Power meter: delta of values over real month

I have a permanently increasing power meter (how could I not) :slight_smile:

And I want to have monthly buckets of consumed energy via PromQL e.g.:

Aug 22: 257 MWh
Sep 22: 310 MWh
Oct 22: 201 MWh (up to now, since Oct isn’t over)

Some smart ideas with PromQL? It’s important that a month always starts on the 1st at 00:00:00.