What is the basic concept of inverter sizing ?

[André Mermoud]

The inverter power sizing is not a trivial task.

In the system definition dialog, the button "Show Sizing" opens a summary of all sizing conditions, dynamically bound to your definitions.

The upper diagram shows the voltage limits for the number of modules in series: They also show in violet eventual current/power limitations of the inverter specifications, which are not mandatory, except if they are contractual (i.e. if they act on the warranty).

The lower histogram shows an estimate of the power distribution available at the output of the array in your meteo and orientation conditions.

If the inverter is of good quality, it will limit its power simply by displacing its operating point on the I/V curve of the array. In this way, the over-power has not to be dissipated: it is simply not produced. Therefore there is no technical danger to apply an oversized PV array to an inverter, this may operate at some occasions over its nominal power without problems.

The diagram shows that even for undersized inverters by respect to the high-power edge of the distribution, the annual loss may stay very low. The optimal inverter sizing will correspond to a yearly loss between 0.2 and 3%. Above this limit the normal process considers it as unacceptable, but you can change this limit.

The usual values of the PnomArray(DC)/PNomInv(AC) ratio are of the order of 1.25 to 1.30.

This ratio is depending namely on the plane orientation: with a façade system, the maximum power is 2/3 of the maximum at 30° south.

NB1: The nominal power at the DC input of the inverter is higher than the Pnom(AC), by a factor of (1/Efficiency).

NB2: In this tool, the overload loss is only a quick estimation according to this histogram.

The loss calculated by the detailed simulation will usually be lower, as the simulation takes other losses into account in the array behaviour.