However, to combine the apparent power (or current) of multiple ac devices, there is no straightforward way to get an exact total because the currents for each device are not necessarily in phase with each other, so they don’t add linearly. To combine the apparent power of multiple dc devices, volt-amperes add linearly. Thus, you would want to ensure that the wires and associated circuitry supplying this product accommodate at least 2.5 A RMS. If a product is rated for 300 VA (the rating implies this is the maximum VA the product will draw) and is powered from a 120-V RMS ac line voltage, you can calculate the expected maximum current as 300 VA/120 V RMS = 2.5 A RMS maximum (see the figure). For example, the standard residential voltage in the United States is 120 V RMS. Volt-amperes provide insight into the amount of current drawn by a product or circuit, assuming you know the voltage. A standard multimeter usually can make both of these RMS measurements. You can calculate the apparent power in volt-amperes for ac circuits by multiplying the measured RMS voltage times the measured RMS current. The apparent power for dc circuits is the same as real power for dc circuits (for dc, VA = W).įor ac circuits, VA are the product of the RMS voltage (V RMS) times the RMS current (I RMS): The apparent power for dc circuits is simply the voltage (V dc) times the current (I dc): Knowing the RMS current helps you properly size wires and circuit breakers or fuses that supply current to your device. Since VA = RMS volts x RMS amps, you can divide the VA rating by your RMS voltage to get the RMS current the device will draw. The apparent power in VA is used to simplify power ratings, making it easier to calculate current draw. To combine the real power of multiple dc or ac devices, you can just add up the individual power ratings in watts of each device to get the total power (watts add linearly). These ratings are useful if you have to get rid of the heat generated by the device consuming the watts or if you want to know how much you will pay your utility company to use your device since you pay for kilowatt-hours (power used for a period of time). A standard multimeter can’t make this type of power measurement. Even the measurement of real power in watts for ac circuits requires specialized equipment (a wattmeter) because the voltage and current waveforms must be measured over a precise period of time, the measurements must be simultaneous, and the average must be calculated over the measurement time period. So while this may be easy to visualize, it is not easy to calculate. Assuming that the voltage and current are both periodic waveforms of period T, the strict mathematical way to express the power calculation for a periodic waveform of period T is: That gives us the watts dissipated by the device in a circuit with voltage v(t) across it and current i(t) through it for the period of time evaluated. Since this instantaneous power is changing over time, we need to get an average value, so we integrate the power over a period of time and divide by the time period to get the average. When you multiply these together, you get the instantaneous power with time, p(t). To get the power in watts, you need to know the instantaneous voltage with time, v(t), and the instantaneous current with time, i(t). The concept for calculating the real power for ac circuits is straightforward, though performing the calculation is much more difficult. Real power for dc circuits is simply the voltage (V dc) times the current (I dc): For example, a 100-W light bulb left on for 10 hours consumes 1 kW-hour of energy (100 W x 10 hours = 1000 W-hours = 1 kW-hour). You pay your utility company for watts expressed as energy, which is power consumed for a time period, typically shown by your utility company in kilowatt-hours. One watt is one joule (energy) per second (1 W = 1 J/s). Power in watts is the rate at which energy is consumed (or generated). The real power in watts is the power that performs work or generates heat. Each of these values can be used for various purposes. Watts refer to “real power,” while volt-amperes refer to “apparent power.” Usually, electronic products show one or both of these values to provide information about how much energy they will consume or how much current they will draw. This article is part of the Power Management Series: What’s the Difference Between Watts, RMS, and More?īoth watts (W) and volt-amperes (VA) are units of measurement for electrical power.
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