this is deffo the alternator. Happened to me on my astra estate. The electric power steering is shut down when the ECU detects no or low charge as someone said already.
Vauxhall will charge about £1000 to change it believe it or not. The highest quote i got for mine was £1200 supplied and fitted!!!!!
I did the job myself and it was quite easy- about 1.5 hours labour without a ramp or pit. You will need to relieve tension on the serpentine belt, by forcing the tensioner up with a screwdriver - there is a nice convenient hole which lines up when the tension is off the belt and you can slide an allen key in which lock the tensioner up out the way. Remove the belt.
Disconnect the battery cables at the battery and alternator. You will also need to remove the vacuum hoses and oil pipe for the vac pump ( the vac pump is fitted onto the alternator rear shaft stub)
There are 2 long bolts holding the alternator in place which are easy to get to, but might require a bit of bar on the end of the spanner to help get them loose.
The alternator ,once free will appear to be stuck inbetween the radiator and the sump. To get it out of the engine bay you need to remove the horn. All will become clear, as to the access route once the horn is out of the way!
Some replacement alternators come with a new vac pump fitted, others dont. If you dont have one on the new one, remove the vac pump from the old. It on 3 small screws and just slides off. Oil will come out of it so dont worry about that.
Refit is reversal of the removal as haynes would say!
Best to fit a new belt whilst your at it as well. Be careful when you release the tensioner - it can snap down sharpish and trap your finger between it and the belt. I think the belt would fair better in that fight.