Reportedly, what our energy bill is based on? (4-4)

I believe the answer is:

watt-hour

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'reportedly what our energy bill is based' is the definition.
I can't tell whether this definition defines the answer.

'on?' is the wordplay.
I cannot quite see how this works, but
'on?' could be 'at' (I've seen this in other clues) and 'at' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for watt-hour that I've seen is " Electrical energy unit".)

I've seen this clue in The Times.

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