Flat fraction and periods

Method

Inside a shutdown period the system delivers nothing to the grid. The AC losses and the day auxiliaries are zero there, the night auxiliaries are still drawn, and the whole inverter output counts as unavailability. A timestep belongs to a period when its start falls inside it; the start of a period is read in the weather file's local standard time. On every other timestep with a positive energy, the flat fraction takes its share of the energy that would reach the grid, as the last step of the chain.

The math

1. Outside the periods
2. Inside a period

$$E_{grid} = (1 - u)\,E_{avail} \quad (E_{avail} > 0) \tag{1}$$ $$L_U = \max(E_{inv},\ 0),\qquad E_{grid} = \min(E_{inv},\ 0) - E_{night} \tag{2}$$

where

Symbol	Meaning	Unit
$E_{grid}$	the energy injected into the grid in the timestep	Wh
$u$	the flat unavailability fraction	—
$E_{avail}$	the energy of the timestep after the auxiliaries	Wh
$L_U$	the unavailability loss of the timestep	Wh
$E_{inv}$	the output of the inverters in the timestep	Wh
$E_{night}$	the night draw of the auxiliaries in the timestep, zero while the sun is up	Wh

References

1. PVsyst 8 help, Unavailability loss
2. Analysis of PVsyst 8.1.5 outputs shows this convention in its results.

API

ac_losses.use_unavailability = true

Symbols follow the symbol registry.