A currency being backed by something means you can redeem the currency for that thing.

When the dollar was backed by gold, you could go to the issuer of dollars and demand gold. The dollar had value because gold had value, not because there was stored energy or the accounting system or anything like that.

With Bitcoin you have no particular right to go to the miner and demand energy or "blocks". So it is unbacked just as fiat currency now is.

Moreover, the energy used to create Bitcoin does not confer value. This is a variation of the labor theory of value, which has rightly been discarded for subjective theory of value - see my longer post in this thread (replying to Sec Yellen) for a more complete explanation.