Motorised awnings don't retract without power, but there's no risk of damage if you leave them out during an outage — they simply stay open until power is restored.
If your awning has to be operable without power, spec a crank override motor at the point of order.
In the uk where power outage is a rare occurrence, we would always recommend a motorised unit, as this will enable the automated safety features. However where power is not an option a manual awnings is perfectly suitable alternative.
We would suggest that manual awnings are always retracted when left unattended.