What are the structural differences between grid-connected solar systems and off-grid solar systems?

Home or commercial energy storage users often have doubts when choosing between on-grid or off-grid. All right. What are the main structural differences between grid-connected solar systems and off-grid solar systems? The following are several aspects to explain.

First, the composition of the power generation system

Choose the difference between solar panel capacity

Solar panels are solar users have equipment, for grid-connected users, the selection of solar panel capacity is usually based on the grid access requirements and the user’s electricity demand to design. For example, in a small residential grid-connected system, a 3-5kWp (kilowatt peak) solar panel array might be installed. Its main role is to convert solar energy into direct current, and then through the inverter to convert direct current into alternating current, most of the power directly into the grid.

For off-grid users, the capacity of solar panels only needs to consider the power consumption of the load and the charging needs of the energy storage device. If you want to power an off-grid cabin in a remote mountain area, the capacity of the solar panels must be able to meet the electricity needs of all the appliances in the cabin (such as lighting fixtures, small refrigerators, etc.) without external power supplement. It is also necessary to consider factors such as local sunshine conditions to determine the power of the panels, which may be smaller than the panel capacity of the grid-connected system, depending on the size of the load, but it may also increase the capacity to ensure sufficient energy storage.

The difference between inverters

Inverter, its function is to convert the direct current generated by the solar panel into alternating current with the same frequency and phase of the grid, in order to smoothly integrate the electric energy into the grid. For example, in China, the AC frequency of the power grid is 50Hz, and the AC output of the grid-connected inverter must also be 50Hz, and it must match the voltage, phase and other parameters of the power grid, so as to ensure the power quality and the safe and stable operation of the power grid.

Off-grid inverters or charge controllers/inverters are usually used in an off-grid inverter. Off-grid inverters convert direct current into alternating current for load use, and in some systems, they also need to have the function of charge management for energy storage batteries. Unlike grid-connected inverters, off-grid inverters do not need to consider the synchronization problem with the external grid, which mainly provides stable AC output according to the needs of the load.

The difference between energy storage systems (batteries)

Under normal circumstances, energy storage batteries can not be configured in grid-connected systems. When there is surplus electric energy, the electric energy can be transferred to the grid; When solar power is insufficient, electricity can be obtained from the grid. However, in some grid-connected systems with special needs, such as in order to continue to power critical loads when the grid is outage, a small number of energy storage batteries will be configured, but their capacity is relatively small.

The energy storage system is an essential part. Since the off-grid system is not backed by the grid, the energy storage battery needs to store enough electricity to meet the load’s electricity demand at night or when the sun is not enough. Common energy storage batteries are lead-acid batteries, lithium batteries and so on. For example, for an off-grid communication base station solar power supply system, it is necessary to configure a large-capacity lithium battery pack to ensure that the base station equipment can operate normally during the period without sunlight, and the battery capacity may vary from a few kilowatt-hours to tens of kilowatt-hours, depending on the power consumption of the base station equipment and the length of the sunless time.

Two, distribution system part

Grid-connected solar energy systems have special grid-connected access equipment, including electricity meters (two-way meters), grid-connected switches, grid-connected cabinets, etc. Two-way meters record the amount of electricity delivered to the grid by the solar power system and the amount of electricity taken from the grid. The grid-connected switch is used to safely disconnect the solar power generation system from the grid when the system fails or needs maintenance. In the entire distribution process, electric energy can flow bidirectional between the solar power system and the grid, and the design of the distribution line should take into account the security of grid access and power quality and other factors.

Off-grid solar systems have no equipment connected to the grid, and the power distribution is mainly inside the solar power generation system, from the solar panels to the charging controller (if there is one), to the energy storage battery, and finally through the inverter output alternating current to supply the load. Its distribution line is relatively simple, mainly around how to effectively distribute the stored electrical energy to each load, while taking into account the starting characteristics of the load and power safety and other factors. For example, for some high-power inductive loads (such as motors), devices such as starting capacitors may need to be added to the distribution line to ensure normal startup.