When doing household photovoltaic system promotion, people often ask, does the electricity generated by photovoltaic need to be stored? How do you use electricity at night? To understand these problems, we need to understand the concepts of on-grid system and off-grid system.

1. Grid-connected system.

The grid-connected system requires the photovoltaic system to be connected to the power grid, and the grid-connected system can be divided into "self-use, surplus electricity online" mode and "full Internet access" mode.

Residential grid-connected power generation system is the installation of solar modules on the roof of residents, the system is connected to the grid, and the power supply task is shared. In the case that the amount of sunlight radiation reaches the power generation requirements, the inverter in the system converts the direct current generated by the photovoltaic power generation system into alternating current, which directly supplies power to the residential household load, and the insufficient part is supplemented by the municipal power supply, and the photovoltaic power generation is greater than the residential household electricity consumption, the remaining power is incorporated into the grid. This is the so-called "spontaneous self-use, surplus electricity online" mode.

Users can also choose to directly integrate all the power generated by the photovoltaic system into the grid. All the electricity needed for household load is supplied directly by the grid. This is the "full access" model.

Characteristics of grid-connected system:

1) Connect to the power grid, and upload part or all of the electricity to the power grid.
2) Power grid outage, photovoltaic power generation also stopped. Because the inverter has anti-island, that is, the power grid company requires: power grid power failure, photovoltaic power must also be immediately cut off. The main concern is safety.
3) Residents still rely on mains electricity at night.
4) No energy storage device.

Therefore, the grid-connected power generation system needs to rely on the grid, and the generated energy is transmitted to the grid, which does not need to be stored. At night, we still rely on the grid, on the mains.

2. Off-grid system.

Principle and composition of off-grid system. The working principle of the off-grid power generation system is very similar to that of the grid-connected power generation system, except that the power of the off-grid system is not transmitted to the public grid and is directly consumed by the load. Taking into account the volatility of photovoltaic power generation, such as power generation during the day and no power generation at night, good sunny day power generation less or no power generation in rainy weather, residents off-grid power generation system needs to be equipped with batteries (groups). The off-grid solar power generation system consists of photovoltaic modules, controllers, inverters and batteries (groups).

Through the understanding of the principle of off-grid system, we can find that the off-grid system has the following characteristics:

a. Independent systems that do not rely on the power grid. That is, regardless of whether there is mains electricity, as long as the sun is satisfied, the off-grid system can work independently and provide electricity independently.
b. There are energy storage devices. That is, the battery is needed, otherwise it cannot work at night or in rainy days.
c. High cost. Because the battery cost is higher, it increases the cost of the off-grid system. In addition, the design of the battery needs to consider a certain margin, such as how many rainy days and so on. On top of that, batteries have to be replaced every few years, so the cost of an off-grid system is high.

3. The application of grid-connected and off-grid systems.

(1) Application of grid-connected system.

a. Household system. At present, the main form of household photovoltaic in rural areas is the grid-connected system. Now the environmental pollution is serious, especially the winter haze is affecting everyone's health. As a green energy, photovoltaic power generation is environmentally friendly and pollution-free, more conducive to energy saving and consumption reduction, and effectively reduce the reliance on coal for electricity. The household system is mainly a full Internet access system, mainly because the residential electricity consumption is less and the price is relatively low.

b. Industrial and commercial users. Industrial and commercial electricity prices are higher, more choose the "self-use, surplus electricity online" mode. In addition, the grid-connected system also includes some large ground power stations. Such projects cover a wide area, large installed capacity, high voltage level, and high investment costs.

(2) Application of off-grid system.

Off-grid systems are mainly used in remote areas without electricity to solve the most basic local power needs. Off-grid systems are also often used in remote wasteland mountains, islands, mobile base stations and other areas where power is difficult to reach.

a. The most common off-grid system in reality is solar street lights. Solar street lamp system is a typical off-grid system. Solar street lights are mainly composed of batteries, panels, lamp holders and controllers. Solar street lights do not require mains electricity, generate their own electricity, store their own electricity, and convert electricity into light energy at night.

The solar street lamp integrates the two directions of the photoelectric professional application: light generation, electricity generation. Photoelectricity, relying on solar panels, that is, photovoltaic direction; Electricity generates light, relying on LED. Both are ultimately semiconductor pn junction applications in real life.

b. In addition, there is an off-grid system for agricultural applications that is more important - photovoltaic pumps. Photovoltaic water pump is mainly composed of inverter, panel and water pump. It can achieve "water with light", and can also cooperate with water storage facilities, photovoltaic pumps are mainly used in some areas that need irrigation, but lack of power resources.