Cal. Pub. Util. Code § 30643

Oral evidence shall be taken only on oath or affirmation. The hearing need not be conducted according to technical rules relating to evidences and witnesses. Any relevant evidence shall be admitted if it is the sort of evidence on which responsible persons are accustomed to rely in the conduct of serious business affairs, regardless of the existence of any common law or statutory rule which might make improper the admission of such evidence over objection in a civil action. Hearsay evidence may be used for the purpose of supplementing or explaining any direct evidence but shall not be sufficient in itself to support a finding unless it would be admissible over objection in a civil action.