You can leave it on all the time or have it shut off after it reaches temp. It doesn't really use much energy either way because they are so well insulated. Mine has vacuum insulation that can keep the water to temp for about half a day, and if you want to bring it back to temp you just have to reheat it a few degrees. The vast amount of energy used to bring water to a boil in the beginning of the heat cycle.

It can be turned off after boiling, but it also has much better thermal insulation, so the water will stay hot for much longer than in an ordinary kettle