PV energy storage technology can not only solve the problem of unstable PV power plant output through proper charging and discharging control but can also bring certain benefits to users by adopting intelligent control strategies. PV energy storage systems can be divided into grid-connected energy storage systems,  off-grid energy storage systems and micro-grid energy storage systems.

This paper focuses on how to design a household grid-connected energy storage system.

    I. Introduction to grid-connected and off-grid systems

    Grid-connected and off-grid systems are characterised by the fact that they can operate either in grid-connected operation or individually to supply essential loads when the grid is down. They are suitable for applications where the power system is unstable, self-generation cannot be connected to the grid, and where the price of electricity for self-consumption is higher than that of electricity connected to the grid. 

The system generally consists of a photovoltaic array of solar modules, an integrated machine, a battery bank, loads and the grid.

    Solar PV modules

    The solar photovoltaic module is the central part of a solar power system and the most valuable component of a solar power system. Its function converts the sun's radiant energy into DC electrical energy.

    Grid-connected all-in-one machine

     The MPPT controller regulates and controls the power generated by the solar modules, maximises the charging of the battery and protects the battery from overcharging and over-discharging. The DC/AC inverter converts the DC power from the modules and the energy storage lithium batteries into AC power for loads.

    Battery banks

    The main task of the battery bank is to store energy to keep the load powered at night or during rainy days.

    II. Key points in the design of household and off-grid energy storage systems

     Determine the power of the grid-connected inverter according to the type and power of the load.

     Household loads are generally divided into inductive and resistive loads. Loads with motors such as washing machines, air conditioners, refrigerators, water pumps and range hoods are inductive loads, and the starting power of the motor is 5-7 times the rated power, so the starting power of these loads should be taken into account when calculating the power of the inverter.

    The output power of the inverter should be greater than the power of the load. However, for the average household, the sum of the load powers can be multiplied by a factor of 0.7 to 0.9 to save costs, considering that all the loads cannot be switched on at the same time.

    Starlight's energy storage lithium batteries, including rack-mounted, wall-mounted and cabinet-mounted, are suitable for small and medium-sized optical storage systems, with a beautiful appearance, compact structure, easy installation and support for various monitoring methods.

    Confirming module power based on electricity consumption

    The design principle of the module is to meet the daily power demand of the load under average weather conditions, which means that the annual power output of the solar module should be equal to the annual power consumption of the loads. Because weather conditions are below and above average, solar modules should be designed to meet the needs of the worst light seasons; that is, the battery can be fully charged every day in the worst light seasons.

    Determining battery capacity based on night-time power consumption or desired power consumption

    The needs of energy storage systems are diverse, and the design of energy storage systems does not necessarily have to be based on the above principles but rather on the user's specific needs. "PV + energy storage" is currently the most reliable and promising distributed solution with tremendous potential for large-scale application.