In my experience with baking bread, buying from the supermarket is cheaper just because of the price of flour and other ingredients (however, even if I'm not as consistent as a good professional baker, my bread is always better than that I can find at the supermarket).

From an energy point of view, well, probably buying is also better, because cooking lots of bread at the same time reduces the amount waste per serving (although in the winter that waste heat is surely appreciated :D ); here a bread machine is going to be better than cooking in the oven like I do.

However, the bread machine is probably *never* going to use 550W at the same time, and I'm sure not for a significant amount of time.

I believe that that energy allowance is to be divided between the motor for mixing and the resistance for heating; a typical bread machine cycle will be:

* around 10-20 minutes mixing (only the motor is running, and there is no need for heating)
* hours of rising time: the heating elements may turn on sporadically, to keep the temperature a tiny bit above room temperature, but is should never go above 35°C (iirc, it may be 40°C or so), otherwise the yeast dies, and higher temperatures are useful to get shorter times, but the taste of the bread improves with longer times.
* 30-60 minutes cooking times: this is when the heating elements will work most, and even here once the bread has been brought into cooking temperature it has enough thermal mass that there is no need for the heating to be continuously on.

There may be an additional rest / mixing cycle after the initial mixing, and the times are very approximated, and also depend on the size of the machine (bigger machines will have longer mixing and cooking times, but the rising time won't be shorter)