There is a long held belief with some electricians that turning on an electric device requires a "startup" cost, or an overhead in the amount of power needed for it to start functioning normally. I have known people who make sure to only turn devices on or off when they feel they really must because they want to avoid this cost. Is this a misunderstanding of how a capacitor works or a real phenomenon?

Edit: By electric device I mean anything that uses electricity. A good example to answer my question would be a lightbulb since you don't have to think about a complex system. Does it take some extra amount of electricity to get the lightbulb lit, or is it simply on when the circuit is closed and off when the circuit is open?

  • This is really only a concern for appliances. For example, I bought a window AC that needs 15 amps to start but only about 10 to stay running. I noticed that turning on the AC while the TV was running would trip the breaker, but turning on the TV while the AC was running would not. The TV needs much less amperage to start apparently. – fredsbend Feb 16 '16 at 3:44
  • In theory, yes. An appliance in startup can draw more power to get to its operational state. In practice, the actual cost would be so negligible as to be irrelevant. – GordonM Feb 16 '16 at 9:48
  • As for power consumed: how long is the startup period? -> For househould appliances typically milliseconds to seconds. During that time the maximum amount of current you can draw is limited by your electrical system (you do not even need to know the actual power consumption of the appliance). If you calculate the power involved you'll see it is better to switch them off. – user22865 Feb 16 '16 at 10:17
  • Another argument could be: wear and tear. I cannot answer for that. – user22865 Feb 16 '16 at 10:19
  • @GordonM I don't think a difference of 5 amps at 120 volts is negligible. You must mean money cost is negligible. Yes, you're right because it only lasts a second or two. At 10 cents a kwh this few seconds costs less than a penny. – fredsbend Feb 18 '16 at 15:32

Yes, some electronics do have a startup cost (ie. when the power draw is higher during startup than during normal operation). This is observable in PCs, particularly from hard disk drives. Specs from your typical run-of-the-mill desktop hard drive are as follows:

WD Caviar Green 5th Generation Specs

2.5TB and 3TB capacities
5400 RPM
64MB cache
3Gb/s SATA
Power Consumption
Spin Up - 10.75W
Seek - 6.25W
Idle - 5.5W
Standby - 1W
Average Read Seek - 15ms
Average Latency - 5.5ms
Spindle Start Time - 17s
Error Rate - <1 in 1015

During the spinup, which is typically in the 5-15 second range, the drive will draw a larger amount of power (nearly 11 watts in this case) than while in normal operation (from 5.5-6.25W). However, this is only for a brief period of time, and the amount of extra power used is negligible compared to the power needed to keep it running for an extended period. In short, you'll save much more electricity by turning your devices off than by trying to avoid startups.

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