Sources of electricity in prison? (5)

I believe the answer is:

cells

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'sources of electricity in prison?' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Perhaps there's an association between them I don't understand?

This is all the clue.

Can you help me to learn more?

(Other definitions for cells that I've seen before include "Mobile phone in the US" , "Basic structural units of all organisms" , "Dungeons" , "Accommodation for prisoners" , "terrorist groups" .)

I've seen this clue in the Evening Standard.

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