Last year was my first year using the idea and we tried making it a take-home project. There wasn't a way for stuck student to become unstuck. If at all possible, I'd still like this to be a take-home assignment. I want students to be able to take a real-life non-contrived problem and be able to make some headway on it.
The real difficulty probably comes with comparing the incandescent light bulbs to the CFL and the LED light bulbs. Students have a hard time making the equations. Finding a yearly rate of change is hard with the incandescent bulbs because you have to purchase more than bulb in a year if you buy the cheap ones. That makes coming up with a slope and y-intercept harder for an Algebra 1 student.
So... dear internet, I'd love a little help finding some way to provide a bit of structure to help students along in the process. I don't know, maybe I'm just too attached to the problem. I want them to work for it... but I want it to be within reach.