: Andrew, this is an interesting app! The Allegro series of Hall-Effect current sensors
seems to be the best solution. Since this is an on/off application, there is no need for a microprocessor, or a need to digitize the actual AC current. Just sensing the presence or absence of an AC current is all you need to do, and this can be accomplished with an analog comparator, whose output can operate a relay to turn on a light, sound a buzzer or bell, or instruct a computer to send you an e-mail. You will
need a toggle switch to arm and disable the alarm however.
OTOH, if you are comfortable (or want to get comfortable) working with Microchip PIC microprocessors, that could be the cheapest and easiest way to interface to the Allegro ACS712.
You will need to provide low-voltage (5V) DC power for the Allegro chip and the comparator (or PIC microprocessor), but this can be provided by a small DC "wall wart" power supply. If you want to place the circuit in its own case, with an outlet receptacle on one side and a receptacle plug on the opposite side, that would be feasible. It would also allow you to use a so-called "capacitor divider" power supply to rectify and reduce AC mains power to 5V DC power. Capacitor dividers are normally not recommended in this forum, but if the circuitry is entirely enclosed in a molded aluminum box it should be okay.
If a PIC is programmed for the task, it can sound an alarm for a fixed interval of time and then shut the alarm off until the Allegro senses that the electric kettle is turned on again. Same-o, same-o with an electric toaster. Please let us know what your capabilities are with regard to building circuit boards and mounting them a suitable metal enclosure.
If you have zero experience with PICs, we can help you choose one and also help program it. The Allegro is available in several different current sensitivities, so it helps to know what amperage your electric kettle and your electric toaster uses, The Allegro is very tolerant of currents higher than its rated "linear output" current, so choosing a model that covers the ten to twenty ampere range is probably good enough for your application.
Please let us know how you want to proceed with construction. Be aware that, if successful, the design will be copied by Asian manufacturers and sold on eBay for less than the cost (to you) for the parts required to make it. Or just wait until someone living in a Pacific Rim country reads this thread and offers a product for sale next week or sooner... I say these things not to deter you. It is a worthwhile project, and you will learn a lot by completing it. Just don't expect to retire early on the profits you make from selling it. This, after all, is a hobby forum. We do electronics here because we love to do electronics.