Many modern energy sources and storage systems operate with direct current (DC). Photovoltaic modules generate DC, batteries store energy as DC, and fuel cells also supply DC. However, our power grid and most larger electrical consumers operate on alternating current (AC). This is where the inverter comes into play. It serves as the interface between these two worlds, converting DC into AC. Without this conversion, for instance, the energy from a solar system or battery could not easily be fed into an existing 230 or 400-volt AC grid.
What happens in an inverter?
An inverter doesn’t simply mechanically reverse the current. Inside, power semiconductors work by electronically turning the DC on and off thousands of times per second. A precisely controlled alternation of these switching operations initially creates a pulsed electrical signal. This is then filtered and regulated to produce a clean sinusoidal alternating voltage at the output.
A modern inverter must do much more than just convert DC to AC. It must precisely regulate voltage, frequency, and power, respond to load changes, and exactly synchronize its output voltage with the power grid in grid-tied systems. At the same time, it must protect itself and the connected components against overload, short circuits, overheating, and other faults.
The quality of this regulation significantly affects how efficiently and reliably the entire energy system operates.
Every conversion costs energy
In no electrical conversion does the same amount of energy exit as enters. Part of the energy is primarily lost as heat. For high-quality modern inverters, the efficiency at the optimal operating point often lies in the range of approximately 96 to over 98 percent. From 100 kWh of DC, for example, around 96 to 98 kWh of usable AC can be produced.
However, these values do not automatically apply across the entire power range. Especially at very low load, efficiency can significantly drop. Thus, it is not only the maximum efficiency that matters, which may look good on a data sheet. What is crucial is how efficiently the device operates at the power levels that frequently occur in actual use.
What does low efficiency mean in practice?
The difference between 98 and 94 percent initially does not sound very large. However, over longer operating times, this can result in a significant amount of energy lost.
Assuming a system processes 1 million kWh of electrical energy over its lifetime. At 98 percent efficiency, about 20,000 kWh would theoretically be lost during this conversion. At 94 percent, this would already amount to about 60,000 kWh. A mere four percentage points of difference in this example means additional losses of around 40,000 kWh.
This lost energy does not simply disappear. A large part of it is converted to heat. This heat must be dissipated from the inverter and in turn burdens semiconductors, capacitors, fans, and other components.
What happens with very cheap inverters?
A low price does not automatically mean poor quality. The problem arises when the price is achieved by cutting costs on critical components. Simpler power semiconductors, smaller heat sinks, cheaper capacitors, improperly sized filters, or less precise regulation can impact efficiency, lifespan, and reliability.
With low-quality devices, conversion losses under real conditions can be significantly higher. Instead of, for instance, 2 to 4 percent, depending on the device, load condition, and design, losses of 5, 8, or even more percent of energy can occur in unfavorable operating points. Particularly critical can be a poorly sized device under partial load. A large inverter that is frequently operated at only a few percent of its rated power can waste unnecessary energy despite having good maximum values.
Therefore, one should not generalize that every cheap inverter causes a specific loss. Efficiency curves, self-consumption, design, and the actual load profile must be analyzed together.
Heat is lost energy and a burden
Every percentage of loss is ultimately predominantly converted into heat. With small systems, this initially appears insignificant. However, it quickly changes with larger outputs.
An inverter with 100 kW output power and 98 percent efficiency already produces about 2 kW of loss at this load. If the efficiency drops to around 94 percent, the loss rises to approximately 6 kW. This heat must be continuously dissipated.
A lower efficiency does not only lead to higher electricity costs. It also results in higher temperatures in the device. High temperatures are one of the main factors for the aging of electronic components. In particular, capacitors and power semiconductors can age faster under sustained high thermal stress.
Thus, poor efficiency can become doubly costly: through lost energy and possibly a shorter lifespan.
The quality of the power is also crucial
An inverter must not only convert energy but also generate as clean an output voltage as possible. Good devices produce an alternating voltage that closely resembles the desired sine wave. With inferior power electronics, stronger harmonics, voltage fluctuations, or electromagnetic interference can occur.
Such disturbances can affect connected motors, transformers, power supplies, communication systems, and sensitive electronics. For example, additional heating, noise, or vibrations can occur in electric motors.
The quality of an inverter cannot be assessed solely based on its efficiency. Regulation quality, grid quality, protective functions, electromagnetic compatibility, and behavior during quick load changes are at least as important.
Cheap inverter and expensive battery
Particularly questionable is the misguided frugality when a high-quality battery storage system is combined with subpar power electronics. Battery, battery management system, and inverter collectively form a system. Ultimately, the inverter determines how the stored energy is utilized and how efficiently it is transferred.
A battery may possess high efficiency. However, if the energy then has to undergo several conversions through poorly functioning power electronics, the benefits of the high-quality battery can be partially lost.
In a storage system, electricity can first be converted from AC to DC for charging and later from DC back to AC. The efficiencies of each step multiply. If each conversion is 98 percent efficient, approximately 96 percent remains after two conversions. However, with two conversions at 94 percent, it only amounts to around 88 percent.
Especially with energy storage systems, the entire system should be considered, not just a single component.
Is the inverter the right place to save?
Of course, cost-effectiveness should also be considered with inverters. It makes no sense to use components whose performance capabilities are not needed for the application. However, a decision based solely on the lowest purchase price can ultimately prove to be the opposite of economic action in the long run.
An inverter processes enormous amounts of energy over its lifetime. A difference of just a few percentage points in efficiency can thus add up to significant energy losses over the years. At the same time, quality and design impact the availability, lifespan, and safety of the entire system.
The correct approach is therefore not to buy the most expensive inverter but to use one that is correctly sized for the actual application, operates efficiently within the relevant power range, and is reliably built.
An inverter is the gateway through which a large portion of the generated or stored energy must pass. Losing energy at this gate just to save some money at purchase can ultimately be a very costly decision.
