I've just checked my Astra. Engine off, volts across battery 12.36 volts. Engine at idle 14.00 volts, with all lights on, rear screen heater and hazards on, 13.40 volts. Omega alternators put out 120 amps, ample to recharge the battery and support all services, vastly superior to dynamos. Trouble is I've known them failing, yet still putting out some volts and amps so that no warning lights come on, and the first the driver knows is when the battery is flat and cannot restart the engine, or when mysterious lights flash briefly.
Fifteen years ago my wife complained her Senator had the occasional flashing light. I took it for a decent test run to Cheddar, no faults, decided she was dreaming. On the way home a light flashed on briefly, I kept going, the engine stopped running 4 miles from home, and would not restart. I walked home and returned with a spare battery to collect the car. Still did not know what the fault was, but changed the alternator next, which cured the fault.
Jonny's Omega 2.2 failed similarly some years later. I swopped cars with him, drove his car home using a spare battery, and fitted a used alternator, curing the fault.