Solar Inverter

Photovoltaic inverter (PV inverter or solar inverter) can convert the variable DC voltage generated by photovoltaic (PV) solar panels into an inverter with alternating current (AC) frequency of mains frequency, which can be fed back to the commercial power transmission system, or supplied to the grid usage of the grid. Photovoltaic inverter is one of the important balance of system (BOS) in photovoltaic array system, which can be used with general AC power supply equipment. Solar inverters have special functions for photovoltaic arrays, such as maximum power point tracking and islanding protection.

Solar inverters can be divided into the following three categories:
Stand-alone inverters: Used in independent systems, the photovoltaic array charges the battery, and the inverter uses the DC voltage of the battery as the energy source. Many stand-alone inverters also incorporate battery chargers that can charge the battery from AC power. Generally, such inverters do not touch the grid and therefore do not require islanding protection.

Grid-tie inverters: The output voltage of the inverter can be returned to the commercial AC power supply, so the output sine wave needs to be the same as the phase, frequency and voltage of the power supply. The grid-connected inverter has a safety design, and if it is not connected to the power supply, the output will be automatically turned off. If the grid power fails, the grid-connected inverter does not have the function of backing up the power supply.

Battery backup inverters (Battery backup inverters) are special inverters that use batteries as their power source and cooperate with a battery charger to charge the batteries. If there is too much power, it will recharge to the AC power supply. This kind of inverter can provide AC power to the specified load when the grid power fails, so it needs to have the islanding effect protection function.
Main article: Maximum power point tracking
Photovoltaic inverters use Maximum Power Point Tracking (MPPT) technology to draw the maximum possible power from the solar panels. There is a complex relationship between solar irradiance, temperature and total resistance of solar cells, so the output efficiency will change non-linearly, which is called the current-voltage curve (I-V curve). The purpose of maximum power point tracking is to generate a load resistance (of the solar module) to obtain the maximum power according to the output of the solar module in each environment.
The form factor (FF) of the solar cell in combination with its open circuit voltage (VOC) and short circuit current (ISC) will determine the maximum power of the solar cell. The shape factor is defined as the ratio of the maximum power of the solar cell divided by the product of VOC and ISC.

There are three different algorithms for maximum power point tracking: perturb-and-observe, incremental conductance, and constant voltage. The first two are often referred to as “hill climbing”. The method is to follow the curve of voltage versus power. If it falls to the left of the maximum power point, increase the voltage, and if it falls to the right of the maximum power point, reduce the voltage.

Charge controllers can be used with solar panels as well as with DC-powered devices. The charge controller can provide a stable DC power output, store excess energy in the battery, and monitor the battery’s charge to avoid overcharging or overdischarging. If some more expensive modules can also support MPPT. The inverter can be connected to the output of the solar charge controller, and then the inverter can drive the AC load.