Does water temperature matter?

More than any other variable except leaf quantity, and the reason is chemical rather than a matter of taste. It is the first equipment upgrade that changes the tea rather than the convenience.

brewing method

What is being compared

Point by point

AspectWestern brewing (teapot or mug)Kyusu brewing (Japanese style)
Why it mattersThe compounds carrying bitterness and astringency extract far more readily in hot water than those carrying sweetness and umami.The same chemistry, exploited deliberately — Japanese practice starts cool and raises the temperature across infusions.
Range that matters most70 °C for Japanese green, 80 °C for Chinese green, 90 °C for oolong and white, boiling for black and dark.50–60 °C for gyokuro, 70 °C for sencha, 85 °C for hojicha and bancha.
Without a thermometerBoiled water left standing a few minutes drops considerably; pouring between two vessels cools it faster.The same, and Japanese practice traditionally cools water by pouring it through the cups first.
What goes wrongBoiling water on green tea is the single most common brewing error there is.Under-heating black tea, which under-extracts and tastes thin.
Is a variable kettle worth itIt is the first purchase that changes the tea rather than the convenience.More so here, since the range is narrower and lower.

The short answer

If you drink green tea and own one thermometer-less kettle, this is the upgrade that changes the most. For black tea alone it changes almost nothing.

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