Its $10 - $12 a month the power company does not get.
Or knowing my yard lights and under house vent fans run for free all year!
Or that I can leave my computer turned on all the time instead of most of the time.
Or that my boiler controls and pumps run for free for the year.
Or that it makes others envious or jealous in some odd way because I have it.
Or that it keeps me out of the chat rooms and actually has me doing something with my skills.
Or its a hobby that keeps me happier and thus slightly more tolerable to be around.
Or that I learned some less common knowledge from the experiences related to designing and building it.
Or that it puts some of my odd collection of industrial junk to work doing something useful.
Or any combinations of the above.
I know people who spend far more on far worse stuff in a year and have nothing to show for it so at least I have something for my time and money that I invested.