Electric Horsepower to Kilowatts for Pump Nameplates

Taking a US Pump Nameplate onto an IEC Pump Curve

Borehole and irrigation pumps travel further than the paperwork that comes with them. A set installed on a farm may carry a North American plate marked in electric horsepower, while the replacement being quoted, the curve you are reading it against and the starter panel are all specified in kilowatts. Because electric horsepower is fixed at exactly 746 W, that particular translation is one of the few in pump work that involves no rounding at all.

Conversion factor: 1 ehp = 0.746 kW exactly. A 7.5 ehp submersible set is 5.595 kW of motor rating — and at a duty of 25 m³/h against 55 m of head it is only putting 3.75 kW into the water.

What the Plate on a Pump Set Is Describing

Motor rating, not water power

The plate states what the motor can deliver to the impeller shaft. The useful power lifting water is flow times head times gravity, and it is always the smaller of the two by a wide margin.

Curves are plotted in kilowatts

Manufacturers draw absorbed power as a kW line under the head curve across the whole flow range, so a horsepower nameplate has to become kilowatts before it can be read against anything on the chart.

Submersible sets are sold by motor size

A borehole pump is a wet-end stack bolted to a standard 4-inch or 6-inch motor, and the motor size is what appears in the model code — so it is the figure that has to be matched when a set is replaced.

Irrigation duty runs for hours

An irrigation set holds one operating point for an entire shift, so the absorbed power at that point — not the peak anywhere on the curve — is what drives the energy bill and the cable sizing.

Landing the Nameplate on the Right Point of the Curve

The order matters: convert first, then find the duty point, then check that the absorbed power at that point still sits inside the motor rating across the whole operating range.

Enter the horsepower marked on the set

Type the nameplate figure on the left — 0.5, 1, 3, 7.5, 25. The kilowatt value appears immediately, with a comma accepted as the decimal separator for fractional sizes and stray spaces ignored.

Read it against the power line on the chart

Find your flow on the horizontal axis, drop to the power curve, and compare that reading with the kilowatt figure you just produced. A pump whose absorbed power crosses the motor rating anywhere in the working range is a non-overloading selection failure.

Turn a kilowatt curve figure back into a nameplate size

Press the swap button (↔) for kW → ehp when the catalogue offers a metric motor and you need the nearest plate marking. An 11 kW motor answers 14.7453 ehp, which in practice means the 15 size.

Carry the value into the selection sheet

The copy button hands over the plain number without a unit or thousands spacing, ready for a pump selection form or a cable calculation. Ctrl + C inside a field does the same thing.

Check the frequency before you trust the curve: a set built for 60 Hz running on a 50 Hz supply turns roughly a sixth slower, which cuts flow by about the same fraction, head by nearly a third and absorbed power by close to half. Every figure on the published chart shifts with it.

Borehole and Irrigation Motor Sizes at Their Duty Points

Common pump set sizes with the plate rating converted to kilowatts, a representative duty for each, and the hydraulic power that duty actually represents for clean water.

Nameplate (ehp)	Motor rating (kW)	Typical duty	Water power (kW)
1 ehp	0.746 kW	3 m³/h at 40 m	0.33 kW
3 ehp	2.238 kW	10 m³/h at 55 m	1.50 kW
5 ehp	3.73 kW	15 m³/h at 60 m	2.45 kW
7.5 ehp	5.595 kW	25 m³/h at 55 m	3.75 kW
10 ehp	7.46 kW	30 m³/h at 65 m	5.31 kW
15 ehp	11.19 kW	45 m³/h at 65 m	7.97 kW
25 ehp	18.65 kW	70 m³/h at 70 m	13.35 kW
50 ehp	37.3 kW	120 m³/h at 80 m	26.16 kW

Water power here runs from about 44 % of the motor rating on the smallest set to roughly 70 % on the larger ones. Part of that gap is pump efficiency, which improves steadily with size, and part is the deliberate margin left so the motor is never overloaded at any point the pump might be run. The gap is why a metered energy bill will never divide neatly into the cubic metres delivered.

What This Page Does at the Selection Stage

Both figures move while you scan a pump range

Each field updates the other as you type, so a whole column of catalogue sizes can be walked through in one pass without clearing anything.

Go from a metric motor back to a plate size

The swap button runs kW → ehp, the direction you need when the pump is offered with an IEC motor and the site paperwork is written in horsepower.

Watts and hp from the same pump rating

Searchable dropdowns on both sides carry every power unit, so a hydraulic figure in watts or a mechanical horsepower spec goes through the same two fields.

Plain numbers for the selection sheet

Values carry up to eight decimals and the copy button strips the unit, which suits a pump selection form or a spreadsheet running a whole irrigation block.

Pump Sizing Questions from the Borehole Yard

Is the motor's kilowatt figure the same as the power delivered to the water?

No, and mixing them up is the commonest error in pump paperwork. Hydraulic power is flow times head times density times gravity: for clean water it comes to roughly flow in m³/h multiplied by head in metres, divided by 367, giving kilowatts. At 30 m³/h and 65 m that is 5.31 kW, delivered by a motor plated at 10 ehp — 7.46 kW. The nameplate is what the motor can give the shaft; hydraulic power is what reaches the water.

Why does the water power come out so far below the motor rating?

Two separate reasons stack up. Pump efficiency accounts for most of it — a small multistage borehole pump may only reach the mid-forties in percent, while a well-selected larger unit gets into the seventies — and the loss appears as heat and turbulence inside the stages. On top of that, a non-overloading selection deliberately leaves margin so the motor is safe even if the pump runs off its design point. Efficiency also falls away either side of the best efficiency point, so a set running far out on its curve widens the gap further.

Are submersible borehole motors rated the same way as surface motors?

The horsepower means the same thing — output at the shaft — but the conditions behind it do not. A submersible motor is cooled by the water flowing past it, so its rating assumes a minimum flow velocity over the casing; a set hanging in an oversized bore or above the screen may need a flow sleeve to stay within rating. The motor is also built to a fixed diameter class, typically 4-inch or 6-inch, which caps how much power can be fitted at all. Starts per hour are limited too, which is why a correctly sized set with a decent pressure vessel outlives an oversized one that cycles.

Why do manufacturers draw the power line in kilowatts rather than horsepower?

Because the rest of the chart is metric. Head is in metres, flow in cubic metres per hour or litres per second, and the hydraulic relation between them produces kilowatts directly with a single constant. Printing the absorbed power in the same system keeps the efficiency curve consistent and lets one sheet serve every market the pump is sold into, with the horsepower equivalent left to a conversion in the model code.

Where does NPSH fit into this power calculation?

It does not — net positive suction head is a pressure margin measured in metres of liquid, not a power figure, and no conversion here touches it. It answers a different question: whether there is enough absolute pressure at the impeller eye to stop the liquid flashing to vapour. Get it wrong and the pump cavitates, at which point head collapses, absorbed power becomes erratic and the impeller starts eroding — none of which a correctly converted motor rating can rescue.

Electric Horsepower to Kilowatts