Say you are transcoding a video. While that is in process, 2 or more drives must be spun up, depending on single or dual parity, source vs destination, etc. The actual transcode is heavily CPU dependent, so the T version will take some arbitrary amount of extra time to complete the task. When the transcode is done, all drives can then spin down. The transcode CPU cycles will be similar, so total power consumed by the CPU will be similar. The drives will consume extra power comparatively, because they were in use longer.
Same task, extra power.