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In electronics, we always calculate power dissipation of components and devices all the time. I understand why we do this (heat issues and power supply), but there is something I never really quite understood.

What normalizes this power we always calculate? By this I mean:

  • Is it like the "percentage" of the power used by a device "per watt" from the source?
  • Power used per second the device/component is operational?
  • If the device is battery-powered , percentage of the total energy of the battery it will drain?
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2 Answers 2

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Fraction of overall power used by a part of the device can be relevant. For example, this can tell you whether it's worth the complexity of turning off a particular subsystem when not in use.

Power per second makes no sense. Power is energy per second.

For battery powered devices, you want to calculate how long the battery will last, or conversely, what battery you need to last a certain time.

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  • \$\begingroup\$ Firstly, thanks for the answers. I knew the second point held no water before even asking it :) but, consider this: we both agree that if it is part of a finite power-sourced system, then, this may determine battery longevity. If the project is later transferred to mains power, it still is said to consume the same power, but now there is no limit. In what sense do we consider this power it now consumes? Also, in power bills, we start paying per Watt-"HOUR". \$\endgroup\$
    – haxkalibrr
    Commented Jun 29, 2015 at 3:36
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Usually there's no need to normalize it: power dissipation is a meaningful quantity by itself. For example, if you have an incandescent light bulb that dissipates 60 W, you know that it will add 60 J/s or 60 Wh/h of heat to the room containing it, and that it will have a similar effect on your power bill. Or, if you're powering it with a battery with a capacity of, say, 120 Wh, you know it'll last two hours on it.

The only case where you would want to normalize it is when optimizing power use. Suppose, for example, that you use a voltage divider as part of a battery voltage monitor in an electronic device, and that the voltage divider dissipates 2 W while the entire device consumes 3 W. In this case you can normalize your voltage divider dissipation by the consumption of the entire device and see that you're losing 67% of your power through one part. This finding would justify putting some time into optimizing it, probably tripling battery life. On the other hand, if your voltage divider only consumed 2 mW while the device consumed 3 W, normalizing it by total consumption would show that it probably isn't worth the effort to reduce power dissipation in the voltage divider any further.

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