The formula
P = 1,000 × 9.81 × flow × head ÷ efficiency
Where it comes from
Hydraulic power — the part that actually moves water — is density times gravity times flow times head. Sixty litres a minute at twenty-five metres comes to 245 watts. But a pump does not turn all its electricity into moving water: a fair amount is lost between the motor and the impeller, and the efficiency of the whole assembly on a domestic pump sits between 40 % and 70 %. At 60 %, those 245 useful watts call for 409 of consumption, and the remaining 164 go to heat.
How to work it out by hand
- Convert the flow rate to cubic metres per second
- Add the friction losses to the static lift
- Multiply 1,000 × 9.81 × flow × head: that is the useful power
- Divide by the efficiency for the electrical power
What is worth knowing
The head to overcome is not just the height difference. Pipework also resists the flow, and that resistance is measured in metres of water column just like the lift: those are the friction losses, and they depend on the diameter, the length and every bend and valve along the route. Forgetting them is the most common reason a pump falls short. As an order of magnitude, in a well-sized domestic installation losses run at 10 to 20 % of the static lift, and with undersized pipework they can climb far higher. An oversized pump is not the answer either: it works away from its best efficiency point, draws more and cavitates.