The most common and most likely answer to this crossword clue is the 5 letter word FARAD
I've seen this crossword clue in The Times
F
A
R
A
D
More clues leading to the result FARAD
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The straight part
We know that "storage unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case storage unit = farad.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 FARAD mean?
FARAD noun- In the International System of Units, the derived unit of electrical capacitance; the capacitance of a capacitor in which one coulomb of charge causes a potential difference of one volt across the capacitor. Symbol: F