This online calculator give instant answers 

Power Number is a dimensionless parameter based on the power in Watt consumed by the agitating rotor = impellers for the SUB and turbines for the SUF. Use the below online calculator to easen up your calculations.

Watt = Power Number x Density x Agitator speed³ x Rotor Diameter⁵

\begin{align*}
    \rm P &= N_p n^3 \rho {D}^5 &&&\rm or &&&&& Np = {P \over pn^3 D^5}
\end{align*}

Where
P = Rotor power, Watt
N_p = Power Number
ρ = Density of liquid, kg/m³
n³ = Rotor/agitator speed measured in revolutions / second
D⁵ = Rotor/impeller diameters in meters

Remember that Power Number decreases with an increase in Reynolds number. Read articles to be downloaded on the first page.