Not sure if this has been posted, but I stumbled on a piece of technology a few months ago that I thought might be an ideal solution for people who want to submeter without hassle.
It's the Neurio Home Energy Monitor. You install it in a panel, connect it to the property's wifi, and it reports real time usage from that panel to either a smartphone app or a web browser.
You can set the cost/kW-hr, and also the billing cycle. So, for me, one of my units isn't separately metered for electrical, but the basement unit does have its own subpanel (each unit should have this at a minimum, for legal reasons). I installed the neurio over there about three weeks ago and it's been faithfully reporting real time and historical usage whenever I look at the app.
It's $180 for the basic kit, and I installed it myself in about 15 minutes. Super simple, but you do need some level of electrical experience. And your tenant needs wifi.
I like it enough I'm gonna get a second one and install it in the garage workshop/studio at my primary residence so I can track that. I do love me some data.