Why is the mains voltage generally above the nominal value?
I am not talking about power spikes, which leave the margins. We are talking about standard operations. By design, the power is set closer to the top margin than to the middle. These are the reasons:
Standard power generators all run with a certain rotational speed which is synchronized with the grid frequency. The rotational frequency of the generator also depends on how many poles it is equipped with, all 4-pole generators in 50Hz grids run with 1500/min, for instance.
Grid frequency is just about the only persistently constant value you can expect from the grid.
At the fixed speed, the power output of a generator is regulated by the excitation of the field coils and the mechanical input at the turbine or engine. Both values must be regulated in unison. If you increase the excitation without increasing mechanical input, the machine will slow down, and come out of sync, which must be prevented.
Some kinds of power plants run asynchronous (flywheel, solar, wind mostly) which means their power output has to be electronically regulated to fit it onto the grid.
For several reasons the power suppliers will regulate towards the upper end.
First, they can react more quickly to reduce power output: Divert some steam, reduce excitation, done. To react upwards, they must first make more steam, which takes time. So it is safer to be on the top limit.
Secondly, the same power can be more efficiently be transported when the Voltage is higher. Losses almost exclusively come from current, higher voltage means less current, so less loss, bigger percentage of voltage arrives at the customer, and only power that arrives will be paid.
Lastly, a part of the used power is pure electrical resistance, which consumes more power with higher voltage, leading to higher consumption and higher sales. I suppose this is not a big deal.
Now the power suppliers know very well how much power will be consumed on average. They know how much more will be needed on special days like thanksgiving (every stove is in action that day), or on superbowl day. They will plan ahead for quite a while.
The quality of the grid lines is taken into consideration here: If they know the voltage drop within a neighborhood rather high, the supply to that neighborhood will be set up so the planned voltage arrives at the customers, if possible. Transformators between the high/medium/low voltage networks can be regulated to some degree. (see ULTC at http://en.wikipedia.org/wiki/Tap_%28transformer%29)
Therefore voltage drops and also phase shifts are the bane of the suppliers: These two factors lead to bigger losses in the lines, which they have to pay for themselves.
1) It is a comparator. It is switching the MOSFET on and off depending whether or not the Vin is present in order to choose to power the board from Vin or from USB.
2) It is a buffer separating between the SCK pin and the LED. The LED is intended to blink while the board is being programmed.
3) It is converted to have 3.3V supply on the board in order to provide it to potential sensors/shields requiring it (and as the reference to the comparator above, but it is not necessary as it can be taken out of voltage divider).
Best Answer
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.