Is a boiling water tap cheaper than a kettle?
On energy alone the difference is small and can go either way; the honest case for one is convenience, and the honest case against is the cost of ownership.
How they work. An insulated tank under the sink holds water at or near boiling, with a small heating element maintaining temperature. Dispensing draws from the tank, and cold water refills it to be reheated.
The energy comparison:
A kettle wastes energy by overfilling. Most people boil considerably more than they pour, and that surplus is heated and discarded — which is the strongest argument for a tap, since it dispenses only what you take.
A tap has standby losses. Maintaining a tank at temperature continuously costs energy even when nobody uses it, and manufacturers' figures for this vary.
Which wins depends on usage. Frequent use through the day favours the tap, since the tank is being drawn down and the standby loss is spread across many uses. Occasional use favours a kettle boiled to the required amount — and a household using it twice a day is likely paying standby losses for nothing.
The costs people do not anticipate:
Purchase and installation are substantial, frequently requiring an electrical supply under the sink and sometimes plumbing alterations.
Filters must be replaced, typically every few months, and this is a continuing cost that is rarely factored in. Skipping them causes scale damage to the tank.
Scale in hard water areas shortens tank life considerably, and filters are essential rather than optional.
Repair or replacement of the tank is expensive, and a failure leaves you without hot water at the tap until it is fixed.
The genuine advantages beyond energy: immediate boiling water with no wait, which changes how you cook as much as how you make drinks; filtered water; reclaimed worktop space; and safety features in modern units, since an unguarded boiling tap is a legitimate concern with children.
The safety point: taps require a deliberate two-stage action to dispense, and the water is typically just below boiling rather than at a rolling boil.
The efficient alternative: boil only what you need in a kettle, which costs nothing to implement.