i could not find a English source, but according to the German wikipedia using a stove top kettle on a gas stove is the cheapest option:
Für das Jahr 2019 ergeben sich in Deutschland[7] für den Gasherd 1,59 ct pro Liter (Gaspreis: 6,34 ct pro kWh, Wirkungsgrad: 38 %[8]) und für den Wasserkocher 3,3 ct pro Liter (mittl. Haushalt-Strompreis: 30,85 ct pro kWh, Effizienz: 89 %[8]). Auch bei Wirkungsgraden von 30 bis 40 % für Gasherde[9] hatte Gas stets einen Preisvorteil.
Zum Vergleich liegt die Effizienz einer Ceran-Kochplatte zum Erhitzen von 1 Liter Wasser um 80 Kelvin bei 56 %.[8]
i could not find a English source, but according to the German wikipedia using a stove top kettle on a gas stove is the cheapest option:
https://de.wikipedia.org/wiki/Wasserkocher#Leistung_und_Effizienz
I’ve also heard that using a stove top kettle on induction electric stove is as energy efficient as a normal electric kettle.
Why do you prefer to use a normal kettle?
I cannot find a reliable source, but it appears the gas prices have since doubled, making it about on par. [https://www.globalpetrolprices.com/Germany/natural_gas_prices/ ]
A kWh is also more like 24 cents these days and not 30,85
Just buying one would be difficult here.