This study seems to verify the obvious: you will use more power no mater what, if you have less daylight per day.
But does this mean that DST is useless? I don't know, but I'd say that one can not judge that based on a simple comparison of power consumption during DST and non-DST periods.
To see if DST does save power (and how much), you must compare it with what we would have consumed if DST were not in effect.
For example, in Greece (where I live), during mid-summer, it gets dark after 20:00. During the winter, even with DST, it gets dark as soon as 18:00. That's a whole 2 hours of more dark per day! Even with DST, I think it's normal we'd consume more power.
Now, had we not used DST, it'd get dark as soon as 17:00. That would be 3 hours less daylight per day. I bet we'd use even more power had DST not been in effect.
Also, how does the study compensate for the increased power demand for heating (spaces, water, etc)?
In short, make sure they're not comparing apples to oranges...