Meters use some energy, but not very much. I have a pre-meter install on a 400A service, like you are anticipating.
My in-depth accuracy comparisons between Sense and my Utility data on an hourly basis indicated that the meter uses something under 2W.
Here’s the more detailed analysis. It turns out data dropouts were the biggest accuracy issue for me with Sense (my networking and the Sense monitor were both to blame - part of the issue is that I have too many smartplugs in use with Sense)
From my most recent measurements.
Sense = -0.001715kW + ( 0.999934 * Utility Reading)