Electrical – Identify when the inverter is pumping energy back into the grid

The most simple and direct answers to the main question depend on how "excessive" it is. Since most equipment is designed to operate within +/- 5% of nominal, the "extra energy" usually gets dissipated as heat, in the device itself. In the case of a light bulb (for example), it produces more light and heat. If the excess energy goes beyond the tolerance of the devices, they will overheat and/or burn (cause damage). These results will be obtained regardless of what causes the "excess energy" on the grid (lightning, solar installations, wind power, etc.).

For the last two questions, if you are charging a 12v battery with a 13v source, the extra 1v will keep the battery "warm" after it is charged to 12v. If you are charging it with an 24v unregulated supply, the battery will overheat, burn up, and possibly explode. If you charge it with an over-voltage and current-limited supply, the battery will be charged to 12v and the extra energy will be dissipated as heat in the supply regulators. One way you can make "efficient" use of any "extra energy," would be to use a bank of batteries and a "smart" charger, which would switch the charging to another battery when one is charged, and shut off (disconnect) when all the batteries in the bank are charged. If there is no interest in saving the extra energy, it can be "dumped" into an appropriate load and converted to heat.

Your understanding has some of the elements, but it is more like this:

1. Monitor the grid wave.
2. Create an inverter wave exactly like the grid matching frequency and phase angle.
3. Connect the two and then no power would flow into the grid.
4. Monitor the current output of the inverter very carefully.
5. Increase the voltage very slightly and dynamically control it to deliver the desired current.

The grid appears to the inverter to be an ideal voltage source behind a complex impedance. The inverter voltage must be raised above the grid voltage just enough to drive current through the source impedance. The ideal voltage source is the voltage measured just before the grid tie is closed. Once the grid tie is closed, it can no longer be measured. The inverter voltage increase is performed by a control system that monitors the inverter voltage, frequency and phase angle. Those parameters need to be controlled and adjusted continuously to keep the inverter synchronized with the grid and deliver the desired power. The grid voltage, phase angle, frequency and impedance can all fluctuate.

The inverter control system essentially makes the inverter act as a current source.

This is a simplified explanation of a grid-tie inverter control system that is actually more complex. The system also must include some protective features. While some small inverters may be designed to be used with any type of input system, others may be designed specifically as solar-PV or wind turbine inverters. "Consumer" inverter designs are different from "utility" inverter designs.