From a power perspective, which is better: an FPGA or a DSP? If you had asked me this question a year or two ago, I would have automatically answered that DSPs are far more efficient. It turns out that the answer isn't as simple as I thought.
To explain why, it helps to draw a distinction between power and energy. Energy is the ability to do work, while power is the rate at which work is done. For most systems, energy is the metric that matters. For example, a battery holds a finite amount of energy. Thus, it is important to optimize energy efficiency in battery-powered systems. The instantaneous power draw usually doesn't matter that much. (For more on this topic, check out the article "Designing Low-Power Signal Processing Systems.")
To get back to the topic at hand, it is true that FPGAs can burn a lot of power. However, FPGAs also offer enourmous computational throughput. Thus, FPGAs can actually be quite energy-efficient. For example, a team at the University of Edinburgh in Scotland claims that their FPGA-based supercomputer is ten times more energy efficient than conventional microprocessor-based machine.
Of course, this may not be news to you. For example, last week's blog referenced a number of articles stating that FPGAs can be energy efficient. However, you may not be aware that FPGAs are also starting to look better in terms of instantaneous power draw. For example, Altera's latest Cyclone-III parts have lowered their static power consumption to 0.5 W.
All this begs the question: Are FPGAs now suitable for low-power applications like cell phones? Some in the industry point out that FPGAs are already used for glue logic in handsets, and argue that it is only a matter of time before they are used for a much broader array of functions. However, I doubt this will happen anytime soon. The entire power budget for a 2G voice call—including the power for the RF components, LCD, etc.—is about 0.5 W. The Cyclone-III burns that much in static power alone—imagine how much it would burn if you added in the dynamic power consumption!
In short, you shouldn't automatically assume that FPGAs are too power-hungry for your application. However, you also shouldn't expect to see FPGAs take over the lowest-power applications any time soon.