I've always maintained that it isn't as cut and dried as low-energy vs high-energy...although the addition of carbon as CO2 or bioavailable organic carbon like Excel is often considered heading down the high-energy route.
In reality, as Jordi says, most of our tanks fall somewhere along an energy continuum - with Walstad tanks at one end of the spectrum and super injected high light tanks at the other...that's why I've tried to coin the term "
hybrid-energy"...there's a fuller definition at the start of my tutorial
http://www.ukaps.org/forum/threads/the-soil-substrate-or-dirted-planted-tank-a-how-to-guide.18943/
Also the terms high-energy and high tech, and similarly low-energy and low-tech are pretty much interchangeable since in the broadest terms there is an underlying and unifying philosophy at work...that is of entropy and energy flows...
...We keep order in our tanks at the expense of chaos in the wider environment....in that it takes an enormous amount of resources (read energy) to produce the fertz, CO2, artificial substrates, glass and plastic components and fuel to get them to market etc to run a single planted tank - even a Walstad tank, it's just relatively less energy demanding than a tank at the other end of the continuum.
So whilst our tanks may look awesome it comes at a considerable down stream cost, just like most other consumer goods. This is where Amano's whole Nature Aquarium philosophy falls down for me...although I also appreciated that one of Amano's main goals was to promote greater understanding and appreciation of nature through the Nature Aquarium concept, so there is perhaps a legitimate trade off.
But in answer to the OP a true Walstad tank will consume considerably less energy/electricity than higher energy alternatives.