In photovoltaic off-grid system, the power of off-grid inverter is generally confirmed according to the load type and power of the user.

The load is classified into resistive load, inductive load and capacitive load.Resistive load: the load with no phase difference between current and voltage is resistive load, such as rice cooker, light bulb, electric stove, electric soldering iron, etc.Inductive load: when the current lag voltage is a phase difference, the load is the inductive load, such as washing machine, air conditioner, refrigerator, water pump, range hood and other load with electric motor and transformer, relay, compressor and so on.Capacitive load: current lead voltage a load with a phase difference is capacitive load, such as compensating capacitor, computer, TV, etc.The starting power of the inductive load such as the motor is 5-7 times of 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 elevator and so on, can not be directly connected with the inverter output terminal, because the elevator in the fall, the motor reversal, will produce a counter electromotive force, into the inverter, the inverter has damage.If the off-grid system must be used, it is recommended to add a frequency converter between the inverter and the elevator motor.

If pv systems used for pumping water only, and can build a water tower, recommend the use of photovoltaic pumping inverter, can save the cost, such as a pump 2 p, if the normal inverter, are more than 5 kw off-grid inverter to drive, but also with more than 200 ah battery, the total cost of more than 10000 yuan, but if use pumping frequency inverter, need only 2 kw.

The waveform of inverter is mainly divided into two categories, one is Pure sine wave inverter, the other is modified wave inverter. Repair sinusoidal inverter, using PWM pulse width modulation method to generate modified wave output, due to the existence of about 20% harmonic distortion, can not bring air conditioning and other inductive load, but can bring lights and other resistance load.The rectifying sinusoidal inverter adopts the unisolated coupling circuit, the device is simple and the efficiency is high.The pure sinusoidal inverter is designed with isolated coupling circuit, which is complex and costly, and can be connected to any common electrical equipment (including television, liquid crystal display, etc., especially inductive load such as refrigerator) without interference.

According to the isolation of the inverter is divided into high-frequency machine and power frequency machine, the inverter back with a 50Hz power frequency isolation transformer, called power frequency inverter;Using high frequency switching technology, high frequency switching elements or high frequency isolation transformer to replace the power frequency transformer in the inverter, called high frequency inverter.The advantages of power frequency inverters are stable and reliable power components, strong overload capacity and impact resistance, while the disadvantages are low efficiency, heavy weight and high price. High frequency inverter has high frequency isolation and non-isolation of two kinds, the advantages of small size, high efficiency, low price, disadvantage is the impact resistance ability slightly lower.If the load is a large impact is the inductive load of the motor, but not often move, it is recommended to choose power frequency inverter, if the load is a small impact resistance load, often move, it is recommended to choose high frequency inverter.

The output power of inverter is larger than the load power.For monitoring stations, communication stations and other occasions with strict requirements, the output power is the sum of all the load power.However, for the average poor family, considering that all loads cannot be turned on at the same time, in order to save costs, the coefficient of 0.7-0.9 can be multiplied by the sum of load power.