That there are new wireless sub metering systems that very accurately allow you to identify how much water, gas, and/or electricity a tenant is using so that you can then bill them for their exact portion. When tenants pay for exactly what they use they almost always consume less. This type of metering can also detect leaks in your water lines.
Metering with green thermostats is becoming an increasingly popular method to install.

In the state of California, to sub meter the water in your units is very prohibitive. The reason is because of the Department of Weights and Measures requires you to uninstall 5% of your meters annually and have them checked for accuracy. This can be a management nightmare.

Typical wireless metering systems will payback within two years or less. In certain states there are tax credits.

If water heaters are used for domestic hot water instead of boilers then we would look at installing a solar thermal system and covering the roofing with solar mats and meter the residents usage appropriately.

The advantage of sub metering is that you can sell it to your residents as allowing them the freedom to control how much energy they choose to use. Importantly, it gets this expense off of the backs of the ownership.