We can estimate the amount of information processed. Visual is like 10mbit [1] plus other senses it might be up to 100mbit. Only doing similar sensor fusion and extracting features in realtime on computer requires more power. But there's also the symbolic processing, doing something similar too requires much more power on computer. Then there is other stuff such as maintaining homeostasis we don't really know how to compute yet.
You can estimate the amount of information being processed in a few different ways. But that's not really relevant; the whole point of solving this problem is that you can do the same job with less computation than it looks like you need.
There is no particular connection between "amount of information processed" and "amount of computation performed".
There is a connection. How can any computation be done without moving information around? In absence of better measure, we can roughly estimate the computational complexity of a black box from looking on the input and output.
If the brain's job could be hypothetically done by some optimized system using an picowatt is irrelevant. We don't have such a system.
The power needs of the human brain are likely to be measured quite accurately.
The same is not true of the "amount of computation" performed by the brain. How are you measuring that?