Fan Shaft Power Calculation

Without seeing your work, it's difficult to find a specific error.

Your formula looks correct if the flow rate and pressure differential are measured at the same time. If the pressure differential is taken at zero flow (blocked fan) and the flow rate is measured with no obstruction, then you cannot multiply them to get power.

Like Rixter, without knowing details it is impossible to answer. Maybe you have a unit conversion problem. Seeing that you are measuring in metric units try using:

P_i = dp q

where

P_i = ideal power consumption (W)

dp = total pressure increase in the fan (Pa, N/m²)

q = air volume flow delivered by the fan (m³/s)

Not sure what type of fan you are looking at but you will need to assume an efficiency (if you don't know, I suggest something around 70%, then multiply Watts by 0.00134 to get HP.

You need to give details of your figure, with inputs, and the figure the dropbox gives before we can say much. But a couple of thoughts.

Your formula only works ignoring gas compression, though for the low pressure rise of a fan, typically < 1000Pa, it's usually near enough.

PSF must be total pressure rise, including velocity pressure 0.5*ρ*V². For velocity say 20m/s, velocity pressure is about 240Pa, which could be a big part of the total rise.

Please share the calculation for the benefit of forum readers.