What Is a Pump's Best Efficiency Point (BEP)?

The pump station checker lets you create a working pump curve from a single number pair — the best efficiency point. This page explains, in plain terms, what that point is, why one point is enough to sketch a whole curve, and exactly what maths the tool uses to do it. For guidance on where the duty point should sit relative to BEP, see the companion article BEP & pump curve selection.

BEP in one paragraph

A centrifugal pump's impeller is shaped for one particular flow rate. At that flow, water travels through the vanes exactly the way the designer intended, and most of the motor's power goes into lifting water. Run the pump faster or slower than that flow and progressively more power is lost to turbulence, internal recirculation and heat. The flow (and matching head) where losses are smallest is the best efficiency point — the BEP. It is printed on every manufacturer's datasheet, which is precisely why the tool asks for it: it is the one point you can always get. Water authorities set hard limits on how far from BEP a sewage pump may run — from ±5% of BEP flow (Watercare, Auckland) to 90–110% (Hunter Water; Water Corporation for submersibles) and 55–110% (South East Water) — covered in detail in BEP & pump curve selection.

The basic maths of pump efficiency

Example: lifting 10 L/s against 20 m of head is 1000 × 9.81 × 0.010 × 20 = 1.96 kW of water power. A pump doing that at 65% efficiency absorbs 1.96/0.65 = 3.02 kW; the missing 1.06 kW becomes turbulence and heat. Plot η against flow and you get a hill; the crest of the hill is the BEP. Since pump stations run for decades, the distance between "on the crest" and "down the side" is an electricity bill.

Why the tool uses BEP

Early in design you rarely have full curves — you have shortlists, catalogue summaries, or a single line in an options report. But centrifugal pump curves have predictable shapes: head falls from shutoff to runout in a smooth arc, and efficiency rises to a single peak. Given one anchor point — the BEP — a realistic curve of the right family can be reconstructed. That turns "we can't run the system-curve analysis until the supplier sends curves" into "we can screen the station today and confirm against the datasheet later".

How the curve is developed — the EPANET method

The tool uses the convention from EPANET, the US EPA's public-domain water network model, which has built single-point pump curves this way for decades. Three anchor points are assumed:

• Shutoff (zero flow): head = 1.33 × BEP head — or the actual shutoff head if the datasheet gives it;
• BEP: the point you entered, honoured exactly;
• Runout: head reaches zero at 2 × BEP flow.

Worked example — enter BEP = 12 L/s at 22 m and the tool generates (these exact figures, reproducible in the checker):

The efficiency curve is a parabola peaking at the BEP — η(Q) = η_bep·(2q − q²) with q = Q/Q_bep — so it is exactly η_bep at the BEP and falls away either side. NPSH required uses a generic rising shape, NPSHr = NPSHr_bep·(0.6 + 0.4q²). Both are standard textbook families, not manufacturer data.

What a synthesised curve is good for — and not

• Good for: screening a station concept, comparing pump sizes, seeing how many pumps a rising main needs, checking velocities and retention before any supplier engagement.
• Not for: final selection, motor sizing, or contractual duty guarantees. Real curves differ from the generic shape — steep or flat, stable or drooping — and only the manufacturer's tested curve captures that.

This is why every synthesised curve in the checker is labelled "synthesised from BEP — verify against the actual datasheet". When you get the real curve, paste its points in (the tool takes Q, H, η, NPSHr lines) and the label disappears along with the assumption.

Summary

• BEP is the flow where a pump wastes the least energy — the crest of the efficiency hill, printed on every datasheet.
• Efficiency is water power over shaft power: η = ρgQH/P.
• The tool reconstructs a full curve from the BEP using EPANET's public-domain convention: shutoff at 1.33 × BEP head, zero head at 2 × BEP flow, power-law fit (≈ quadratic) through all three.
• Synthesised curves are for screening; final design uses the manufacturer's curve — paste it in when you have it.