I don't think so. On my laptop, fully loading the CPU takes it from silent to screaming fans in about two minutes. That's clearly a big change in heat production and therefore power consumption.
You should use a kill-a-watt and measure it. Probably less than you think. Laptops already on the edge of their thermal envelopes, anything above their low steady state (browsing, listening to music) will cause the fans to spin.
The battery time remaining will also drop by a factor of 4 or more. This is large enough that I don't think a more precise instrument is needed to determine that the effect is significant.
If I have time this weekend I will pop the battery on my macbook and do an erlang compile on an encrypted sparse image both on the rotational drive and the ssd. I'll report back on how much energy was consumed for (encrypted,non-encrypted) x (ssd,rotational)
batteries derate rapidly as the current draw goes up. They are not linear. For my own edification it would be nice to know what kind of energy hit encryption takes on both storage mediums.
I would definitely be interested in hearing. I don't expect that the overhead of encryption will cause much additional power draw, simply because encryption doesn't require all that much CPU power. The stuff about fully loaded CPUs was wandering off into the theoretical realm about CPU power consumption variance in general.
For whatever it may be worth, I couldn't find idle power consumption for my notebook's CPU, but it is possible to come up with a worst-case estimate by taking the battery capacity and dividing by the runtime. Apple specifies "up to" 7 hours. While that is of course hard to achieve, I think it's fair to use that figure when looking at idle power use. The battery is 50Wh, so we can figure that the computer as a whole is using at most about 7W when idle. The i7-2677M CPU is specced to use up to 17W all by its lonesome when running flat out, so that's a substantial increase, especially when you take into account the fact that the base 7W idle-ish consumption is for everything in the computer, not just the CPU.
