inverter attached to the solar panel fails. And as a result, the dependence on the weakest link is eliminated.
Optimization
A central inverter consists of solar panels connected in series and treats the entire string of solar panels as one giant system. Any solar panel in that string with bad electrical properties defines the string property. The best part is that each solar panel performs at its maximum potential depending on weather conditions, shading, and electrical properties. The effect of that is one solar panel cannot drag the entire solar array’s performance. In the case of the centralized design, all panels are oriented in the same direction. When a roof has a complex geometrical shape, uniform orientation (Portrait or landscape) may not be achievable. As a result, there’s a possibility that maximum output from each panel is not possible. Hence, a microinverter design philosophy would be the best approach to maximize the solar energy output.
There is a type of optimization that inverters perform, called Maximum power point tracking (MPPT). Maximum power point tracking refers to a technique used by variable power sources to maximize energy output under all conditions. MPPT samples the panels’ electrical output and constantly determines the optimal resistance (load) to maximize total power. Microinverters use MPPT tracking by changing the voltage requirements of each panel since every solar panel always has a different amount of solar radiation falling on it at any given moment due to its placement of the panels.
Longer warranty period
A central inverter is generally exposed to natural elements, as well as high power and heat loads. The effect is that the electronic components inside the inverter start to get worn out and eventually stop working. Hence, you will probably need to replace your original inverter with a new central inverter during the lifetime of your solar panels (20-25 years). The warranty period offered by central inverter manufacturers is generally a ten-year warranty period, with an option to buy a long-term warranty. On the other hand, micro-inverters typically come with a 25-year limited warranty, better matching the lifetime expectancy of your panels. The fine print of the warranty certificate explains in depth what is covered and not covered.
Increased flexibility
Central inverters have size limitations. E.g., suppose you’re looking to install a solar energy system that is 2 kW. In that case, most central inverter manufacturers offer inverters in the 3 kW or above range for any residential design. Some solar energy systems sizes are higher, e.g., a 23kw or 28 kW solar energy system. Then, you may need to purchase multiple central inverters to convert the DC electricity to AC electricity. A 23 kW solar energy system would require two (2) 12 kW inverters to make sure we get the best conversion results. Micro-inverters do not have any size limitations, as you buy one inverter for each solar panel. An additional advantage of a microinverter system allows you to expand your system by adding more solar panels and micro-inverters, which may not be an option if your central inverter is tapped out.
Improved safety
Micro-inverters operate at 240 volts, whereas voltages associated with a central inverter can reach as high as 600V based on the system size and solar panels connected in series before reaching the inverter.
Panel-level monitoring
Micro-inverters come with a standard monitoring service, which shows how much power is produced at an individual solar panel level. A central inverter cannot display this detail unless you install an optimizer attached to each panel to optimize the output and gives you that detail. Most central inverters are not designed to handle optimizers, but few do to make sure you get an optimum output
Disadvantages of Micro-inverters
Price
Micro-inverters are generally considered premium inverters, and solar installers typically present them as a higher-end option. In 2021 the price of a simple residential solar PV system was just under $3 per watt installed. In comparison, for systems with higher upfront costs, such as those using micro-inverters and higher-efficiency panels, the price increased to around $3.2 per installed watt capacity. This doesn’t mean that micro-inverters are 20-25% more expensive than centralized inverters. In general, e.g., different cabling needs, labor costs, etc., may explain the price difference. In general, a Micro Inverter technology is $1000 – 1500 more costly than a central inverter for a five kW system. Also, when comparing the prices of alternative systems, you should concentrate on the total ownership cost and not on the upfront cost of buying the solar energy system. Although there’s a possibility of minimal future expenses after a solar energy system is installed, it’s always better to understand the Levelized cost of solar PV systems.
Technology maturity
Micro-inverter technology has taken off only since 2008. Central inverters have been used ever since solar panels were used to convert DC into AC electricity. Micro Inverters have not been used sufficiently long to produce long-term performance data. A point to be noted is that microinverters are installed at the back of the Solar Panel and are one of the hottest solar panels. There may be an increased risk of failure after several years of heat exposure. Today, though, there hasn’t been a significant crisis in the sector that resulted in a mass recall of micro-inverters.
Maintenance
When a micro-inverter fails, the solar technician/contractor needs to get on the roof and remove the solar panels from the solar energy system to replace the damaged/broken microinverter. Whereas the central inverters are mounted next to the main electrical supply, so maintenance and installation are easier
Advanced features require an internet connection
Microinverters function regardless of whether an internet connection is present or not. The solar panels will generate DC electricity, and the microinverters would convert into AC. They would still show your monitoring performance via your ethernet cabling structure. Still, an internet connection would be needed to use some of the advanced monitoring techniques that come with micro-inverters.
Micro-inverters may be worth the price premium
As the microinverter technology matures and mass production increases, the price is expected to drop further. The units’ reliability is expected to improve. These two factors (declining costs and increased reliability) are likely to increase the market share of micro-inverters shortly.
But what about now? Suppose you are about to make your solar investment decision. In that case, you should do a cost analysis on your solar energy system to determine which is the best option for you. There are few steps to achieve it
1)Shopping around and getting multiple quotes always helps. A higher efficiency panel is always better. Does it always justify the cost of the system, the ROI, and your payback period? There is no clear-cut answer. In a case where you have a standard roof installed and no shading issues present, then a central inverter with traditional solar panels may be the best bang for the buck, provided the right and quality products are used. Suppose you’ve got shading issues at one side of the roof where most solar panels are installed, and you’re not in the right temperate environment. In that case, higher-efficiency panels with micro-inverters will cost you more initially. Still, they may produce more power, and as a result, there would be a decrease in the overall ownership cost in the long run.
Shading, the geometry of your roof, and incentives like (SRECs, tax credits, net metering) offered by the utility, state, or federal government are the main deciding factors whether t to use microinverters or higher efficiency panels. If the shading or geometry of your roof is concerning, then a microinverter would be the standard way to go about having an inverter. As a result, you will boost your production compared to a more conventional standard inverter. If incentives are higher, having a high-efficiency panel would help you get better output and faster ROI on the system.
What things do I need to consider?
Regional electrical standards matter when selecting an inverter type. A typical DC voltage would have been around 12 V back in the day, but nowadays, 24 V or 48 V has become the standard due to wiring costs. This is because high voltage means lower current.
Aside from voltage, knowing the amount of power an inverter can handle is critical. Consequently, there are three levels of power ratings. The first is continuous, a power rating that identifies the power the inverter can take in a long-term setting. The second, known as a limited-time rating, measures the amount of power the inverter can take in a shorter period — generally around 15 minutes. Finally, there’s something known as a surge rating. The surge rating refers to the amount of power it can handle in an instantaneous overload. This is especially important when starting up.
