I really don't think the ROI on charging is all that much. It looks like it's more of a deal to draw customers and make money from your attraction. Hilton Hotel chain, and now Marriot just announced plans to install destination type chargers at many of their locations. Haven't really heard details on what they plan to charge or not charge for the convenience. I have noticed a few casinos that have unattended AC chargers in their parking garages. It looks to be free but I haven't sampled it. Pretty sure they will make up for the cost if folks spend a few extra minutes entertaining themselves in the casino. Major gas station chains are having chargers installed, that I assume will result in increased trips to the convenience stores there.