• Basic electricity formulas

    Apparent power, kVAs, real power

    One kVA is simply 1000 VA (volt-amperes). As we have seen above, one of the ways to calculate power is to multiply voltage in volts by current in amperes. If we divide the result by 1000, we have kVAs.

    V * I
    kVA = ———

    We use the name apparent power for this multiplication of voltage and current in the context of alternating current, since it assumes that voltage and current are in phase. This is not usually the case (the peak of voltage variations does not happen at the same time as the current peak), so that to calculate real power we must take into account the phase difference between them.

    This way we can calculate real power in kilowatts by multiplying the apparent power by the so-called power factor, which is the cosine of the phase difference between voltage and current. By default, a factor of 0.8 is used for the calculation when no measurement tool is available, which means a phase difference angle between voltage and current of about 37 degrees. Therefore:

    kW = kVA * cos(Φ) = kVA * PF

    For example, at 220V a current of 10A gives us 2.2 kVA (220*10/1000) and a real power of 1.76 kW (2.2*0.8) if our power factor is 0.8.

    This formula is valid for single-phase current. If we use three-phase AC we must multiply by the square root of the number of phases, which in this case is three, and therefore the factor will be 1.7320508076.

    kW = kVA * PF * RAIZ(3) = kVA * PF * 1,7320508076

    Thus, for our example of 220V and 10A, we would have 3.05 kW.

    Heat emission

    As we know, energy is neither created nor destroyed, it is only transformed. In our case, it is mainly converted into heat where the equipment is located. It is often necessary to calculate the heat generated by the equipment to determine the needs of the air conditioning system.

    Heat is usually measured in kilocalories per hour or in the Anglo-Saxon BTU (British Thermal Unit) per hour. To do this we multiply the real power by the time in seconds, specifically 3600, which is the number of seconds in 1 hour. This will be multiplied by 4.184 to convert it into calories. Therefore:

    kCal = Real_Power * 3600 / 4,184

    In the above example, for single-phase current we would have 1514 KCal/h (1.76*3600/4.184).

    If we need the result in BTU/h, we multiply the result by a factor of 3.96566683. Thus;

    V = BTU/h = kCal/h*3,96566683
    Thus, our single-phase equipment with a consumption of 5 amps and a voltage of 220V, has an apparent power of 2.2 kVA, a real power of 1.76 kW and generates 1514 kilocalories (or, what is the same, 6005 BTU) every hour.

    (It should be taken into account that people also emit heat. Obviously this depends on the level of physical activity, but averaging the one who dances intensely and the one at the bar with a drink in hand, 125 kCal/h (500 BTU/h) may be a reasonable amount per person).

    In our Calculators you can find a calculator that uses the above mentioned laws as well as one that calculates kVA, real power and thermal equivalents.