[Les]

Originally Posted by UberBlue

Pumps.org's (aka Hydraulic Institute) definition and formula 1.2.6.3

That formula yields results as water horsepower. From there I convert to watts. 1 water horsepower = 746.043 watts.

(metric) P = Q*H*s/366

where:
Q = flow M^3/hr
H = Head in meters
s = specific gravity
......

The formula
(metric) P = Q*H*s/366

is the formula for "Pump output power (P w )" in Watts
where:
Q = flow lph
H = Head in meters
s = specific gravity.

The source makes the, perhaps, misleading statement that "Pump output power (P w )" is "also called water horsepower." They also use "Q = rate of flow, m 3 /h" for another formula on the same page.
These two facts could lead to your interpretation of the equation.
However the correct interpretation is as shown.

Here lolito_fr gives a derivation of (Pw) in Watts:
"Derived formula: P=0.16Q.H
P in Watts
Q in lpm
H in m"
These are (allowing for approximations) the same formula.

My usual statement of the formula is "Watts ~ m(H2O)*LPM/6". The " ~" used to imply an approximation.
My non-rigorous derivation for "Pump output power (P w )" gives:
Power(Horsepower) = 0.00365 x Head(mH2O) x Flow(m^3/h)
I stand by this as being numerically correct.