how to choose inverter for off grid solar system?

Loads are classified into resistive loads, inductive loads, and capacitive loads according to their impedance properties. Resistive load: A load with no phase difference between current and voltage is a resistive load, such as rice cookers, light bulbs, electric stoves, electric soldering irons, etc. Inductive load: The load when the current lags the voltage by a phase difference is an inductive load, such as washing machines, air conditioners, refrigerators, water pumps, range hoods and other loads with motors and transformers, relays, compressors, etc. Capacitive load: A load with a negative phase difference of current ahead of voltage is a capacitive load, such as compensation capacitors, computers, TVs, etc., which are used in switching power supplies at home. The starting power of inductive loads such as motors is 5-7 times the rated power. When calculating the power of the inverter, the starting power of these loads should be taken into account.

If the load is an elevator or the like, it cannot be directly connected to the output terminal of the inverter, because when the elevator is descending, the motor reverses, which will generate a back electromotive force, which will damage the inverter when it enters the inverter. If an off-grid system must be used, it is recommended to add a frequency converter between the inverter and the elevator motor.

If the photovoltaic system is only used for pumping water, and a water tower can be built, it is recommended to use a photovoltaic pumping inverter, which can save costs, such as a 2P water pump. If a common inverter is used, an off-grid inverter of more than 5kW can be used. Drive, but also with a battery of more than 200AH, the total cost of more than 10,000 yuan, but if the pumping water inverter is used, only 2kW is required, and the price is less than 3,000 yuan.

The waveform of the inverter is mainly divided into two categories, one is a sine wave inverter, and the other is a modified wave inverter. The modified sine wave inverter uses the PWM pulse width modulation method to generate the modified wave output. Due to the harmonic distortion of about 20%, it cannot carry inductive loads such as air conditioners, but can carry resistive loads such as lamps. The modified sine wave inverter adopts non-isolated coupling circuit, the device is simple and the efficiency is high. The pure sine wave inverter is designed with an isolated coupling circuit, which is complicated and expensive. It can be connected to any common electrical equipment (including TVs, LCD monitors, etc., especially inductive loads such as refrigerators) without interference.

The inverter is divided into high-frequency machine and power-frequency machine according to the isolation. There is a 50Hz power-frequency isolation transformer behind the inverter, which is called power-frequency inverter; using high-frequency switching technology, high-frequency switching elements or high-frequency isolation The transformer replaces the power frequency transformer in the inverter, which is called a high-frequency inverter. The advantages of power frequency inverters are that the power components are stable and reliable, and the overload capacity and shock resistance are strong. The disadvantages are low efficiency, relatively heavy weight and high price. There are two types of high-frequency inverters: high-frequency isolation and non-isolation. The advantages are small size, high efficiency, and low price. The disadvantage is that the impact resistance is slightly lower. If the load is an inductive load such as a motor with a large impact, but it does not move frequently, it is recommended to choose a power frequency inverter. If the load is a resistive load with a small impact, it needs to be moved frequently, and a high-frequency inverter is recommended.

The output power of the inverter is greater than the power of the load. For the occasions with strict requirements such as monitoring stations and communication stations, the output power is the sum of all load powers. However, for ordinary poor families, considering that all loads cannot be turned on at the same time, in order to save costs, the sum of the load power can be multiplied by a factor of 0.7-0.9.