The most common and most likely answer to this crossword clue is the 7 letter word WATTAGE
I've seen this crossword clue in The Guardian
W
A
T
T
A
G
E
Buddy explains!

The straight part
We know that "Power" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case power = wattage.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does WATTAGE mean?
WATTAGE noun- An amount or power (especially electric), expressed in watts, kilowatts etc.
- The power requirement of some electric appliance.