Calculating KW from Hp or from Amps & Volts?

Question

I a trying to figure out how much KWH a pump costs me to run all day. The sticker on the pump says:

8 Amps

230 Volt

1.5 HP

I know that KW = HP x 0.746 so that is:

1.5 HP x 0.746 = 1.1 KW

but I also know that Watt = Volt x Amp so that is:

230 Volt x 8 Amp = 1840 W = 1.84 KW

Why are these not equal? What am I missing here to get KW??

THANKS.

Answer 1

u made a mistake! 1hp=1kw *0.7457

check its just very near.current may not be sharply 8 amp.

Answer 2

This must be a reactive load....  

In DC world, you are correct.  V*A=W

In AC world, when the load is reactive, (ie. not purely resistive), that is not so simple.

VA = V*A

Watts=VA * power factor

Power factor gets involved because V and A does not peak at the same time in reactive load under AC.  

You are missing the power factor.  In fact, you can do backwards and figure it out.

Answer 3

cos theta!!

i mean power factor (it is less than 1)..coz real power in watt = V*I*cos theta

Answer 4

1.5 HP x 0.746 kW/HP = 1.1 kW

230 V x 8 A = 1840 VA or 1.84 kVA

Power Factor, PF = kW ÷ kVA = 0.60 (looks rather low)

PF is the cosine of the phase angle between the voltage and the current.

For a pump I would expect a PF closer to 0.85.

I suspect the reason the discrepancy between kW and kVA is that the motor's nameplate has inflated the actual current draw.  Perhaps they rounded it up the next whole number.  If the current was closer to 7.5 amps, the kVA value would be 230 x 7.5 = 1725 VA or 1.725 kVA.

1.5 kW ÷  1.725 kVA = 0.87 (a more reasonable power factor)

Answer 5

There are two things missing.

One is efficiency. The HP rating of a motor is its output power. The current and voltage are input.

The second is power factor. A motor is an inductive load so its power factor will be something less than 1.

The equation is P(out) = V X I  X pf X eff

Your conversion from HP to KW is correct.

You cannot work out running cost from the nameplate but you don't know how heavily loaded the motor is anyway. The only way to do what you want is measure it, either with a power meter or switch everything else off in your house and use the supply authority's meter. Anything else would be an estimate.

Answer 6

Whencalculating KW from Hp, you will get only the output kw since pump ratings are in output Hp with the efficiency excluded. When calculating kw from Hp rating always including an assumed pump efficiency of 0.5 or higher.

When calculating kilowatt from its output rating always include an assumed power factor of 0.8.