With a good challenge question, you shouldn't be too focused on a specific solution to the problem. You want your statement to be as broad as possible, not focusing on a specific method or approach. It helps to center the question around the goal or the people benefiting from the solution you are looking for. Make the question specific enough to keep people focused on the problem at hand while keeping it open enough to allow for creative solutions that you wouldn't even expect. For example How might we [help homeowners] by using [new technologies] to [reduce their energy use]?
It is also a good idea to follow up your innovation challenge question with some context to tell the necessary stakeholders why it is important and what can happen if nothing is done.