If you're a coffee or tea connoisseur, you already know that different varieties require specific water temperatures to optimize flavor. Green tea, for example, is sometimes thought of as bitter but that's often because it's steeped incorrectly. If capturing your hot beverage's subtle flavors is important to you, you'll want the best variable temperature kettles.
Black teas: 200 to 212 degrees (except for some more delicate blends, like first-flush Darjeelings, which do best at 180 to 190 degrees)
Coffee: 195 to 205 degrees
In addition to temperature range, consider these other factors that make a good variable temperature kettle:
Fast heating: The higher the wattage the better, so look for something with at least 1,000 watts or more.
Long heat retention: Once your water is hot, it's helpful if the kettle can keep it warm for at least 30 to 60 minutes so you can keep refilling your cup.
High-quality material: Stainless steel and glass are best (although plastic can be a good budget choice). Stainless steel is exceptionally durable, although it can sometimes transfer a metallic taste. Glass is best at preserving taste but is susceptible to shattering.
Easy pouring: A well-designed pour spout and smooth, ergonomic handle make the kettle easier to use and prevents spilling.
Other features: Things like automatic shutoff functions, easy-read LCD temperature displays, and water filters are great bonus features.