If an accident has knocked out our electricity, do we have any right to a rental offset?

There is no hard and fast answer about  how long the utility has to make the repair; that is because the situations vary so much. Sometimes damage does take a week--or weeks--to repair. While  consumers may have a cause of action if the utility does not take reasonable steps to restore service--e.g. to recover for food ruined because of an unnecessarily prolonged delay--as long as the utility is doing what it reasonably can, it is unlikely that there would be fault, and so liabilty.

The landlord did not cause the problem, so holding the landlord liable is more problematic. Again, liability depends on fault; there is no fault here. Courts would not required the landlord to pay  (to give a rebate) because of an act of god (a storm) which happened to a service provided by another (the utility). If service were to disrupted for weeks or months, you might well have grounds to terminate the lease without penalty, on grounds of impossibility (you can no longer, due to a change beyond anyone's control, get the rental you are paying for) or constructive eviction (effectively forced out). However, a week loss of service is highly unlikely to rise to that level, unless there are exigent circumstances (e.g. someone in your family needs electirically powered medical equipment).

In short, a week's loss of power is a tremendous inconvenience, but it happens every year to millions of Americans without providing grounds for recovery. It is part and parcel of life.