The costs of producing BTC or USD are not their backings. You can't exchange your BTC back into electricity.
The USD (and usually most other currencies) aims to be indirectly backed by the corresponding amount of goods they represent within the currency zone and their legal status.
BTC isn't even indirectly backed. It just exists. And that's okay for its purpose.
Re your comment about currency production costs: A spike in electricity costs would directly effect existing bitcoins' value. A spike in cloth paper prices would not cause a meaningful change in the value of already-printed dollars. And the price of the dollar would spike with electricity costs indirectly. They're not the same, it's an invalid comparison that neatly evades their inherent technological and conceptual differences.
"Store of exchange-value" != currency, and BTC is currently behaving best in that role. Admirably, really- see how it's spreading like hotcakes in China. That's as a store of exchange value, not a currency.
You can't turn your gold "back into" ore or your diamonds "back into" carbon. The cost of an airline ticket is mostly determined by the current price of fuel.
When BTC starts to be used as a currency in earnest we can revisit the issue. Although at that point, you will be able to exchange your BTC back into electricity or other commodities through speculation or arbitrage. Which you can actually do directly right now if you know the right people, anyway. Profitably.
For up to about two weeks, until it adjusts to the new, lower norm, and de/inflation continues at the same rate as before. But technically true over that period, yes.
Which really puts the lie to the idea of currency being "backed by" a scarce resource at all, which is the main premise of goldbugs. I'm totally okay saying that a currency is backed by the entire economy of goods being exchanged for it myself.
The meaning of a backed currency is that it's backed by the issuer with a specific commodity (or service). You can go to the issuer and exchange the money ("symbol of value") into the real thing ("value").
With BTC you can't go to some miner (=issuer) and exchange it back into the electricity that was used to produce it.
But I can't expect that I will be able to exchange the BTC for a fixed amount of electricity, which is the same as the fixed amount of electricity I originally exchanged for BTC. Hence BTC is not convertible, and not "backed"
I can buy gold with the US dollar, but it doesn't mean the US dollar is backed by gold.
The USD (and usually most other currencies) aims to be indirectly backed by the corresponding amount of goods they represent within the currency zone and their legal status.
BTC isn't even indirectly backed. It just exists. And that's okay for its purpose.