This was a serious event. NATS spends a small fortune on making their power supplies uninterruptible and for such a failure to occur is alarming.
Sure it was planned maintenance and at a fairly quiet time but imagine if it had been a real power failure at LACC or LTCC on a Monday morning and the back-up hadn't worked.
I don't think it's a question of back to the drawing board but no matter what it costs it has to be sorted.