What do you mean by "backed by"? When we say "paper receipt is backed by gold" it means you can get gold in exchange for receipt at any time. In case of Bitcoin, you are NOT getting back electricity in exchange for your bitcoins, blocks or transactions. Please clarify what you mean by this.
One definition of "backed by" is "guaranteed exchangeable for."
Another is "supported by." As in 'Bitcoin is supported by the scarcity of energy available from natural resources.'