valhalla360 wrote:
A key formula to remember Watts = Volts * Amps. Electric motors generally pull the same wattage, so if the volts go down, the amps must go up to compensate. Items like a toaster draw a steady amperage, so if the voltage goes down, they just don't get as hot.
A purely resistive device, like a toaster, does not consume a constant current regardless of voltage; rather, it varies proportionally with the voltage, as described by Ohm's law. This means that, with low voltage, the current consumed also is lower, and the power consumed (and hence degree of heating) doubly lower.
The power consumed by an electric motor is a bit complex and depends on what the motor is driving, but for comparatively small changes in voltage around nominal it is often true that the power is roughly constant and so the current varies inversely with voltage.
Most modern electronic devices with switching power supplies will behave quite precisely as constant power loads: voltage and current are pretty much exactly inversely proportional, over a wide voltage range, assuming the device is doing constant work. Many are specified to operate on any global utility supply, from about 100V to 250V nominal supply voltages, and in any case fluctuations in line voltage are of little importance with them.
(Technically, for AC power, wattage = voltage * amperage * power factor; the power factor is there to account for situations where the current and voltage waveforms are not in phase with each other. The power factor isn't all that important for these discussions. If it's ignored, you're talking about what's termed apparent power, rather than actual power.)
