Where does “ηλιακός” come from?

ηλιακός (Greek) comes from Ancient Greek Ἥλιος, from Ancient Greek τύπος, from Ancient Greek τύπτω, from Proto-Indo-European (s)tewp-, from Proto-Indo-European (s)tew- — to push, to hit.

ηλιακός (Greek): solar, sun; solar water heater