And liability for the party guest kid with alcohol poisoning. 

I've been there and done that, I was in my mid 20's so I didn't get hosed financially really but (it's not for public consumption type stuff) I got screwed pretty well.

Basically any SINGLE lever system - shower, kitchen/bar/lav faucet with a single lever there is a mixing valve, if the mixer fails or leaks, allowing hot to enter even when you turn the lever to cold, could trigger consumption of hot water. 

I wanted to get it out to BP though for consumption.  

Make sure it can handle the AC power consumption of a window AC unit!!

The two most annoying problems with badly designed low flow toilets are (1) Low water level in the bowl, which means when you drop heat you may create steak marks on the side walls unless you hit the right spot; (2) Very strong flush action with low water consumption sometimes create water splash backs.

Now, what I really wish I could do is an LED energy consumption equivalent  in electric heaters- that would make a difference but everything is a huge cost commitment.

I feel it is a consumption issue (she has a big family) and she should pay it.

In the SFH we have all tenants pay for their own water, and sometimes sewer and trash also.For any utility that is meter based, where the bills increase with more consumption, having the tenants pay will not only reduce your expenses, but also reduce the overall consumption. 

Another angle, many times the listing agent or the property manager has interior pictures of the home that aren't necessarily ready for public consumption.