If you are powerless, you cannot stop or control what is happening. You may lack strength, lack the legal right, or simply face something too big — a storm, a rule, someone else’s decision. People say I felt powerless when they watch harm or unfairness and cannot step in. It is about not having power, not about the lights going out (that is a power cut, or the power is out). Common in both speech and writing; a little more serious than everyday chat, but not stiff. You can be powerless to do something (powerless to help) or powerless against something (powerless against the flood). Helpless is close, though actually helpless leans toward “cannot do anything for yourself,” while powerless leans toward “have no control or authority here.”