Panhandel's answer is right, but incomplete.
A final scan can waste more energy, by simply increasing the DPI resolution of the scan. Increasing the resolution can make the roller run at lower speeds. as the tube is constantly lit until the scan ends. Another situation can be that the roller runs twice (I've experienced this somehow with Xerox photocopiers).
On the subject of photocopiers, most of them don't have a display large enough to make previews. Those who do show a very little thumb. I suppose it can be enlarged, but the point is that those instruments usually produce good results, thanks to the circuitry inside.
Regardless of everything, the amount of energy a scan wastes is almost a residue of what the output source (the computer) wastes. (source .pdf, read page 2). In most cases, I need a preview to make sure that certain alterations (increasing contrast, luminosity, gamma adjustments) or that the final size is well adjusted. I have scanned several documents and I only need to preview once, as the rest of the pages can usually follow the settings defined at preview.
To sum it up, a preview is but a very small percentage of the full scan or even the whole process.