I consulted my Daughter over this.
She is head of HR at her company (staff of 600),and received the following advice.
Neither the Company nor the Agency is liable for the pay you would not receive due to the Power Failure.As you are paid by the hour those missing due to the power failure are not due to you.More especially since the Company would not probably pay the Agency for the hours you did not work.
The Company could claim that this was due to "circumstances beyond it's control" and not something they could have allowed for.
However,my Daughters says HER Company WOULD pay you,as it wasn't something that you did that resulted in the missing hours,but that her company's policy is not one that is applied overall.
The people tp push are the Agency,they should be able to get a definite answer (one way or the other) from the Company.
If they can',then you may have to do as the Agency says, and fill in a full day on the timesheet.However most companies check the hours on the timesheet before signing it,so I think you should know then whether or not they will OK it?