Basically, a homeowner installs PV solar and some kind of battery/inverter system such as the Tesla PowerWall. They then sign up to be a Virtual Power Plant (VPP). In doing this they agree to sell power back to the grid when required to meet peak demand.

As grid demand varies utilities need to do something to meet peak demand. Traditionally this has been to run expensive generation plants -- typically run on natural gas or diesel -- to meet the peak demand. These plants are many times more expensive to run than systems that support the base load.

By being able to buy power from the VPPs they can save a lot of money and pass on that saving to the homeowner. Additionally, there is more saving because the power is produced locally decreasing transmission loss.

This is available in parts of California and expanding to other states. This could easily become the future.