Hello. Just to get things rolling, maybe try this query to see if it gives you the results grouped by day?
SELECT mean("totalkWh")*24 FROM "modbus" WHERE $timeFilter GROUP BY time(1d) fill(none)
Also, in this thread, the poster said he also wanted to convert cumulative values to daily/hourly/monthly values, but he actually wanted Stat panels showing these values. Just to clarify, do you want the same, or do you want several regular time series graphs, e.g. one showing bars for hourly power consumption, another for daily power consumption, etc.