Power and efficiency

On a bike, in a gear that’s too low you pedal like crazy and go nowhere. In a gear that’s too high you strain and wobble. In between there is a gear that feels just right.

Pumps have a “just right” too: the best efficiency point, where the most of the motor’s power ends up in the water.

The useful power a pump gives the water is its hydraulic power: density × g × flow × head.

For water there is a shortcut: kW ≈ flow (m³/h) × head (m) ÷ 367. Our pump at 40 m³/h and 30 m gives the water about 3.3 kW.

Now the chart also shows efficiency (green dashed line). Move the valve and watch efficiency and motor power.

The green band is where pump makers want you to run: roughly 70–120% of the BEP flow.

At 75% efficiency, a quarter of the power is wasted: friction of water on the vanes, swirl in the casing, water leaking back past the wear rings, and the bearings and seals.

All of it becomes a little heat and noise. The motor itself wastes a few percent more.

Far to the left (too little flow), water swirls back at the eye and the outlet — called recirculation. The pump gets noisy, hot and shakes.

Far to the right (too much flow), the water rushes in too fast and cavitation begins. Near the BEP the flow meets the vanes smoothly: less noise, less wear, lower bills.

Over its life, a pump usually costs many times more in electricity than it cost to buy. A few percent of efficiency, over 20 years of running, is a lot of money — and a lot of CO₂.