The simplest might be for the drone company to act as an intermediary. They'd bill drone users for charging and have contracts with utilities. The drone company could do some authentication / DRM / etc. so that you'd basically have to jailbreak your drone to charge without paying.

Yes, I'm sure the markup would be large as a percentage, but for most customers the convenience would be worth it. Most of the customers are probably commercial and don't want to risk getting banned or sued.