How does net metering or net billing affect the economics of a project?

In the case that your clean energy source has produced excess energy, you can sell that energy back to the grid, which can improve project economics.

Net metering is a clean energy friendly policy set by the utility that pays out the same rate for energy that is exported as it is for energy that is consumed or imported. 

Some states and utilities that have in the past supported net metering like California are transitioning to a net billing policy which compensates for exported energy but at a lower rate than is consumed or imported. See if your state has supportive net metering policies here