Electronic – How does peer-to-peer energy trading work

'How' is a question that's impossible to answer.

You have to start with the observation that when you connect a battery to a resistor, energy is transferred.

Now there are several levels of model we can invoke to summarise how much is transferred. We can also push those models to 'explain' what's going on in other terms that we feel good about, that we think we understand.

The simplest model is circuit theory. The battery delivers a current, from a voltage, and the product is power. It doesn't tell us what voltage is, or current consists of, but it's neat for doing calculations ranging from the very simple to the very complicated. The hydraulic analogy is often invoked to make it more familiar.

Next we could think about mobile electrons. Voltage is a measure of how much energy per unit charge is involved when they move. It doesn't tell us what electrons are, or how energy is transferred. There are a lot of electrons in the conductor, some move, most don't, why the difference? They gain energy by moving 'down' the field from the battery. How does that get transferred to the resistor? Is there friction between the electrons and the atoms in the resistor? Do they hit the atoms? Do they get scattered? All three are defensible explanations, depending on the level of model, and what you want to assume.

At the lowest level of our understanding is quantum mechanics, where electrical effects are mediated by photons (yes, even at DC). Frankly, that does my head in, because I lack the imagination and skill to work with those models. But it underpins the simpler models, and it still doesn't tell you what's 'really' going on.

Just get used to the idea that energy transfer happens, and learn enough equations at the simplest possible level of model to allow you to figure out how much.

There is more than one way to skin this cat

Manufacturers over the years have employed several solutions to the problem of how to power switch "smarts" (in occupancy sensors and the like) without using the neutral wire (as the mandate for having the neutral at the switch location first appeared in the 2011 NEC, and installations prior to it often omitted it in order to reduce cost and conserve truck-space, aka the electrician's version of BoM space, when switch loops were used):

• Some devices (like the ones in the OP's post) use the equipment ground as a return path. This means they require a grounding conductor in the box in order to install successfully, and are largely limited to retrofit applications in homes and other small buildings to avoid excessive current on grounding conductors. (It also means that you shouldn't put one downstream of a GFCI to minimize the risk of nuisance tripping.)

• Other devices "trickle" their operating power through the load, relying on circuitry inside to make sure that current flow through the load is not excessive and to maintain a powered state whether the internal switching elements are on or off. This means that they require a minimum load to guarantee functionality and often cause sensitive loads like LED lights to light dimly if such a minimum load (such as an incandescent bulb, or a dedicated return capacitor such as a Lutron LUT-MLC) is not present. However, they can be used in older installations where ground wires are not present.

• Last but not least, there are a few wallbox devices that rely on a long-life primary battery (such as a CR123) for power. (Some of Intermatic's timers do this, most notably, and so do some wall-mounted fan remotes.) This, of course, means you have the downside of having to change a battery every few years in a device that you normally wouldn't expect to have a battery to change.

There is one more point to discuss with the Lutron Maestro occupancy sensor the OP asked about, and that is the distinction between the green and bare wires. Many devices that use the grounding conductor for return have a single connection to the grounding system, shared between the yoke ground connection and the return path from the internal circuits. However, the Lutron devices separate these, with the bare wire connected to the yoke and the green wire connected to the internal circuits. This allows them to be converted to neutral-based operation simply by slipping a sleeve of white spaghetti tubing over the green wire and nutting it in with the neutrals instead of the grounds (some higher-end Lutron Maestro devices already come with the tubing piece and instructions for its use, even).