Without the data telling us how efficient this particular supply is, it isn't terribly beneficial to talk to about the losses from the transformer. What would be interesting is the Kill-a-Watt measurements from both a Premiere and Roamio under the same conditions. Throwing a Plus into the mix would be a good idea too. But, the fact that the Premiere's supply was 120W peak, and the Roamio's supply is 24W peak is still good news. That's quite an improvement and will show up on the power bill.