It has a tank - you can get different sizes, mine is 4 litres (though usable is 3 I think).
Energy efficiency is about the same as a kettle - the tank is super insulated, and you only use what you need rather than boiling more in a kettle (and that then cooling down), that balances out the small tank losses. If you have solar it’s more friendly for that as it’s lower peak wattage so more likely to stay within your solar generation.
That comment got me curious so I went searching, looks like one supplier is called QETTLE and they have many videos on YouTube. And a FAQ that mentions energy use: https://www.qettle.com/support/faqs , although the prices are probably from before the 2022 UK Government Omnishambles.
Here's another brand's innards of the reservoir: https://youtu.be/Q7TV73Y5kmo?t=614 . If I had to guess they keep it under high pressure so the water can be hotter than 100 deg C but still remain liquid.
I guess it uses steam pressure to push the water out of the tap anyway.
I see several examples where the tap seems to be integrated into the sink or actually just another setting on the regular tap. That seems dangerous to me.
Energy efficiency is about the same as a kettle - the tank is super insulated, and you only use what you need rather than boiling more in a kettle (and that then cooling down), that balances out the small tank losses. If you have solar it’s more friendly for that as it’s lower peak wattage so more likely to stay within your solar generation.