Electronic – How to power grid suppliers control the voltage and frequency

I think what might be missing here in your understanding is that when we send information on a computer bus, we are sending digital information. Bits are represented with two states, '1' or '0' - or simply "on" and "off". A string of bits represents a number in base 2. We represent these two states (1 and 0) with discrete voltage levels, for the sake of simplicity, something like +5V and 0V respectively (different devices may use different levels to represent their bits but the principle is the same).

So when we talk about a computer bus having a frequency, we're talking about how quickly it can toggle a bit - or a single wire in this case - from a logic '1' to a logic '0' or vice versa as this will determine the maximum bandwidth the bus is capable of (at the bit rate level). To further divide computer buses into categories, there are parallel buses and serial buses. A parallel bus breaks out the individual bits of a base 2 number (typically an address or data value) and gives each one a wire. Thus, the calculation that was quoted holds true for parallel buses. Then there are serial buses for something like USB like you mentioned. On a serial bus, all bits are sent on a single wire (two wires in USB's case, but they are a differential pair, so you can think of it as one wire). The bit rate calculation for that would simply be its maximum "toggling" frequency.

You should not try to relate the above with a sinusoidal AC signal frequency found in AC power. Yes you are correct in that a DC voltage should have a theoretical frequency of 0 - but in the context of digital communication we are not really interested in that.

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.