Why accept the limitations of the house wiring? You don't need to *source* 336 Kj of energy in 5 seconds, you need to *deliver* that much energy to the water in 5 seconds...

If your kettle had a large capacitor that charged up over, say 30 minutes, and maintained that charge. You could pour water into it, retreat to a minimum safe distance, and trigger the kettle remotely with an app on your phone....