Does it cost a lot to keep Oyu on all the time?
No. A lot of the energy in a traditional kettle is wasted because of poor insulation, as water is converted to steam before it switches off. There is also excess water which is heated and left in the kettle but not used - most people overfill their kettles.
Zojirushi water heaters we sell also have to store and keep water at a specific temperature, thereby using more energy than an Oyu.
Oyu only heats the exact amount of water you want and dispenses it thereby saving energy and, most importantly, money
For customers in the UK see https://yum-asia.com/uk/energy-use-guidance/
For customers in Europe see https://yum-asia.com/eu/energy-use-guidance/