Funny, I just put a somewhat similar site online. It depends on people putting in their current meter count regularly by hand and obviously can't be as precise. Still, for most people who don't have a smart meter (in Germany that's everyone), it can work well, and provide at least a monthly usage chart.

I developed it to scratch my own itch: we share a flat and had to pay a lot for electricity at the end of one year because the the advance payments we pay monthly didn't cover our usage. Since then I noted the meter count monthly so we could split the extra charges every month.

It's only in German for now, ugly, and probably still buggy. Feedback is greatly appreciated, though: http://www.stromio.de