In a photovoltaic off-grid system, the inverter power is generally confirmed based on the user's load type and power.
Load Classification
Loads can be classified according to their impedance characteristics into: resistive loads, inductive loads, and capacitive loads.
Resistive Load: This type of load has no phase difference between current and voltage. Examples include rice cookers, light bulbs, electric stoves, and soldering irons.
Inductive Load: This type of load has a phase difference where current lags voltage. Examples include washing machines, air conditioners, refrigerators, water pumps, exhaust fans (which have motors), transformers, relays, and compressors.
Capacitive Load: This type of load has a phase difference where current leads voltage. Examples include capacitor compensators, computers, and televisions, which commonly use switch-mode power supplies.
Inductive loads such as motors may have a starting power that is 5-7 times their rated power, so when calculating the inverter power, the starting power for these loads must be taken into consideration.
If the load is something like an elevator, it cannot be directly connected to the inverter's output because when the elevator descends, the motor reverses and can generate back electromotive force, which could damage the inverter. If using an off-grid system is necessary, it is recommended to add a variable frequency drive (VFD) 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 suggested to use a photovoltaic water pumping inverter to save costs. For instance, a 2P water pump would require an off-grid inverter of over 5kW if using a standard inverter, along with a battery of over 200AH, leading to a total cost exceeding 10,000 yuan. However, if using a water pumping VFD, only a 2kW inverter is needed, costing less than 3,000 yuan.
Types of Inverter Waveforms
Inverters can primarily be divided into two categories: pure sine wave inverters and modified sine wave inverters.
Modified Sine Wave Inverter: It uses PWM (Pulse Width Modulation) to generate modified wave output. It may have about 20% harmonic distortion and cannot support inductive loads like air conditioners but can run resistive loads such as lights. It utilizes non-isolated coupling circuits, is simpler, and more efficient.
Pure Sine Wave Inverter: It employs an isolated coupling circuit, making it more complex and costly, but it can connect to any common electrical appliance (including TVs, LCD displays, and especially inductive loads like refrigerators) without interference.
Inverter Isolation Types
Inverters are classified into high-frequency and low-frequency (industrial frequency) types.
Industrial Frequency Inverter: This type includes a 50Hz industrial frequency isolation transformer at the back.
High-Frequency Inverter: This utilizes high-frequency switch technology to replace the industrial frequency transformer with high-frequency switching elements or transformers. High-frequency inverters come in isolated and non-isolated types. Their advantages are compactness, high efficiency, and lower cost, while their drawback is generally lower impact resistance.
If the load is highly inductive, such as motors that are infrequently moved, it is advisable to choose a low-frequency inverter. Conversely, if the load consists of low-impact resistive loads that need to be frequently moved, selecting a high-frequency inverter is recommended.
The output power of the inverter must be greater than the power of the load. For strict applications like monitoring stations and communication stations, the output power should equal the total power of all loads combined. However, for general low-income households, considering that not all loads can be on simultaneously, it is cost-effective to use a factor of 0.7-0.9 on the total load power.
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