In states that allow net metering, excess energy produced from a solar PPA will be sold to the utility at the net metered rate.
Through a Solar Power Purchase Agreement (PPA), you will be purchasing the energy produced by a third-party-owned solar system at the rate set by your PPA, which is usually lower than your current utility cost of electricity.
If the solar system produces more energy than you need at any given time, and your state allows net metering, the excess energy will be sold to the utility at the net metered rate. The net metered rate is can be anywhere between “at the same rate that you purchase electricity for”, “at a lower rate” or “no compensation at all” (see if your state has supportive net metering policies here). If, on the other hand, your solar system produces less energy than you need at any given time, you will pull the additional energy needed from the utility, just as you would without Solar PPA in place.
If your state does not allow net metering, then they won't pay you to send excess energy back to the grid. This means in states without net metering, it does not make sense to size your solar system to allow for any excess production, or, you can opt to install battery storage, so that you can store your excess energy onsite.