The New Scientist report says that the current is about 500W and the operating temp. is about 35deg.C. (~92deg.F). It does stay on all the time.

The strings are tuned slightly sharp beforehand - so in a warmer environment some modification would be needed.    I get the impression that it does stay on all the time the piano is in use - and the very slight sound is scarcely audible.

...now let me see: 500w per piano.... concert halls.... ... schools..... music colleges... theatres..... private homes.....    probably one or two new power stations will be enough. ;-)

Steve