Question-and-Answer Resource for the Building Energy Modeling Community
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This answer is no:
you cannot set an arbitrary reporting frequency for an Output:Variable: the Reporting Frequency can only be Detailed, Timestep, Hourly, Daily, Monthly, or RunPeriod=Environment=Annual, (see here),
you cannot have more than one Timestep object (in the Energy+.idd it has the \unique-object property).
I would run with Timestep=60, report both variables at the "Timestep" level, and resample (downsample) the power consumption down to 15 minutes. Note that this would still bear the extra cost of reporting the power consumption every minute by E+, but maybe you have a good reason to get it to 15-minute (such as determining electric demand...)
