Are kilowatts equal to kilovolt-amperes? Yes & no!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

19 March 2006

Last week’s article explained that a horsepower is equivalent to 33,000 foot-pounds of work per minute. It went on to state that one pound of force is equal to 4.45 newtons and at that point, many readers were lost. One question was left unanswered, namely: What is a newton? So let’s start there.

The newton (named after Sir Isaac Newton) is the metric standard unit of force. It is defined as the amount of force that would increase (or decrease) the speed of a one-kilogram mass by one metre per second in one second. Now, near the surface of the Earth, gravity accelerates everything by about 10m/s every second (about 9.7 m/s per second in most parts of Kenya).

Therefore the amount of gravitational force “felt” by any object near the surface of the Earth is approximately 10 newtons per kilogram. If you weigh, say, 50kg, you will feel a gravitational force of about 500 newtons, and so on. Thus, in metric units, the horsepower is also equivalent to approximately 4,572 kilogram-metres of work per minute.

Going back to Patrick Githinji’s question: it is understandable why he has trouble with kilowatts (kW) and kilovolt-amperes (kVA). He probably remembers that, in school, they teach that power in an electrical system is calculated by multiplying the voltage and the current, that is, volts times amperes. The result is stated in watts. Thus if you multiply kilovolts by amperes, the answer should be in kilowatts. Is it therefore correct to say that kW and kVA are the same?

The answer is yes AND no! In some situations, kilowatts are equal to kilovolt-amperes and, in other conditions the two quantities are not the same. It all depends on the type of equipment being powered and the nature of the electricity used – that is, whether it is direct or alternating current.

If the electricity is direct current type (like that from a battery), the watts and the volt-amperes are equal. If the equipment being power is a heater (e.g., a light bulb, an iron box, a water heater, etc.), again, the watts and the VA are the same (in this instance, it does not matter whether direct or alternating current is used). If the equipment has a coil and it is using alternating current (e.g., a motor), then the kW is NOT equal to the kVA.

The reason for this is that when a coil is connected to alternating current electricity, it “reflects” some of the power back into the supply system. This reflected energy is not useful; it only heats up the power lines. Therefore, in order to generate a certain amount of power (kW), the equipment must be supplied with a larger quantity of volt-amperes (kVA).

The ratio of kW to kVA is called “power factor” and many manufacturers will indicate it on the equipment. Many motors and generators have power factors of between 0.8 and 0.9. Thus if the quoted rating is, say, 30kVA and the power factor is 0.85, then the useful power is 25.5kW (30 times 0.85).

There you have it, it’s that easy.