[simonsquiff]

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.