Here is what I did:
I own a duplex that only has one meter and one furnace for both units. Because of this I include the cost of utilities in the rent. A few years ago I got new tenants in the upstairs unit and my utility bills suddenly got really expensive. After a few months of doing nothing but feel resentful, I approached them about the situation and offered them two options.
Option A) I would do a month by month comparison of their power bills to the bills from the previous year. If the usage was lower than it had been that month in the previous year I would credit them the difference, but if it was higher I would charge them the extra.
Option B) I would do the same comparison except that we would split any savings equally and I wouldn't charge them for overages.
I guess they were nervous about overages because they chose option B. As it turns out they've gotten a rent reduction every month since we started the system. Every month I take a few minutes to do the calculations and then send an email saying how much we've both saved. I think this incentivizes them to save even further since the bill has been reduced by as much as $90.
Maybe you could try something similar.