Sunlight + LED on a windowsill, but how do you know if plants actually get enough light in a day?

In a mixed light setup (sunlight + LED), a single PAR reading doesn’t really mean much.

PAR only shows the intensity at that exact moment.
But plants respond to the total photons they receive over the whole day — that’s DLI.
The reading keeps changing as the sun shifts.

PAR tells you the instant brightness,
DLI tells you whether the plant actually got enough light today.