John Crane handbook and a couple other references give the friction loss equations for packing based on surface area. Under ideal conditions, the friction is converted to heat in the flush water.
Mech seals use 2 contact surfaces. You would have to know all the info on the seal and operating conditions. Best to contact your seal vendor and see if they have a value.
"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein
If you look in many of the mechanical seals manufacturers general handbooks you will find graphs for power loss per RPM versus seal diameter and differential pressure generally presented seperatly for balanced and unbalanced seals.
Although as sprintcar said the power loss is dependant on the seal face design / contact area, running conditions and amongst other things the spring load.
However the graphs can give you a ball park figures for losses.
Since John Crane has already been mentioned - in their general mechanical seal catalouge these graphs are towards the back that cover this.
what is your concern regarding power consumption of a packed stuffing box / versus mechanical seal.
A properly packed stuffing box will have low friction loss, therefore a low power consumption, an overtightened gland, resulting in high friction of the packing on the shaft will increase the power consumption and also accelerate the wear on the shaft / shaft wear sleeve. Therefore the power consumption of a gland / stuffing box is mainly a maintenance problem.
The decision to use a packed stuffing box or a mechanical seal is usually based on other considerations than power consumption.
Also if you are looking at overall power consumption - don't forget to add in the cost of pumping, collecting and processing any sealing water supplied to the packing or mech. seal.
