Cisco Power Calculator Discrepancy

Unanswered Question
Aug 5th, 2010
User Badges:

I'm working on calculating the BTUs generated by a 6509 for a new IDF buildout and I'm trying to determine how the Cisco Power Calculator calculates it's values.  With all of the blades I put in, POE load, and redundant 6000 Watt Power supplies, the Calculator says that there is a total output power of 5348.67 Watts, a Typical Power Output of 4278.93 Watts and a heat dissipation of 9154.08 BTU/Hr.  If I do this manually and say 5348.67 Watts * 3.413 Btu/ Watt =18,255 BTU/ Hr

Where does this discrepancy come from?

Also what is the difference between Total Output Power vs Typical Power Output?



  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
gephelps Thu, 08/05/2010 - 10:46
User Badges:
  • Cisco Employee,

Are you using the power calulator linked here:

The reason I ask is I do not see a listing for Typical Power output anywhere.

Can you also post the config you are putting into the calculator? I want to see if I get the same output. I threw together a configuration where the total output was 1281 Watts and 5146.61 BTU/Hr. If I do the math by hand 1281 * 3.413 = 4426 BTU/Hr. I do not know how the number reported for you was so low.

Joe Kukis Thu, 08/05/2010 - 12:15
User Badges:


I contacted TAC and they replied back with a good explanation.It looks like the biggest difference comes in with the efficiencies of the components, combined with the fact that the majority of power and heat loss for the phones occurs at the phone itself and not at the switch:

Below is the explanation from TAC:

"The calculations are broken down into the Data BTU and PoE BTU.
Data BTU:
Output power of 1290.24 is what the linecards require from the power source. The efficiency of the power source is considered to be 85% therefore the input power must be higher than this number. Thus the heat dissipation is calculated as:
1290.24/0.85 * 3.415=5183.74
This is calculated as the amount of heat dissipated between the power source and the switch ports (the rest of the heat is dissipated at the phones themselves).
12.9/0.89/0.85 - 12.9) * 280 * 3.415=3970.35
12.9 = power delivered to switchport per phone
280 = number of phones
89% is the efficiency of poe daughter card
Total heat dissipation within switch is 5183.74 + 3970.35 = 9154.08.
The important distinction is that while the output power in the tool is the total power required by the components from the power supply, the heat dissipation is what occurs within the switch. Since majority of power for PoE is consumed at the PoE devices themselves, the rest of the heat dissipation will occur there instead of at the switch. "
gephelps Tue, 08/10/2010 - 05:19
User Badges:
  • Cisco Employee,


Thanks for taking the time to post the resolution. I agree that the explanation makes sense. Originally I didn't include any PoE devices which is why I didn't have the typical output column in the results. The typical wattage makes more sense when considering PoE since all of the ports may not be used and only a subset of the connected devices would use PoE.


This Discussion