I'm currently running some power dissipation analysis or thermal calculation on a system and the way I do it is by looking at each part's data sheet and see if they list its power dissipation. Now some part's data sheet list power consumption and some other show the power dissipation.

My questions are:

  1. How can I estimate the power dissipation if the only power listed is the power consumption? I think that power consumption is different from power dissipation, isn't that right?

  2. Is the power consumption the power at the input of the electrical part? For example: how would I calculate the power consumption of a 30 watts power supply (120VAC input and 24V/1.25A output)?

  • \$\begingroup\$ What do you use that 30 W output for? Unless it's battery charging or similar, everything will be dissipated where it's used. Say you use that 30 W for a laptop. You will burn ~3 W in the adapter alone which is dissipated as heat but your computer will waste everything as heat in the end, so you end up with 33 W dissipation total in the room which will heat it up. \$\endgroup\$
    – winny
    Nov 16, 2016 at 9:31

2 Answers 2


Consumption is really not a good term to use for power, but it probably refers to the power input to the device under consideration. Power dissipated probably refers to the losses associated with the device that are usually dissipated as heat. The output power of the device is transferred elsewhere in some useful form. The losses are calculated as input power minus output power.

It may be difficult to calculate loss dissipation from data sheet information. The input power listed in the data sheet is usually the maximum value that may not occur under normal operating conditions. The output power is usually the maximum value that can be tolerated continuously under the most adverse permissible values of other conditions such as input voltage and ambient temperature. Depending on the purpose of loss dissipation, you may want to consider either typical actual operating conditions or worst case conditions.

  • \$\begingroup\$ I use AM100S/5-A and specs: Output Power (max) of 100W, output current is 20A, output voltage is 5 volts, and its efficient is 75%. All I know for the load the power supply connects to is that it(the load) dissipates 15Watts. How do I calculate power dissipation on this supply? I used the worst case scenario because I don’t know how much current is drawn by the load. So the worst case scenario I use the max output voltage and max output current to find the output power which is Pout = 5 x 20 = 100W and consequently the power dissipation is Pout ((1/0.75)-1) = 33.33W. What do you think? \$\endgroup\$
    – user124011
    Nov 15, 2016 at 14:33
  • \$\begingroup\$ The power supply losses are 33.3 W at full load. If the load has 15 watts of losses and its efficiency is 85% than it's input is 100 W and the power supply is fully loaded. You need to know the nature of the load. If it transmits energy to other equipment, 85% is a reasonable efficiency. If it is something like a heater or computer it dissipates all of the input energy as heat, the efficiency is zero and the input is 15 watts. \$\endgroup\$
    – user80875
    Nov 15, 2016 at 15:23
  • \$\begingroup\$ It's a computer in this case. The efficiency is zero only for the heater, but not computer, right? \$\endgroup\$
    – user124011
    Nov 15, 2016 at 16:05
  • \$\begingroup\$ You would have to consider that the efficiency of a computer is zero or almost zero. All of the energy input is dissipated as heat. It does not have any energy output except for the possibility that it transmits data someplace else, but that would be a very small amount of energy. If the computer includes a display screen, some of the electrical energy is converted to light, but the amount is small and it is converted back to heat as soon as it is absorbed by objects in the room. The same is true of sound energy. \$\endgroup\$
    – user80875
    Nov 15, 2016 at 16:18
  • 1
    \$\begingroup\$ Power supplies, transformers and electric motors convert electrical energy to other forms of electrical energy and to mechanical energy. For other equipment, input power or "consumption" is essentially the same as dissipation. All of the energy that they receive is dissipated as heat in or near the equipment. \$\endgroup\$
    – user80875
    Nov 15, 2016 at 16:33

A power Supply transforms input to output with idle losses and dynamic losses dependent on load.

  • Thus Input Power = Consumption max = Internal Dissipation + Output Pwr
    • but Vac and Fuse rating is not necessarily Input power

Unless you have a true RMS meter for input and output, you can't tell unless you have efficiency specs. You might expect 80% at max load as typical. or 5W at 30W out

There is no way to guess efficiency without actual measurements or asking the supplier. But there are industry norms and new standards for PC ATX supplies.

Here is just one typical example of a DC-DC converter. Then there is the AC-DC converter.

enter image description here

  • \$\begingroup\$ Also, is there any way you can correlate the power dissipation of the load to figure out the power dissipation in the power supply that feeds it? I know that this load will not draw the max current from the power supply, but if I had to guess to avoid using the worst case scenario what would you say the this load would draw? In a good design, are expecting to use 80% of the current from the source or what is the threshold? \$\endgroup\$
    – user124011
    Nov 15, 2016 at 14:34

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