In the context of the global energy transition driven by the "dual carbon" goal, Residential Energy Storage Systems (RESS) are shifting from "optional configurations" to "essential needs" for household energy management. As a key device connecting photovoltaic power generation, power grid, and household loads, inverters undertake the core tasks of converting electrical energy forms, coordinating system control, and optimizing energy distribution, making them the "heart" of household energy storage systems.
According to the International Energy Agency (IEA), in 2023, the cost of inverters will account for approximately 15% -20% of the total installed capacity of household energy storage worldwide (even higher if mixed inverter functionality is included). Their performance directly determines the system's power generation efficiency, power supply reliability, and level of intelligence. This article will deeply analyze the working logic of inverters in household energy storage systems from the dimensions of technical principles, functional classification, working modes, and future trends, and reveal how they achieve efficient collaboration of "light storage load grid".
1、Basic concepts and technical principles of inverters
1.1 The core requirement for energy conversion
The electricity generated by photovoltaic modules is direct current (DC), while household appliances such as refrigerators, air conditioners, lighting, etc. require alternating current (AC); Meanwhile, the power grid also transmits alternating current. In addition, the charging and discharging of energy storage batteries (such as lithium batteries) require direct current, but if they are to be used at home or fed back to the grid during discharge, they need to be converted to alternating current. Therefore, the core function of an inverter is to "convert direct current into alternating current" and ensure that the converted electrical energy meets the requirements of the power grid or load such as voltage, frequency, and phase.
From the perspective of energy flow, inverters in household energy storage systems need to handle four typical scenarios:
• Photovoltaic power generation → household load: photovoltaic DC → inverter → AC → load;
• Photovoltaic power generation → energy storage battery: photovoltaic DC → charging management → battery DC (no need for inverter, but requires DC-DC conversion);
• Energy storage battery → household load: battery DC → inverter → AC → load;
• Energy storage battery/photovoltaic → power grid: DC → inverter → AC → power grid (subject to grid integration standards).
1.2 Basic circuit structure of inverter
The core of an inverter is the "inverter circuit", which chops DC power into approximately sinusoidal AC power through the switching action of power semiconductor devices such as IGBT and MOSFET. Modern household inverters typically use "full bridge inverter circuits" or "half bridge inverter circuits", and control the switching frequency (usually several thousand to tens of kilohertz) through PWM (pulse width modulation) technology to generate high-frequency pulse signals, which are then smoothed into sine waves by filtering circuits (inductors, capacitors).
Taking the full bridge inverter circuit as an example:
• DC input (such as 48V DC output from photovoltaic modules) is divided into two groups (such as+24V and -24V);
• Four power switching transistors (Q1-Q4) are turned on/off in a specific order (e.g. Q1/Q4 outputs a positive half cycle when conducting, Q2/Q3 outputs a negative half cycle when conducting);
• By adjusting the conduction time (i.e. duty cycle) of the switching transistor, the amplitude and frequency of the output voltage can be controlled.
1.3 Key technologies and performance indicators
Household inverters need to meet requirements such as high efficiency, high reliability, and low harmonic distortion, and their core technologies include:
• Efficient power conversion: using soft switching technology (such as LLC resonance, ZVS zero voltage switching) to reduce switching losses, the overall efficiency can reach over 98% (under the EU EN 50530 standard);
• Low Harmonic Distortion (THD): By using PWM modulation and filtering circuits, the harmonic content of the output AC power is controlled within 5% (meeting the IEEE 519 standard) to avoid interference with household appliances;
• Wide input voltage range: Suitable for photovoltaic output under different light intensities (such as 40V-600V DC), ensuring that the system can still operate under low light conditions;
• Island protection: When the power grid is cut off, the inverter needs to quickly detect and stop supplying power to the grid (response time<20ms) to prevent the "island effect" from threatening the safety of maintenance personnel.
2、Functional classification of inverters in household energy storage systems
According to the degree of functional integration, household energy storage inverters can be divided into three categories: unidirectional inverters, bidirectional inverters, and hybrid inverters, with significant differences in their technical logic and applicable scenarios.
2.1 Unidirectional inverter（DC-AC Inverter）
Only realize the conversion from DC to AC, usually used for pure grid connected photovoltaic systems (without energy storage) or independent off grid systems (with diesel generators).
• Working logic: DC generated by photovoltaic modules → inverter → AC → household load or grid;
• Features: Simple structure, low cost, but unable to handle the charging and discharging of energy storage batteries (requiring additional DC-DC converters);
• Applicable scenarios: early photovoltaic systems or scenarios that only require grid connected power generation (such as some rural areas).
2.2 Bi-directional Inverter
It can simultaneously achieve bidirectional conversion from DC to AC (inverter) and AC to DC (rectifier), and is the core component of household energy storage systems.
• Work logic:
Energy storage charging: grid AC or photovoltaic DC → inverter rectification → DC → battery (matching battery voltage, such as 48V DC);
Energy storage and discharge: battery DC → inverter inverter → AC → household load or power grid;
Photovoltaic Direct Supply: Photovoltaic DC → Inverter Inverter → AC → Household Load (Prioritize the use of photovoltaics to reduce battery charging and discharging losses).
• Features: Integrated charging management, discharge control, and grid interaction functions, supporting flexible scheduling of "photovoltaic+energy storage";
• Applicable scenarios: Mainstream household energy storage systems (such as Tesla Powerwall with inverters).
2.3 Hybrid Inverter
On the basis of bidirectional inverters, further integration of photovoltaic MPPT (maximum power point tracking) function can be achieved by directly connecting photovoltaic modules, realizing the full chain management of "photovoltaic → energy storage → load/grid".
• Work logic:
Photovoltaic input: Photovoltaic DC → MPPT module (optimizing photovoltaic output power) → DC bus → inverter (distributed to battery or load/grid according to demand);
Energy storage control: Unified management of battery charging and discharging through DC bus to avoid energy diversion (such as redundant conversion of photovoltaic → DC-DC → battery → DC-AC → load in traditional systems);
Multi mode switching: supports multiple operating modes such as grid connected, off grid, and backup power supply (such as automatic switching to off grid power supply in case of power outage).
• Features: Reduce energy conversion steps (reducing losses by about 3% -5%), simplify system topology, and improve overall efficiency;
• Applicable scenarios: High end household energy storage systems (such as Huawei Sun2000 series, Sunac SG series).
3、Analysis of the working mode of inverters in household energy storage
The operation of household energy storage systems highly relies on the intelligent control of inverters, and their working modes can be divided into grid connected mode, off grid mode, and hybrid mode. The control strategies of inverters vary significantly in different modes.
3.1 Grid-connected Mode
The system is connected to the power grid, and the inverter needs to simultaneously meet the requirements of "self use", "surplus electricity grid connection", and "grid interaction".
• Typical scenario: When there is sufficient sunlight during the day, photovoltaic power generation is prioritized for household loads; Excess electrical energy can be stored in batteries or fed back to the grid; At night or on rainy days, the battery is discharged for load use; When there is a power outage in the grid (if allowed), some inverters can be switched to off grid mode (pre configured).
• Inverter control logic:
Photovoltaic priority: Real time adjustment of the operating point of photovoltaic modules through MPPT algorithm (to maximize output power), supplying photovoltaic DC → AC to the load for use;
Load distribution: If the photovoltaic power is greater than the load demand, the excess electricity will be charged by the battery (rectified by a bidirectional inverter) or fed back to the grid (in accordance with the grid connection protocol);
Grid synchronization: The inverter output AC needs to be strictly synchronized with the grid voltage, frequency, and phase (error<1%) to ensure seamless grid connection;
Security protection: Real time monitoring of the power grid status (such as voltage dip, frequency offset), and if the power grid is abnormal (such as power outage), immediately stop grid connected output (islanding protection).
3.2 Off-grid Mode
The system is disconnected from the power grid, and the inverter needs to independently support household load power supply, relying on battery energy storage as the only source of energy.
• Typical scenario: Remote areas without power grid coverage or power outages (such as during natural disasters).
• Inverter control logic:
Battery power supply: battery DC → inverter inverter → AC → load;
Power balance: Real time calculation of load demand and available battery capacity. If the load power is greater than the maximum discharge power of the battery (such as a battery capacity of 5kW but a load demand of 8kW), the inverter needs to limit output (to avoid overload) or activate unloading protection (to cut off non critical loads);
Photovoltaic supplement: If the photovoltaic modules still generate electricity, the battery can be charged through a DC-DC converter (while being powered by an inverter) to extend the battery life;
Voltage stability: The inverter needs to have a built-in voltage regulation module (such as Boost/Buck circuit) to maintain stable output AC voltage (220V ± 5%).
3.3 Hybrid Mode
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• Typical scenario: When the power grid is normal, the system operates in grid connected mode; When the power grid is cut off, it automatically switches to off grid mode (seamless switching supported by the inverter); During peak electricity prices in the power grid, priority should be given to using batteries for power supply; Charging and energy storage during low electricity prices in the power grid.
• Inverter control logic:
Intelligent scheduling: dynamically adjust charging and discharging strategies (such as "valley charging, peak discharging") based on electricity price signals (such as time of use electricity prices), photovoltaic forecasts (such as weather forecasts), and load demand;
Seamless switching: Using static transfer switch (STS) or virtual synchronous machine (VSG) technology, switch to off grid mode within 10ms after power outage (ensuring that the load is not powered off);
Virtual Power Plant (VPP) Collaboration: Some high-end inverters can be connected to the virtual power plant platform to receive grid dispatch instructions (such as feeding back electrical energy to the grid during peak hours to obtain economic compensation).
4、The Intelligence and Future Trends of Inverters
With the development of the Internet of Things, artificial intelligence, and power electronics technology, household energy storage inverters are upgrading from "single conversion devices" to "intelligent energy management centers", and their future trends are mainly reflected in the following aspects:
4.1 Function Integration
• Multi in one design: The inverter will integrate photovoltaic MPPT, battery BMS (battery management system), energy storage PCS (energy storage converter), energy management EMS and other functions, further simplifying the system topology (such as Huawei's "1+X" modular inverter).
• Communication interface extension: supports Wi Fi, Bluetooth Zigbee、 Ethernet and other communication protocols enable real-time data exchange with smart homes (such as smart meters and temperature controllers) and cloud platforms (such as power grid dispatch centers).
4.2 Control Intelligence
• AI algorithm optimization: Predicting photovoltaic power generation, load demand, and electricity price changes through machine learning, dynamically adjusting charging and discharging strategies (such as "short-term power prediction based on LSTM model").
• Adaptive protection: Real time monitoring of battery health status (SOH), inverter temperature and other parameters, automatic adjustment of operating parameters (such as reducing maximum power output to extend lifespan).
4.3 Application Scenario Expansion
• V2H: The inverter supports bidirectional interaction with electric vehicles (EVs), using the onboard battery as a backup energy storage for the home (such as Nissan Leaf+bidirectional inverter solution).
• Microgrid collaboration: In community microgrids, multiple household inverters can form a "virtual power plant" to participate in auxiliary services such as grid frequency regulation and peak shaving (such as Tesla Autobidding platform).
