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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