Yes, that's exactly right.
That's why it's (usually) helpful to think of your task on Nec Assump as
"Which answer, if negated, most weakens"
And remember that while about
70% of correct answers will be doing some form of idea math (Bridge ideas or other missing ideas),
30% of correct answers will be ruling out an objection that will definitely be bringing in new ideas (Defender ideas)
So be willing to entertain things that weren't mentioned yet, always asking yourself whether these new things would have any impact on the Conclusion.
Since the conclusion's truth value here rests on whether or not these generators would greatly reduce their electric bill, thereby saving money.
If it had just said "greatly reduce their electric bill", then the upfront costs that (C) is talking about wouldn't be relevant. (I don't think your purchase and installation costs would usually show up on your electric bill)
But since the conclusion is saying ".... thereby saving money", the overall cost/benefit of the thermophotovoltaic generators is relevant.