It is interesting to think about energy efficiency, but we should also consider carbon efficiency, which often conflicts with energy efficiency. For instance,

It is interesting to think about energy efficiency, but we should also consider carbon efficiency, which often conflicts with energy efficiency. For instance, if this old machine is connected to a solar panel, then its operational emissions will be 0 gCO2eq/kWh (grams of carbon dioxide equivalent per kilowatt-hour of electricity) if its consumption is kept lower than the solar generation. If it is connected to a greener grid like CAISO, its emissions will be mostly green (~ 0 gCOeq/kWh) during afternoons (~ 12-6pm). Sure, at browner times (6pm - 6am), this old machine should be turned off (unless there is wind coming to the rescue). But at greener times, it should use the excess energy that comes from renewables and that can't be stored to be useful elsewhere.