In letters, E (LH:4), is the 5th letter in the Greek alphabet, with the name epsilon (έψιλον) (NE:#865), symbol E (upper case) or ε (lower case), the second vowel, pronounced: "eh", of seven vowels, with a number value of "5".
In 2500BC, in Egypt, in hieroglyph of a man with his arms raised, symbolic of a man in action, as shown below, became, via Proto-Sinaitic and Phoenician cultural transmission, the root etymology of the Greek letter epsilon "E", symbolic of a person ‘in action’, or doing ‘work’ of some sort, i.e. expending ‘energy’ as we would say:
In 1,000BC, during the formation of the Greek alphabet, letter E became, as the letter epsilon (ε or Ε), the second vowel of the seven vowels, thematic to or named after the sun, the second planet in Greek cosmology.
In 110AD, Plutarch, The E at Delphi, informs us, in the sense that the letter "E" , in the Greek alphabet, is the second vowel of the seven Greek vowels, symbolic of the sun being the second planet of the seven wandering stars of Greek cosmology. 
- “Ammonius smiled quietly, surmising that Lamprias was expressing his own opinion on the matter and was repeating a story from hearsay for which he could not be held responsible. Someone else in the group commented that this was the same explanation that a Chaldean stranger had been prattling about earlier; that there are seven vowel sounds amongst the letters; seven stars in the sky with autonomous and independent movements; the E is second in the list of vowels; amongst the seven planets the sun comes after the moon, which is first; and that nearly all Greeks identify Apollo with the sun. ‘But all these ideas’, he concluded, ‘come from star books and the rumors chattered about at the gates of the sanctuary.”
- — Plutarch (c.110), ‘De E apud Delphos’
- “An anonymous member of the group reports the remarks of a Chaldean visitor who said that the significance of the E lay in its second place amongst the seven vowels and compared this to the second place of the sun amongst the seven wandering stars.”
- — Judith Alexander (2018), “Plutarch’s ‘De E apud Delphos’: Translation and Commentary” (pg. 20) 
Moeller connects this to the four Es at the center of the Sator square as follows:
- “The E [ε] had special quality among the ancients and was associated with Delphi, the place sacred to Apollo, the sun god. In fact, Plutarch devoted an essay to the subject, ‘De E apud Delphos’, in which he has as one of the reasons for the relationship that ‘ε’ was the second letter of the Greek vowel system and therefore represented the sun, the second of the seven planets (De E 4).”
- — Walter Moeller (1973), The Mithraic Origin and Meanings of the Rotas-Sator Square (pg. 18)
Many E-based terms, according to the view that E-terms are solar themed, accordingly, are energy-based or heat-based: ergon, energon, enthalpy, entropy, existence, engine, engineer, equation, experiment, explosion, etc., typically have deep etymological significance, in respect to a person in action, powered by the sun.
- Thims, Libb. (2020). Human Chemical Thermodynamics — Chemical Thermodynamics Applied to the Humanities: Meaning, Morality, Purpose; Sociology, Economics, Ecology; History, Philosophy, Government, Anthropology, Politics, Business, Jurisprudence; Religion, Relationships, Warfare, and Love (§2: Alphabet) (pdf) (§36: Joule [§§36.1: | E = Sun (etymology)]). Publisher.
- Plutarch. (c.110), ‘De E apud Delphos’. Publisher.
- Plutarch. (c.100AD). Isis and Osiris; in: Plutarch's Moralia, Volume Five (pdf) (pg. 25-27; Osiris tomb, pg. 53-54; §:The E at Delphi, pgs. 193-253) (Introduction: Victor Hanson). Harvard University Press.
- Alexander, Judith. (2018), “Plutarch’s ‘De E apud Delphos’: Translation and Commentary” (synopsis of: §4: 386-386C; pg. 20) (pdf), Master’s Thesis, University of Calgary, Nov 19.
- E – Wikipedia.