Total dynamic head (TDH) is what a pump is actually asked to deliver: the sum of the static head (the elevation the fluid must be lifted), the friction head (every loss in pipes and fittings at the design flow), and the velocity head v²/2g. A system moving 5 L/s of water through 50 m of 100 mm pipe against a 10 m lift adds about 0.31 m of friction and 0.02 m of velocity head to reach a TDH of roughly 10.33 m — the number the pump curve must meet at that flow.
Only the static component is fixed; the friction part is computed segment by segment with the Darcy-Weisbach equation hf = f·(L/D)·v²/(2g), the friction factor coming from Swamee-Jain in turbulent flow or 64/Re in laminar, plus minor losses from the summed fitting K-values. Because friction grows with flow, plotting TDH across the operating range produces the system curve, and the pump’s duty point sits where its own curve crosses it.
TDH converts directly into the power bill: the required shaft power is P = ρ·g·Q·H/(η_pump·η_motor), rounded up to the next standard motor rating so the drive carries margin above the hydraulic duty at the design point.