Unfortunately, the industry isn't regulated in regards to energy consumption. You will not be able to find apples to apples comparisons. The way one manufacture figures their efficiency is different from the other. Even supposed side by side comparisons have been conducted under different parameters so the results are questionable.
However, if you stay with some of the top manufacturers, you will be fine. The top manufacturers are very diligent in building energy efficient hot tubs. Some of those brands are Jacuzzi, Hot Spring, Caldera, Sundance, Marquis, Dimension One. There are certainly others that fit in this category, but this list will give you a good start.