The core characteristic of an off-grid photovoltaic power generation system is its independent operation. It is disconnected from the mains power supply, allowing it to use either photovoltaic power generation or energy storage, or mains power. Furthermore, it can operate independently even in the event of a mains power outage.

The main system equipment includes photovoltaic modules, batteries (energy storage system), off-grid inverters, reverse current protection devices, surge protectors, etc.

Functions of Each Component:

① Photovoltaic Modules: Photovoltaic modules are one of the core components of an off-grid solar photovoltaic (PV) power generation system. Their function is to directly convert solar radiation energy into direct current (DC) electricity. The power output of the modules directly determines the overall power generation of the PV system. The power and quantity of PV modules can be selected according to actual needs. The electricity generated by the PV modules can be directly supplied to the load through an inverter or stored in batteries for later use.

② Surge Protectors: These protect against surge voltage impacts caused by lightning strikes or voltage fluctuations in the PV modules, ensuring the safe and reliable operation of the system.

③ Off-grid inverter: One of the core components of an off-grid solar photovoltaic (PV) power generation system, it is the “brain” of the system. Its main responsibilities include:
1) Converting the direct current (DC) generated by the PV modules into 220V alternating current (AC) usable by household appliances (loads);
2) When the PV power generation exceeds the load’s power requirement, storing the excess power in a battery (energy storage) for later use;
3) When the sum of the PV power generation and the battery power (i.e., when the battery is depleted) is less than the load’s power, drawing power from the mains to supplement the load’s operating capacity.

These switching actions are performed automatically by the inverter without manual intervention.

④ Battery: An energy storage device whose main function is to store excess energy when the PV module’s power generation exceeds the load’s demand, and then supplying it to the load when the PV power generation is low or nonexistent. While off-grid PV power generation systems may not require batteries, installing suitable batteries can significantly improve the system’s efficiency.

⑤ Anti-backflow device: Photovoltaic power generation is intermittent and fluctuates. When the generated power exceeds the load power and the battery is fully charged, the excess energy may flow back into the grid, a phenomenon known as “backflow.” Backflowing energy causes voltage fluctuations and increases harmonics in the grid, affecting power quality, interfering with normal grid operation, and potentially causing safety hazards such as localized overheating and short circuits. Therefore, large-scale photovoltaic power plants need to be equipped with anti-backflow devices, and it’s best for residential off-grid photovoltaic power generation systems to also be equipped with them. (If a photovoltaic power generation system experiences backflow and is caught by the grid, you will be fined!)

⑥ Grid meter: This measures the amount of electricity the power generation system draws from the city’s power grid.

The core characteristic of a grid-connected power generation system (full grid connection) is its connection to the public power grid, with all generated electricity sold to the grid. Photovoltaic power generation is entirely fed into the public power grid through a grid-connected inverter, and users still purchase all their electricity from the grid, i.e., the municipal electricity supply.

System components: Photovoltaic modules, grid-connected inverter, grid-connected meter, surge protector, etc. Sometimes, anti-reverse current devices are also included to prevent grid current from flowing back to the photovoltaic modules when they are not generating electricity.

The equipment already introduced will not be repeated here. Instead, we will briefly introduce some equipment that differs from the previous descriptions: Grid-connected meter: This measures how much electricity the photovoltaic (PV) system sells to the grid, serving as a crucial basis for PV grid-connection settlement!

The difference between a grid-connected PV system and an off-grid PV system is that the electricity generated by the PV modules is converted to DC by an inverter and sold directly to the grid, without the system itself using any electricity! Its biggest advantage is that it does not require batteries and has high utilization efficiency; theoretically, every kilowatt-hour of electricity generated can be sold to the grid, generating revenue with no waste.

However, the disadvantages are also obvious. When the public grid experiences a power outage, the system automatically shuts down to ensure the safety of grid maintenance personnel (to prevent islanding), and cannot continue to supply electricity. In other words, even if your PV system is generating electricity, your home appliances will not be able to use PV power when the mains power is out. Furthermore, the returns are relatively low; the grid-connected electricity price is usually lower than the residential electricity price, and far lower than the industrial and commercial electricity price.

Taking Yunnan as an example, the grid-connected price of photovoltaic electricity is 0.33 yuan/kWh, while the grid electricity price is 0.5 yuan/kWh. If you spend 12,600 yuan to install a photovoltaic power generation system, it can generate 3,650 kWh of electricity per year. If you sell all of it to the grid (at a price of 0.33 yuan/kWh), you can earn about 1,200 yuan a year, and it will take 10 years to break even. However, if you use all of it yourself, and the electricity price is calculated at 0.5 yuan/kWh, you will save 1,820 yuan a year, and you can break even in 7 years.

The core characteristics of a grid-connected power generation system are: connection to the public power grid; priority is given to using the energy generated by photovoltaic power plants; any surplus is sold back to the grid; and power is drawn from the grid when needed.

System Components: Photovoltaic modules, grid-connected inverters, etc. A small amount of energy storage can be configured with a dual-power switch (referred to as a “grid-connected/off-grid energy storage system”) to provide power to some critical loads during power outages.

A two-way meter, as the name suggests, can both measure how much electricity a photovoltaic power generation system supplies to the grid and how much electricity it draws from the grid. A photovoltaic power generation system that feeds surplus electricity into the grid treats the grid as a “free power bank,” eliminating the need for energy storage batteries. Excess electricity generated can be sold back to the grid without waste. Of course, energy storage batteries can be optionally installed, typically for systems with high electricity consumption, especially at night.

In conclusion, observant readers may have noticed that both off-grid and grid-connected photovoltaic (PV) systems require the grid power to be connected to an inverter before being supplied to the load. However, the power output of typical household inverters is limited to around 6-12 kW. If the power of household appliances exceeds the inverter’s capacity, it can easily cause the inverter to shut down or pose a safety hazard. High-power inverters are also quite expensive. This leads to another type of grid-connected PV power generation mode known as “unlimited load capacity.” This mode directly connects the AC power output from the inverter to the household grid’s inlet line. However, due to space limitations, we won’t go into detail here. Interested readers can search for it on Google.

Grid-connected photovoltaic (PV) power generation is actually a type of off-grid power generation. The reason for mentioning “unlimited load capacity” PV power generation systems is that there are various grid connection and installation modes for PV power generation systems, and the required equipment also differs. Therefore, there are bound to be omissions and incompleteness in this article, and we hope you can understand.

Finally, each of the three PV power generation modes has its advantages and disadvantages. If your local power grid is unstable and experiences frequent power outages, an off-grid PV power generation system is recommended. If you can install a high-capacity PV power generation system, the grid is stable, and your own electricity consumption is low, then a fully grid-connected mode is recommended. If the grid is stable, and your daytime electricity consumption is high (especially for users with high electricity prices), then self-consumption with surplus electricity fed into the grid is recommended.