When did Britannic's lights go out?


I've not been able to find any information on the topic of when Britannic's lights went out/electrical system failed during the sinking.
I've read that some of the lights below decks were knocked out from the explosion but as for the power going completely out I'm not sure it did until the very last minutes. The Captain said that right before he simply stepped off the ship as she was sliding under he gave one last blow of the whistles. The whistles are electrically operated from a switch on the bridge.