The most common and most likely answer to this crossword clue is the 8 letter word KILOVOLT
I've seen this crossword clue in Financial Times
K
I
L
O
V
O
L
T
Buddy explains!
The straight part
We know that "Amount of energy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case amount of energy = kilovolt.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 KILOVOLT mean?
KILOVOLT noun- One thousand ( 103 ) volts. Symbol: kV or KV.