Usually for indoor grows this is not a concern. Thermal imaging detects heat not light and heat is the byproduct of the electricity. It is relative to the insulation and materials such as metals in your home as well as the structural integrity of the roof itself.
Thermal Infrared cameras(FLIR) are quite expensive usually upwards for 2k for basic ones and go much higher from there. Ones mounted on helis/planes are several times that making it cost prohibitive to police forces for SMALL grows.
Unless you're using about 6000+ watts of lighting you really shouldn't have anything to worry about. Space heaters use around 2.5k-3k watts of electricity and like the previous poster said using it in the middle of your house is a great idea. The more barriers(walls, insulation, piping, wiring) in between you and the camera the lighter the footprint and the more likely it would be dismissed.
Now, I really don't know how nosy power companies get but it would stand to reason they use the "don't ask- don't tell" philosophy considering the more power they get out of you the more money they make.
That said for the paranoid and EXTREME diy'er:
1. .Build a faraday cage to lessen the heat signatures. Given the high frequency of heat ~300ghz a mesh isn't going to cut it but one to two layers of galvanized steel most likely would do the trick. Here is a link that demonstrates the concept:
http://www.youtube.com/watch?v=t23iXhEiQUc
2. Take your grow off the grid...
Purchase the diesel generator(or build, but I won't get into this- in this post) sized to your operation then build a biodiesel processor:
http://www.instructables.com/id/Make-Your-Own-Biodiesel-Processor/
Essentially what this does is it converts used vegetable oil(usually obtained for free from a restaurant) into Biodiesel which is MORE EFFICIENT, less emissive and overall better quality than standard diesel. Some ingredients include sodium hydroxide(lye) and methanol. But when done you can have your own biodiesel fuel that will operate your diesel generator MORE EFFICIENTLY than standard diesel for anywhere between
.50 - $1 per gallon.
Keep in mind gasoline engines CAN be converted, however diesel requires no conversion due to the variable compression rates between to the two systems.
Remember I said EXTREME DIY'er lol...
-potlike