Heating water in a 4-plex

[Robert Gunther]

I've got a 4-plex, two units share a water heater.  Currently the water heater tanks are 13 and 9 years old, I plan to swap out the 13 year old one as at that age it is a time bomb.

I pay for the water and the electricity to heat the water.

Is it possible to pass some of the electric bill onto the tenants by installing some type of point of use water heater in the individual units (1 bathroom in each unit).

I was thinking it might be possible to keep the water heaters at a very low temperature (say warm water, but not hot enough to have a shower for example).  Then when there is demand by the tenant the point of use water heater in their unit comes on and is able to heat the warm water from the tank to hot water for use.

Possible, stupid, drawbacks?