Sultangc - you declare the level of emergency you need in this case - there is no 'fixed rule'. If you try a PAN and no-one understands, then you call Mayday. If ATC and everything are going along nicely taking you to a nearby diversion airfield, you may elect to delay declaring such. Not a bad idea though at some point to focus minds.
This 30/60 thing is confusing everyone unnecessarily - it is a customer option unless a standard fit on later models. Flying for 90 minutes on a 60 minute battery is just as it always was, Denti. Switch off the electrics until you need them again. Assuming you would not dispatch?? with less than 3 electrical sources on ER you are building up a very bad day to lose three at the same time, and something that is not planned for. Why not short out the batteries as well and have a really good day?