Yes, HERTZ is a Scrabble word!

Letter values in Scrabble US: H 4, E 1, R 1, T 1, Z 10.

Is it a valid word?

HERTZ is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the unit of frequency; one hertz has a periodic interval of one second
German physicist who was the first to produce electromagnetic waves artificially (1857-1894)
German physicist who with James Franck proved the existence of the stationary energy states postulated by Bohr (1887-1975)

Also: cycle per second, cycles/second, heinrich hertz, heinrich rudolph hertz, gustav hertz, gustav ludwig hertz.

Not playable in Scrabble US: HZ, CPS.