While it is up for debate if certain abstract characteristics of BTC give it value (such as its scarcity, its usefulness and what not) you can certainly say that BTC is backed by the power required to mine it.
That's incorrect. The price of creating a thing does not determine its value. If there is no demand, then it is worth nothing no matter how hard it is to create.
To be crude, human poop takes effort to create, but the worth of it as a tradable good is significantly less than the effort involved.
The cost of mining bitcoin is dependent on the value, not the other way around. This is because the rate of creation of bitcoin is essentially fixed. If the price rises, mining becomes more profitable, so more resources go into mining, but the difficulty also increases. So the same amount of bitcoins will be created, but the energy cost of mining a bitcoin will increase proportionally with the price.
To be crude, human poop takes effort to create, but the worth of it as a tradable good is significantly less than the effort involved.