Energy is described for Magic on page 236 of Basic Set: Characters, it is a cost for spells which can be spent from either FP or HP

GURPS Magic apparently at some point indicates:

1 kilowatt-hour = 10 energy (360 kilowatt-seconds or 360 kilojoules = 1 FP).

According to Bruno at

The technology college spells begin on page 175 with Machine Spells, the note is found at the start of Energy Spells on 178

The conversion between electrical energy and FP is useful to know for NPCs like Roger in Hellboy Sourcebook and Roleplaying Game who is powered in FP by an electrical battery and was previous Unhealing while lacking it.

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