A Comprehensive Guide to Residential Energy Storage ...

Source: GoodWe Solar Community, Haitong Securities Research Institute

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The AC coupling system is 100% compatible with the power grid, easy to install and easy to expand. Standard household installation components are available, and even relatively large systems (2KW to MW level) are easily expandable and can be combined with grid-connected and stand-alone generator sets (diesel units, wind turbines, etc.). Most string solar inverters above 3kW have dual MPPT inputs, so long strings of panels can be installed in different orientations and tilt angles. At higher DC voltages, AC coupling is easier, less complex and therefore less costly to install large systems than DC coupled systems requiring multiple MPPT charge controllers.

AC coupling is suitable for system transformation, and it is more efficient to use AC loads during the day. Existing grid-connected PV systems can be transformed into energy storage systems with low investment costs. It can provide users with safe power protection when the grid is out of power. It is compatible with grid-connected PV systems from different manufacturers. Advanced AC coupling systems are often used for larger off-grid systems and use string solar inverters combined with advanced multi-mode inverters or inverter/chargers to manage batteries and grids/generators. Although relatively simple to set up and powerful, they are slightly less efficient (90-94%) when charging batteries compared to DC coupling systems (98%). However, these systems are more efficient when powering high AC loads during the day, reaching more than 97%, and some systems can be expanded with multiple solar inverters to form microgrids.

AC coupling is less efficient and more expensive for small systems. The energy going into the battery in AC coupling must be converted twice, and when the user starts using that energy, it must be converted again, adding more losses to the system. Therefore, when using a battery system, AC coupling efficiency drops to 85-90%. AC coupled inverters are more expensive for small systems.

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The off-grid household photovoltaic + energy storage system is generally composed of photovoltaic modules, lithium batteries, off-grid energy storage inverters, loads and diesel generators. The system can realize direct charging of batteries by photovoltaics through DC-DC conversion, and can also realize bidirectional DC-AC conversion for battery charging and discharging. During the day, photovoltaic power generation first supplies the load, and then charges the battery; at night, the battery discharges to supply the load, and when the battery is insufficient, the load is supplied by diesel generators. It can meet the daily electricity demand in areas without power grids. It can be combined with diesel generators to enable diesel generators to supply loads or charge batteries. Most off-grid energy storage inverters do not have grid connection certification, and even if the system has a grid, it cannot be connected to the grid.

Off grid inverter

Source: Growatt official website, Haitong Securities Research Institute

Off-grid home photovoltaic + energy storage system

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