Watt -> kWh: what am I doing wrong?

I am trying to convert the watt values of a sensor (a regular dryer) to kWh. If I graph the energy in a graph, then it looks pretty reasonable (see below)

Now if I transform the watt values into a kWh total per 24 hours based on the query below I get values which cannot be right.
SELECT mean("value") *24/1000 FROM "W" WHERE ("entity_id" = 'energy_dryer') AND $timeFilter GROUP BY time(24h) fill(none)

Values of 13 kWh per day are, obviously, wrong. What am I doing wrong here?