## Mythbusting

26.12.2005 19:03

I heard several (mostly older) people say that the electric current is "stronger" in the evening. This was usually said in reference to cooking on electric stoves - for example you need to set the stove on a lower setting in the evening to get the same effect as with a higher setting in the morning. Recently I got into an lengthy argument about whether this is true or just an urban myth, so I decided to check it myself.

The UPS on my server keeps logs of the RMS voltage in the electricity distribution network so this seems to be a good point to start.

As you can see the voltage in the socket can vary as much 15V during a day and is far from the declared 240V.

On the other hand the digital voltmeter in the UPS can't be trusted, since it wasn't designed for such precise measurements (its only purpose is to tell the UPS when to switch to battery power). Because of this I double checked these values with a true-RMS voltmeter and it looks like the UPS measurements are constantly off by 10 volts (UPS shows 228.0V, voltmeter shows 238.0V), however the relative values seem to be quite accurate.

So let's say that I want to boil 2 liters of water on a 1kW electric stove. Such an electric stove has a heater with resistance of R=(240V)2/1kW=57.6Ω. To heat 2 liters of water from a room temperature of 20ºC to 100ºC you need Q=m·c·ΔT=4200J/kgK·2kg·80K=672kJ of heat. If the electric stove is operating at its declared power it will provide this quantity of heat (if we ignore the heat loss) in t=Q/P=672s=11.2min.

Now let's see how much time it would take to boil the water if we take into the account the swings in the line voltage. The graph shows that the lowest voltage measured is 222.2V. This is Umin=232.2V when corrected for the UPS measurement offset. At this voltage, the stove has a power output of Pmin=Umin2/R=936W. It will heat 2 liters of water to boiling point in tmax=12.0min.

On the other hand the highest measured voltage is Umax=247.5V. This gives the power output of Pmax=1063W and the water will boil after tmin=10.5min.

The maximum difference is obviously 1.5min or around 13%. This is value falls somewhere in the middle: I can't say that I would notice if the water for my spaghetti would begin to boil 90 seconds too early, but it is quite possible that some people can notice that. On the other hand most modern electric stoves have thermostatic control. This means that the temperature of the stove is controlled with some kind of a feedback loop. This loop should attenuate any effects of the line voltage swings to such a degree that I'm sure noone could notice (even if with a stopwatch I don't think you could measure any effects but I'll try to prove that at some later time).

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