IPA: /ˌɛlɛk.trəˈθɜːr.məl/
KK: /ɪˌlɛk.trəˈθɜr.məl/
Relating to the production of heat using electricity.
The electrothermal system efficiently heats the water for the building.
Electrothermal is formed from "electro-" (from Greek "ēlektron", meaning amber or electricity) and "thermal" (from Greek "thermos", meaning heat). The word describes something that relates to or produces heat through electricity.
Think of 'electricity' ('electro-') being used to create 'heat' ('thermal') — that's what electrothermal means.