HP to kW Calculator

The formula#

P(kW) = HP × 0.7457

One mechanical horsepower is 745.7 W, which almost everyone rounds to 746. The rounding is a 0.04 % error and it has never mattered.

There are two horsepowers, and that one does matter#

Mechanical horsepower — the imperial one — is 745.7 W. It comes from James Watt's figure of 33,000 foot-pounds per minute, which is where the horse in the name came from.

Metric horsepower — written PS or CV — is 735.5 W, defined as 75 kgf·m/s.

They are 1.4 % apart, and European and Indian motor practice has long quoted the metric one. So a pump listed as 5 HP is 3.729 kW in one catalogue and 3.677 kW in another, and neither is wrong.

That difference is invisible in a rule of thumb and awkward when somebody is checking a delivered nameplate against a purchase order. The calculator shows both so you can see which one your figure matches.

The mistake that undersizes cables#

Here is the one that costs real money.

A motor's HP rating is its mechanical output — the power at the shaft. It is not what the motor draws from the supply. The motor has losses, so the electrical input is always higher:

Input (kW) = Shaft (kW) / efficiency

Take that 5 HP motor at 88 % efficiency:

Shaft output = 5 × 0.7457 = 3.73 kW
Electrical input = 3.73 / 0.88 = 4.24 kW

4.24 kW, not 3.73 kW. Fourteen percent higher. Size a cable or a breaker from the 3.73 and you have undersized it by that margin before you have applied a single derating factor — and derating is what makes an already-tight cable fail. The cable sizing calculator works from current, and the current follows the input power, not the output.

And a third term for current#

To get from horsepower to amps you need power factor as well, because the motor draws reactive current to magnetise its core:

I = HP × 746 / (√3 × V × PF × η)

At 415 V three-phase, 0.85 power factor and 88 % efficiency, that 5 HP motor draws about 6.9 A. The HP to amps calculator does this directly, and adds the starting current — around six times full load on a direct-on-line start, which is what the protection has to survive rather than what the cable has to carry.

Quick reference#

Efficiency varies with size and with load: a small motor is worse than a large one, and any motor running lightly loaded is worse than the same motor at its rated point. Take the figure from the nameplate where you have it.

Which figure to specify#

If you are buying, specify in kW. It is the SI unit, IS and IEC motor standards use it, and it removes the metric-versus-mechanical ambiguity entirely. Horsepower survives in pumps, compressors and vehicle engines, and there is no harm in it — as long as everyone in the conversation means the same 746.

Where the conversion meets the tariff#

Converting a motor rating into kW is not only a sizing exercise. On most Indian commercial and industrial connections, the sanctioned load on the service agreement is expressed in kW or kVA, and the sum of connected motor ratings is what the utility uses to assess it.

That has two consequences worth knowing before the paperwork is filed. Exceeding sanctioned load attracts a penalty that is typically applied to the excess demand and can be substantially higher than the normal demand rate, so a plant that quietly adds a 15 kW compressor to a connection sanctioned for 50 kW is buying an ongoing charge rather than a one-off fee. And sanctioned load frequently determines which tariff category the connection falls into at all — crossing a threshold can move a supply from a low-tension to a high-tension tariff, which changes the metering arrangement, the transformer ownership and the rate structure together.

Where the conversion sits near a boundary, it is worth being precise about which horsepower is being converted, because the difference between metric and mechanical horsepower across a large motor list can be enough to cross one.

Input power, not shaft power, is what the utility meters#

The nameplate figure is output at the shaft. The connection draws that plus the losses, and the losses are what the meter records along with everything else.

A 22 kW motor at 91 per cent efficiency draws about 24.2 kW at full load. Across a plant with a hundred motors, the gap between the sum of the nameplates and the actual demand is not a rounding error — it is a couple of hundred kilowatts, and it is the reason a load list built from shaft ratings consistently under-predicts the maximum demand charge.

Efficiency also falls away from full load. A motor at 50 per cent loading holds most of its rated efficiency; at 25 per cent it does not, because the fixed iron and friction losses are unchanged while the useful output has quartered. An oversized motor therefore costs money twice: once in the purchase, and again in every hour it runs at a load it was never matched to.