Electrical power loss

Could not unlock my car with remote nor with key, left windows cracked so able to unlock. Started car no gauges on dash, windows won't come down or up, signal lights don't work, got my car home.
Radio, headlights, brake lights work.
Drove to a mechanic, he says needs alternator. Could this be true?

Side note: during winter this last year my signal lights or buttons on steering wheel wouldn't work, until the car is warmed up.