How accurate do you need to be?

Quick and dirty method: Use the agitator motor HP as the heat input to the batch. This is a high-spot estimate unless you are overamping the motor.

More involved method: Google “mixing power”, or look in Perrys. You’ll need to calculate Re for the impeller, determine if impeller flow is laminar or turbulent, select the correct equation, and hopefully have a standard blade design so the empirical agitator constant is readily available.

Most involved method: Programs like Visimix, which I have used (no affiliation), can give mixing power after you enter the geometry of the vessel/agitator and liquid properties. It also many other mixing evaluations. However, you need to purchase a license for it.

Just measuring the actual electrical kilowatts used by the motor would be most accurate. Nameplate horsepower is the maximum continuous rating of the motor. It is the load on the motor that determines the actual hp.