Smarter power when the supply changes
As I wrote before, I want power plugs and jacks to be smart, so they can negotiate how much power the device needs and how much the supply can provide, and then deliver it.
However, sometimes, what the supply can provide changes. The most obvious example is a grid power failure. It would not be hard, in the event of a grid power failure, to have a smaller, low capacity backup system in place, possibly just from batteries. In the event of failure of the main power, the backup system would send messages to indicate just how much power it can deliver. Heavy power devices would just shut off, but might ask for a few milliwatts to maintain internal state. (Ie. your microwave oven clock would not need an internal battery to retain the time of day and its memory.) Lower power devices might be given their full power, or they might even offer a set of power modes they could switch to, and the main supply could decide how much power to give to each device.
Of course, devices not speaking this protocol, would just shut off. But things like emergency lights need not be their own system -- though there are reasons from still having that in a number of cases, since one emergency might involve the power system being destroyed. However, battery backup units could easily be distributed around a building.
In effect, one could have a master UPS, for example, that keeps your clocks, small DC devices and even computers running in a power failure, but shuts down ovens and incandescent bulbs and the like, or puts devices into power-saving modes.
We could go much further than this, and consider a real-time power availability negotiation, when we have a power supply or a wire with a current limit. For example, a device might normally draw 100mw, but want to burst to 5w on occasion. If it has absolutely zero control over the bursts, we may have to give it a full 5w power supply at all times. However, it might be able to control the burst, and ask the power source if it can please have 5w. The source could then accept that and provide the power, or perhaps indicate the power may be available later. The source might even ask other devices if they could briefly reduce their own power usage to provide capacity to the bursting device.
For example, a computer that only uses a lot of power when it's in heavy CPU utilization might well be convinced to briefly pause a high-intensity non-interactive task to free up power for something else. In return, it could ask for more power when it needs it. A clothes-dryer or oven our furnace or other such items could readily take short pauses in their high power drain activities -- anything that uses a cycle rather than 100% on can do this.
This is also useful for items with motors. A classic problem in electrical design is that things like motors and incandescent lightbulbs draw a real spike of high current when they first turn on. This requires fuses and circuit breakers to be "slow blow" because the current is often briefly more than the circuit should sustain. Smart devices could arrange to "load balance" their peaks. You would know that the air conditioner compressor would simply never start at the same time as the fridge or a light bulb, resulting in safer circuits even though they have lower ratings. Not that overprovisioning for safety is necessarily a bad thing.
This also would be useful in alternative energy, where the amount of power available changes during the day.
Of course, this also applies to when the price of power changes during the day, which is one application we already see in the world. Many power buyers have time-based pricing of their power, and have timers to move when they use the power. In many cases whole companies agree their power can be cut off during brown-outs in order to get a cheaper price when it's on. With smart power and real-time management, this could happen on a device by device basis.
These ideas also make sense in power over ethernet (which is rapidly dropping in price) which is one of the 1st generation smart power technologies. There the amount of power you can draw over the thin wires is very low, and management like this can make sense.