Advancements in Solar Inverter and Energy Storage System Technologies
The global energy landscape is undergoing a significant transformation, driven by an increasing focus on carbon neutrality and the development of green energy. Solar energy, particularly when combined with Energy Storage Systems (ESS), is at the forefront of this shift, offering a promising future for a more sustainable and clean energy infrastructure. This article examines the evolving trends, design considerations, and prominent topologies in residential, commercial, and utility-scale applications.
Solar Energy Market Trends: A Growth Imperative
The solar energy market is experiencing robust growth, with global installed capacity projected to increase by 10% to 655 GW in 2025, reaching 930 GW by 2029, with the Compound Annual Growth Rate (CAGR) remaining in the low double digits[1]. On the other hand, global ESS installed capacity is set to expand from 175.4 GWh in 2024 to 221.9 GWh in 2026, up 26.5% year-on-year[2].
Central to this shift are solar energy systems and battery Energy Storage Systems (ESS), which together represent the cheapest form of electricity generation when considering the Levelised Cost of Energy (LCOE). This hybrid solution not only creates new market opportunities but also significantly enhances the reliability and efficiency of solar energy. They are vital for temporarily compensating grid fluctuations, offering short-term balancing, and facilitating long-term energy storage during the day. Favourable government policies and incentives are further bolstering the growth of these markets, and the increasing demand for high energy from green sources, particularly from data centres and communication systems, is a significant driver.
Solar Inverter Type: String, Micro or Central
There are different types of solar inverters, each with distinct characteristics and applications.
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- String inverters are best suited for simple installations and are typically the most cost-effective option. It only takes one string inverter, which is wired to a series of solar panels, a ‘string’. The single inverter also means a smaller chance of circuit failure. Another advantage is that they do not need to be located close to the solar panels, making resistance to heat and humidity less of a concern. The main drawback is that the panel with the lowest power output determines the total power output of the system. For example, if one panel is shaded, the output of the entire string will be limited to that shaded panel’s output, which can significantly affect the overall power output. This system, therefore, is less suitable for properties, both residential and commercial, with multiple panel angles or significant shaded areas.
- Micro-inverters, on the other hand, are attached to each solar panel in a system. This approach means the power output is not affected by voltage differences between panels. If one panel is shaded or underperforms, it does not significantly reduce the output of the other panels in the system, leading to a higher overall electricity yield (potentially up to 30% more output compared to string systems). They are primarily used on residential rooftops, which often feature dormer windows or flat roof sections, as well as for city infrastructure, such as streetlights and traffic lights. However, they are generally more expensive than string inverters because multiple inverters are needed for a system. Their proximity to the solar panels requires them to be designed for resistance to humidity and heat. Having multiple inverters also slightly increases the risk of circuit failure compared to a single-string inverter.
- Central solar inverters are typically installed in utility-scale solar farms and handle ultra-high capacities. While historically significant, their total newly installed capacity has been surpassed by string solar inverters in recent years due to limitations on installation locations.
Typical Solar Inverter Requirements: Power and Voltage
Solar inverters are fundamental components in any solar energy system, responsible for converting the direct current (DC) electricity generated by solar panels into alternating current (AC) electricity, suitable for grid consumption or home use.
Figure 1: Types and parameters of solar Inverters and ESS (Source: onsemi)
The typical power and voltage requirements for solar inverters are highly dependent on the number of solar panels, their connection configuration, and the specific grid requirements of the deployment.
In terms of DC input voltage from solar panels, systems can handle ranges from up to 1000 V to over 1500 V. For AC output voltage, requirements typically range from 480 V up to 800 V. A key trend in solar inverter design is the shift towards higher bus voltages, which inherently leads to higher efficiency and reduced energy losses. This optimisation is crucial for maximising the energy harvest from solar installations.
Different solar inverter types are categorised by their power levels and corresponding voltage capabilities, Figure 1.
- Residential solar inverter systems typically have power levels below 20 kW, although the trend is shifting towards 30 kW. Single-phase residential inverters are rated from below 1 kW up to 10 kW, depending on the region, with DC link voltage levels typically between 300 V and 600 V. Three-phase residential systems also exist and operate at a DC link of 1100 V.
- Commercial solar inverters range in power from 150 kW to 350 kW. Similar to residential inverters, they also manage DC input voltages of up to 1000 V or 1100 V; however, higher-power commercial systems are trending towards 1500 V. These systems primarily provide a 3-phase AC output.
- Utility solar inverters are designed for large-scale applications, ranging from 200 kW up to 5 MW. These utility-scale solar inverters typically require DC input voltages exceeding 1500 V, with 1500 V being a common limit influenced by semiconductor and cable isolation technologies. However, actual trends aim for even higher voltages. They deliver 3-phase AC output.
Energy Solution Planning: Solar Inverter or Hybrid Solar Inverter with ESS
When planning an energy solution, a fundamental decision involves choosing between a standalone solar inverter connected directly to the AC power grid or a solar inverter integrated with an ESS before connecting to the grid. The choice has a significant impact on system functionality, reliability, and economic viability.
A standalone solar inverter converts solar-generated DC electricity to AC and feeds it directly into the grid or powers local loads, Figure 2. This setup is more straightforward, primarily focused on immediate energy consumption or grid export.
Figure 2: Standalone solar inverter converts solar-generated DC electricity to AC (Source: onsemi)
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Solar Photo Voltaic Technology Steps Up
Worldwide electricity consumption is growing fast, expected to rise by circa 4% annually from now to 2027. Solar PV generation is forecast to provide about half of this extra electricity, driven by falling PV prices and the need for zero-emissions.
Unlike standalone solar systems, which might restrict energy generation when demand is low or the grid cannot accept more power, battery ESS store excess energy for later use, thereby enhancing energy independence and resilience. This ability to store energy also allows for decentralised power generation, meaning power is produced closer to where it is consumed, thereby reducing transmission losses and improving grid efficiency.
AC-coupled ESS is a separated system that can be added to an existing solar/energy generation system/grid, making it an easy upgrade, Figure 3. However, it requires additional power conversion stages to achieve full charging/discharging, resulting in higher losses.
Figure 3: AC-coupled battery ESS (Source: onsemi)
Figure 4: DC-coupled battery ESS (Source: onsemi)
On the other hand, DC-coupled system, commonly employed in residential hybrid solar inverters, offer extra energy storage capacity by connecting to the DC bus, Figure 4. It involves single DC/DC conversion step but requires a decision during product design, as DC bus voltage is often high and may pose safety or retrofitting challenges.
The integration of ESS enhances the reliability and efficiency of solar power systems by enabling features such as enhanced monitoring and control, providing real-time data and grid support functionalities, and facilitating smart grid integration. ESS is capable of temporarily compensating for grid fluctuations, offering short-term balancing, or storing energy for longer durations, such as during the day for night-time use. This flexibility is vital for maintaining grid stability and energy independence.
Design Trends for Solar Inverter and ESS
Various topologies are available for solar inverters and ESS, designed to meet different power, voltage, and application requirements, including standalone or hybrid inverters and residential, commercial, or utility-scale systems. There are DC/DC boost converters, DC/AC inverters, bidirectional DC/DC converters, and bidirectional AC/DC converters.
DC/DC Boost Topologies are often used between solar panel strings and the DC link to elevate voltage and implement Maximum Power Point Tracking (MPPT). The primary function of MPPT is to maximise the power output from the solar panels by continuously adjusting the electrical operating point of the panels to ensure they are always working at their peak efficiency. This optimises power generated under varying environmental and sun irradiation conditions. The two-level single boost topology is the simplest boost circuit, Figure 5. It is easy to control, and offers a low Bill-of-Materials (BOM) cost, low failure rate, and normal efficiency. It has moderate size and EMI characteristics, requiring power components with full voltage capabilities.
Figure 5: DC/DC two-level single boost string inverter (Source: onsemi)
Figure 6: The H6.5 topology is suitable for residential solar inverters (Source: onsemi)
DC/AC solar inverters convert DC electricity generated by solar panels into AC electricity for the grid. They can be constructed using a variety of topologies, like, for example, the High-Efficient and Reliable Inverter Concept (HERIC) H6.5, Figure 6. It is a 3-level topology primarily used in single-phase solar inverters. This design offers better efficiency compared to half-bridge designs and can reduce common-mode leakage current when the transformer is removed. This topology is widely used in residential applications, targeting power levels typically below 10 kW.
ESS require bidirectional operation to store energy from the grid or renewable sources and release it when needed. For AC-coupled ESS, where high efficiency and power density are critical, the single/three-phase totem pole converter topology improves efficiency compared to traditional designs, offering enhanced EMI performance and total harmonic distortion (THD), Figure 7. It reduces the number of switches that conduct per cycle, leading to high power density. However, it requires wide bandgap components to minimise recovery losses and may have issues with zero-crossing point noise and common-mode noise.
Figure 7: Single/three-phase totem pole converter (Source: onsemi)
Figure 8: Bidirectional DC/DC buck-boost converter (Source: onsemi)
Bidirectional DC/DC converters are used for integrating battery ESS, especially in DC-coupled systems where energy is stored directly from panels, reducing conversion losses. The Buck-Boost converter is a basic converter that can step the voltage up or down, Figure 8. It is used to extend the charging/discharging voltage range, improving battery usage and realising bidirectional power conversion. It has few components and is easy to control. It is an optional component in ESS, depending on the battery voltage, and is suitable for both residential and commercial scales.
Higher bus voltages in solar inverters, such as shifting from 1100 V to 1500 V, reduce interconnection costs and losses by lowering current for a given power, which drives the development of higher voltage switches or multi-level topologies. The trend is also towards higher power and power density, with a shift to higher switching frequencies using SiC power integrated modules (PIMs) and SiC discrete components.
Common Challenges and Technological Solutions
PIMs vs. Discrete Components: The choice between Power Integrated Modules (PIMs) and discrete components is critical. PIMs offer advantages in design cost and space saving, simplified manufacturing, easier PCB design (requiring smaller area), reduced part count, lower thermal impedance, stable isolation via Direct Bonded Copper (DBC), and optimised matching among devices. They are generally more reliable as a single device. However, discrete components offer potentially lower BOM cost and easier second sourcing.
For high-power products, module solutions are highly recommended to simplify challenges such as imbalanced current, heat distribution, switching timing, and wiring connections when paralleling multiple MOSFETs or IGBTs.
SiC vs. IGBT: Compared to IGBTs, SiC devices offer higher efficiency, faster switching capabilities, and allow for smaller passive components (like inductors), leading to increased power density. This means higher power can be achieved in equipment of the same size and weight. SiC diode replacement is becoming common in the DC/DC stage due to affordability and improved system performance.
However, a full SiC system requires an entirely new system design, including driving circuits and protection approaches, as SiC components have a smaller short-circuit withstand time (SCWT) than IGBTs. IGBTs remain the first choice for high-power products (>200 kW) where high operating switching rates are not critical, as they perform well with high currents. Hybrid PIMs integrate SiC boost diodes to optimise losses.
Gate Driver Considerations: Gate drivers are crucial for turning switches on and off efficiently. Key factors for selection include current driving capability (higher source/sink current means quicker switching and lower losses), fault detection (e.g., Under Voltage Lock Out (UVLO) and Desaturation (DESAT) for short-circuit protection), common mode transient immunity (CMTI) for high-frequency switching systems, propagation delay for accurate timing, and compatibility for simplified design.
onsemi's Solutions for Solar Inverter and ESS
onsemi provides an extensive portfolio of products, including discrete SiC and IGBT devices, PIMs, isolated gate drivers, and power management controllers, all designed to enhance system power density and efficiency.
Power Integrated Modules: onsemi offers PIMs for 1100 V and 1500 V applications in power ranges from 30 kW to 350 kW (and up to 5 MW). These PIMs are designed for light and compact systems, simplifying infrastructure deployment and PCB design. They feature lower thermal impedance, stable isolation through Direct Bonded Copper (DBC), optimised matching among devices, and integrated temperature sensors. Modules in the F5BP package offer superior thermal performance, combining Si and SiC devices for design flexibility, and support both solder and solderless press-fit pins. The QDual3 module package, also available with solder (default) or press-fit (under request) pins, is suitable for utility BESS and central solar inverter solutions.
Discrete Components: onsemi’s discrete offerings include SiC MOSFETs (up to 1700 V, 2000 V under development), SiC Cascode JFETs (up to 1700V), IGBTs (up to 1200 V), and SiC diodes (up to 1700 V). These components are manufactured with an in-house supply chain, ensuring high quality and guaranteed delivery. They offer minimum switching and conduction losses, high-efficiency operation, and reduced EMI.
Comprehensive Support: onsemi supports design efforts with a range of resources, including System Solution Guides (SSG) for Solar Inverter and ESS, interactive block diagrams, application notes, and simulation tools such as the Elite Power Simulator and Product Recommendation Tools (PRT+).
In conclusion, the integration of solar energy with Battery ESS is pivotal for a sustainable future. The choice between SiC and Si MOSFETs, or the use of hybrid PIMs (combining Si and SiC devices), depends on the specific design requirements, including voltage levels, switching frequency, desired efficiency, and cost considerations. Through continuous innovation in system design, power electronics, and component technologies, onsemi, together with the power experts at Avnet Silica, is at hand to enable engineers to meet the growing global demand for clean, reliable, and cost-effective energy solutions across all scales.
References
[1] Global Market Outlook for Solar Power 2025-2029 (6 May, 2025) https://www.solarpowereurope.org/insights/outlooks/global-market-outlook-for-solar-power-2025-2029/detail
[2] Global Energy Storage Market: Review and Outlook (24 January, 2025) https://www.infolink-group.com/energy-article/energy-storage-topic-global-energy-storage-market-review-outlook
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