Measure input winding direct current resistance Ri
Measure output winding direct current resistance Ro
Measure input current Ii
Output current is 0.1A = Io
Pin = Ri x Ii**2 in watts
Pout = Ro x Io**2 in watts
1 Watt = 1 Joule/second = 0.2388 Calories/Seconds = 9.478 x 10**-4 BTU Intenational/second
where Calories are IT (international)

Thank you very much jbartos.
The power supply in question is a low cost switcher manufactured overseas, the stateside distributor does not have the DC Ri and Ro.  I will order a sample and measure the values myself.  For preliminary design purposes, do you think it is safe to assume the power lost (given to heat) will be < 5W?
thanks again in advance.

I am sorry that I misinterpreted your posting for a practical implementation. Heat dissipation of electrical loads and equipment are often available in catalogs or from manufacturers. If the circuit is known, the heat dissipatioin may be estimated/calculated/analyzed.
Please provide:
1. Distributor name and contact or
2. Manufacturer name and contact and
3. Efficiency of the unit
Some enclosure manufacturers may support electrical equipment heat dissipation data to increase the sale of their enclosures.
Often, the efficiency may provide a solution for the equipment heat dissipation. That power supply may have 65% efficiency only. This would imply the higher power dissipation than 5W at full rating. Then, one would need a curve of heat dissipation or efficiency curve versus the output current.

Manufacture is "Meanwell", model# S-25-24.  Distributor is Jameco. Jameco (Cust. service/sales) is of no help.  
The spec sheet does include efficiency (80%), but the efficiency is given @ 100% load.  Because I will at most use only 10% of PS output capacity, I discounted using the efficiency in my estimation.  I do not have a good feel for efficiency curves of these types of circuits.  At such a low level of usage, is it possible to predict if the efficiency is likely to increase or decrease?  
Also, there is another variable: PS will operate an absolute encoder; current draw will vary 20mA-100mA depending on usage and duty cycle of the equipment.
As you can see there are a lot of dynamics to this application, I need to provide a supplemental anti-condensating heater to the controls enclosure.  My question regarding the heat generated by the PS is needed only to gauge how much additional heat I will need to supply.

Suggestion:
Try to contact the manufacturer at info@meanwell.com
as a potentially accurate source of information.

David Tung from Meanwell was kind enough to provide the following:
1. Heat dissipation at 0.1 A (at 115VAC) output current is 2.7 Watts obtained by measurements
2. Efficiency at 0.1 A (at 115VAC) output current is 46.9% obtained over measurements and calculations.
A copy of the David Tung email is available.

Thank you jbartos.
I would greatly appriciate it if you would forward the Meanwell info to me. Email: joeskrab@stratos.net