Again thanks to all for the comments.
One final post regarding some of the answers.
There seem to be some confusion as to the original question. Some seem to not understand whether I want to measure "Power" or "Energy". Others indicated that although I want to measure "Power" (as in [kW]) I should be measuring "Energy" (as in [kWh]) instead.
The original question clearly states that the objective is to measure "Power" (as in [kW], power intake to the loaded motor, for instance).
As to the reason for wanting to measure that, I suggest reading (among the many references in literature) the Fact Sheet "Determining Electric Motor Load and Efficiency" from the Motor Challenge Program of the US Dept. of Energy.
The Power Intake of a loaded motor can be measured directly or calculated as: Pi = f (V,I,PF) where Pi is the input Power in [kW].
Once Pi is obtained, the "% Load" (output power as % of rated power) of the motor is calculated as: (Pi/Pir)x 100 where Pir is the input Power at full rated load.
Once the % Load is known, the Efficiency at which the motor is operating can be estimated.
That and the difference between "% electric load" and "% mechanical load" (e.g. in the motor shaft) are also well explained in the Fact Sheet.
HOWEVER, directly measuring the Power intake or the variables needed to calculate it (V, I and PF) in the case of a highly variable load like the Pump Jack (as indicated in my original question) requires a particular type of instrument, one capable of recording the values of the variables vs. Time (my original question mentions one such instrument, there are many in the market).
In loads of this type, the I and PF are varying constantly and rapidly. Hence the need for this particular type of instrument.
There are MANY articles and literature references about this. One that I think covers the subject rather well is a paper from the Society of Petroleum Engineers (paper SPE 16363: Electrical Savings in Oil Production by J.E. Bullock, Pacific Gas & Electric Co.. Presented at the Society's regional meeting in California in 1987). Regarding the proper equipment for these measurements in the particular case of Pump Jacks, the paper indicates that meters whose output is instantaneous values of the measured parameters are "not suited for measuring pumping unit motors" due to the cyclic nature of their operation (e.g. highly variable % Load condition).
The foregoing I hope clears that the objective is to measure POWER intake of the loaded motor (e.g. [kW] (to be used in determining % Load and Efficiency of the motor) and that a particular type of instrument is required in the case of the Pump Jack and similar types of loads.
Should anybody question any of that, I'd suggest taking the case up with the authors and/or organizations providing the material. I am merely a reader/user of it.
About the reason for wanting to measure/calculate those parameters:
This is also covered in the two references mentioned (and there is a score of others...). There are a number of reasons, all having to do with Optimizing Energy Use in Oil Production: advisability/convenience of replacing a motor with one of higher Efficiency, installing any of various options for energy savings, determine which pump jacks are the better candidates for any of these actions... etc etc.
That said, let me go back to the ORIGINAL QUESTION:
Does anyone know of any specific STANDARDS from any reputable organization or professional society (e.g. IEEE, API, SPE etc etc.) relevant to the case of pump jacks, referring to the measurement of the Power intake of electric motors (or their Efficiency) in the specific case of pump jacks or similar highly variable loads ?? (e.g. type of instrument, minimun length of recording time or any other aspect of this task).
(NOTE that I have made reference to publications, articles and papers but NOT to Standards. That's what I’m trying to find out).
Thanks and best regards to all.
All the meters around here have wheels or fake wheels just as you describe.