He is the god of the firmament, in whose hands are the thunder and the lightning; at whose command the refreshing showers fall to render the earth fruitful.
In the Vedas, Indra is the king of Svarga (Heaven) and the Devas. He is the god of the heavens, lightning, thunder, storms, rains and river flows. [Wikipedia]
Reference: Hindu Mythology, Vedic and Puranic
To prove this we consider one more context during Dwaparyuga, in Nanda Gaon when Lord Krishna intentionally told everyone to stop worshiping Indra to overcome Indra proudy nature and that led to heavy raining and lightening in Nanda Gaon . But later Lord Krishna saved everyone by taking all the people of Nanda Gaon under the Govardhana parvat.
According to Brihadaranyaka Upanishad:
कतम इन्द्रः कतमः प्रजापतिरिति । स्तनयित्नुरेवेन्द्रो यज्ञः
प्रजापतिरिति । कतमः स्तनयित्नुरित्यशनिरिति । कतमो यज्ञ इति ।
पशव इति ॥ ६॥
6. "Which is Indra and which is Prajapati?" asked Sakalya. "The thunderclap is Indra and the sacrifice is Prajapati." "Which is the thunderclap?" "The thunderbolt." "Which is the sacrifice?" "The animals."
Another translation from Vedanta Spiritual Library:
III-ix-6: 'Which is Indra, and which is Prajapati?' 'The cloud itself is Indra, and the sacrifice is Prajapati'. 'Which is the cloud?' 'Thunder (strength).' 'Which is the sacrifice?' 'Animals'.
In the book of Adi Shankaracharya Bhashya on Brihadaranyaka Upanishad by Gita press Gorakhpur, the Sanskrit word स्तनयित्नु is taken as विद्युत्
So, Indra is equated with Vidyut i.e Indra is the deity of cloud, thunder and electricity.