We are currently testing a way of connecting a rotary power source, like the Human Dynamo, directly to the power grid. This system is called the "FireWheel Inter Grid" or "FIGG" for short. Installed at at several gyms in the United States, FIGG's early results are promising.
With FIGG, no special DC wiring or inverters are needed to hook up machines to the grid. They simply "plug in" to a standard socket and the power flows through the cord into the circuit. If the power in the building goes out, the system stops producing power, making it entirely safe. (For safety reasons, utility companies don't want power flowing "out" of a building during outages.)
As for your electricity usage, the electric meter slows down in relation to the inside power generated. If you have enough machines producing more than you are using, the meter will turn backwards during those times!