Calculation of daily total precipitable water vapor depth (mm) based on hourly TCWV data

ERA5 provides hourly Total Column Water Vapor (TCWV) data with the unit of kg/m2, namely mm. However, it does not mention the time interval over which this variable represents the water vapor content. Now I need daily total precipitable water (PW) to calculate Precipitation Efficiency (PE) in the daily timescale. By definition PE is the ratio of daily total precipitation and daily total precipitable water.
My question today is whether the daily total PW is derived from the average of hourly PW values or from their sum. In my opinion, the daily total precipitable water should be greater than the hourly precipitable water values, hence, daily total PW is the sum of hourly PW values. While, TCWV data is an instantaneous variable, rather than cumulative variable, which is a bit strange for me. We look forward to hearing from you soon.