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Discussion of Watts, Volt-Amps, Power Consumption, and BTUs

  • 1.  Discussion of Watts, Volt-Amps, Power Consumption, and BTUs

    Posted 11-08-2013 21:41
    Article ID: 2404

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    The relationships among the following Alternating Current (AC) terms are discussed in this document:
    Watts (W)
    Volt-Amps (VA)
    Power Factor (PF)
    Volts (V)
    Amperes (Amps) (A)
    British Thermal Units (BTU)[/code]Watts is a unit of measurement that describes the total amount of power a unit "actually" consumes. So if we say a device consumes 500 Watts, this is what the customer is charged for by the power company and can be used to determine the ventilation requirements of the device's environment.

    Volt-Amps is a unit of measurement that describes how much power must be provided to the equipment for it to function properly. For AC powered equipment with either inductive or capacitive components that the AC power source sees, Volt-Amps will equal the actual power consumed in Watts, plus an amount of power that must be provided but not actually burned. It is not possible to calculate Volt-Amps from the voltage and current figures. This requires a special piece of test equipment to measure what is called Power Factor, which will be some positive number having a maximum value of 1.

    If you place a pure inductor or capacitor across an AC power source, it will draw current during the first half of the AC cycle. If you multiply this current (in Amps) by the Voltage, you will get a value measured in Watts. During the second half of the AC cycle the inductor or capacitor gives this power back. The result is what can be called a Wattless Watt. What is important here is, even though no power is actually consumed, the power supply must still provide the current.

    All modern electronics use switching power supplies which have a relatively high inductive input so they will require some of these Wattless Watts. This is where the term Volt-Amp comes into play. A hypothetical 500-Watt device with a documented Power Factor of .89 has a requirement of 562 Volt-Amps. What this means is that 500 Watts of actual power is consumed and must be provided by the AC source, plus 62 Wattless Watts that must also be provided due to the inductive input of the power supply.

    Power Factor is the ratio between Watts and Volt-Amps, and is very important to the power companies. If they are faced with a load with a low Power Factor it means they must have the generation capacity and distribution network to support both the real power consumed plus these Wattless Watts. Power Factor can be improved by putting capacitive loads across the AC lines to counteract the inductive loads. Companies that use motors extensively will sometimes have banks of capacitors on site to improve the Power Factor. The large capacitor/transformers visible at the top of utility poles function to improve the Power Factor from things like washing machines, refrigerators, etc.

    Some AC conversions:
    Volt-Amps = The AC energy that must be available to operate a device
    = Watts / Power Factor

    Watts = The amount of AC energy consumed by a device
    = Volt-Amps x Power Factor
    = Volts x Amps

    BTU/hr = The amount of heat generated by a device as it is operated
    = Watts x 3.413[/code]Notes:

    • When calculating the UPS (Uninterruptable Power Supply) needs for a device, the value you must take into consideration is Volt-Amps, not Watts.
    • When calculating the cooling needs for a device, the value you must minimally take into consideration is Watts. However, it would not be unreasonable to compute BTUs using Volt-Amps instead of Watts - if that's all you have to work with.