Inverter and clipping

The DC power of the strings is aggregated per MPPT input and converted to AC in the inverter.

Efficiency

With a measured curve (.OND)

The efficiency depends on the DC power and the input voltage of each hour (the file contains efficiency profiles for several voltages). The engine interpolates between them. It includes:

• the start-up threshold: below a certain power the inverter does not deliver;
• the night-time consumption (stand-by);
• the device's maximum output power limit (not just the nominal one).

Without a curve (PVWatts)

NREL PVWatts model with the inverter's European efficiency and a reference efficiency of 96.37 %: the efficiency varies with load (worse at low power) according to an empirical expression.

Clipping

When the available DC power exceeds what the inverter can deliver, it is clipped. With a high DC/AC ratio, clipping appears in the middle hours of clear days; it is a loss that often pays off because for the rest of the year the inverter works closer to its nominal power.

• With an inverter with independent MPPTs, each input is limited to the nominal power divided by the number of inputs (as PVsyst does).
• The clipped energy appears in the waterfall as «Inverter: clipping at nominal power».

Several inverters

Each inverter is calculated separately with its inputs. In shared MPPT mode, the inputs contribute to a single set.

Night-time consumption

The inverter consumes a little when there is no sun. It is treated as one more load (from the grid or the battery) and recorded separately; it does not alter the user's consumption.

How to reduce clipping

• Reduce the DC/AC ratio (fewer modules or a larger inverter).
• Spread the modules over more MPPT inputs.
• Add a battery (it stores the energy that would be clipped).
• Assess it with the scenario comparator: if clipping is small, the savings of a smaller inverter usually pay off.