Best estimate of a power number for an elephant ear impeller in a tank with 4 baffles is between 5.5 and 6.0.  Although details of the impeller, like number and shape of the blades may give different results.
 
If torque measurement is not an option, then power measurement is about the only other way.  Electric measurement devices are available that will give motor output power.  Amperage and voltage measurements will not give satisfactory results.  Wattage measurement takes into account motor power factor.  The only problem is finding a power measurement device for the appropriate range.

The answers by this expert are based on the best available interpretation of the information provided.  The consequences of the application of this information are the responsibility of the user.  If clarification is needed, please submit a further question.