The building could bill or the utility could bill. How often does power go out anyway?
If the owner of the building bills, then it MIGHT be possible for the utility to install a meter for generated power.
backup power costs money, and in reality the tenants should be charged MORE under backup power. Basically it fuel costs that are variable and maintenance costs are in the "condo" fee.
I have no idea how metering is dome, but there are plenty of systems that can do it. ION comes to mind.
There is also a lot of issues relating to power factor and peak power. If the complex is large, they probably employ power factor correction, but then agin, it depends. We might just be talking about how many washers are running at the same time. If there are 200 air to water heat pumps, then that's a different story.