There are a number of variables to consider when choosing the best inverter for a specific purpose. One of the most important aspects is the power rating of the PV array. The inverter's DC (direct current) power rating should not be more than ten percent less than the power capacity of the PV array.

While the DC value indicates the power that can be absorbed by the inverter, the AC (alternating current) power rating indicates the alternating current that the inverter is able to supply continuously to the power supply grid. The DC value must always be greater than the AC value, as a portion of the electrical power is inevitably lost in the form of heat during conversion.

• There are a number of variables to consider when choosing the best inverter for a specific purpose. One of the most important aspects is the power rating of the PV array. The inverter's DC (direct current) power rating should not be more than ten percent less than the power capacity of the PV array. While the DC value indicates the power that can be absorbed by the inverter, the AC (alternating current) power rating indicates the alternating current that the inverter is able to supply continuously to the power supply grid.
  The DC value must always be greater than the AC value, as a portion of the electrical power is inevitably lost in the form of heat during conversion.
• This applies especially to devices that comprise a low-frequency transformer. The latter adjusts the voltage of the PV array to that of the supply grid and guarantees galvanic separation between the PV array and the power supply grid.
  However, the transformer reduces the efficiency of the inverter. That is why some manufacturers do without this component, also because it makes the inverter bigger, heavier and more expensive.
• Other manufacturers use high-frequency transformers, thus limiting power loss. This type of transformer does not affect the size and weight of the inverter as much as the low-frequency transformer. However, high-frequency transformers increase the work load and the cost of the electric circuits and are not therefore capable of achieving significant reductions in price.
• If no transformer is used, the voltage of the PV array must be higher than that of the power supply grid.
  There are two ways of achieving this: either by connecting several PV modules in series (strings), which makes it more difficult to adapt the geometry of the system to the available roof area, or by using a so-called step-up converter to increase the voltage. This however also results in loss of efficiency.
  Whether to use an inverter with or without a transformer is therefore a crucial decision. There has been considerable progress in the field of transformerless inverters. Their use is, however, subject to different laws in force in the various EU countries.