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Electrical Energy Fundamentals

While energy generally flows in one direction through the network of wires in your home — from the grid, towards your outlets — they don’t have to.

You could plug a generator into any outlet in your home (but don’t do this, it’s dangerous) and energy flows “backwards” from that outlet to the rest of your home.

First analogy: Belts and Wheels

One analogical model that I think is helpful for understanding how energy moves is a system of belts and wheels. In industry, before electric motors on every machine were common, one common way to distribute power to machines was through a system of wheels, axels, and belts. One large energy source — perhaps a steam engine or a water wheel — would spin wheels/axles and belts that went throughout the factory. Each machine would get its power by having a belt come off of the axles spun by the power source.

Let’s say that the energy source is actually 10 people pedaling stationary bikes, and a small fan is added to the system. Those pedalers won’t feel much of a difference when you “plugged in” the fan. But if there’s just one person pedaling and you attach a washing machine, they’ll definitely feel that extra load. They’re even likely to slow down at first while they adjust to the new load — a “brownout” like your lights flickering as a refrigerator starts up.

Another thing you could do is just attach another bike anywhere on any of the axles and start pedaling, adding energy to the system instead — like a generator.

The wheels and axles are agnostic to where the energy comes from and where it’s going. You can tap into the system anywhere and add or consume energy. The electrical network in your home is similar: energy could be added or consumed anywhere.

The belts and wheels (or chains and sprockets) analogy is very helpful and versatile, as there are a lot of good mechanical analogs to electrical components. For example, the belt moving only in one direction is like direct current while alternating current would be like the belt oscillating back and forth; and diodes, which only allow electrical current to flow in one direction, are like freewheels that only allow the wheel/chain connected to it to spin in one direction. Spintronics is a cool toy/game for learning how electricity works using chains and sprockets.

However, there’s another analogy that may be better for understanding the directions that energy flows in the system…

Another analogy: Pressurized air in a network of tubes

For this analogy, there is pressurized air in a network of tubes — a pneumatic system — where pressure inside that system is the energy.

• Things that draw electricity are consuming air from the system and reduce pressure.
• Things that produce electricity are pushing air into the system and add pressure.
• Volts ~= Pressure (atm, PA, or PSI): How strong the air is trying to push out of or into the system. A higher PSI is like a higher voltage.
• Watts ~= Air flow (e.g., CFM or ft3/min): How much air is being produced or consumed.

When it comes down to it, the components of an electrical energy system are simply connected together; i.e., all simple splices that electricity can flow through in any direction. In the analogous pneumatic system, this would be like air being able to freely throughout the system and the pressure is the same everywhere (while it can take time for the pressure to propagate — and air pressure propagates slower than electricity does — that speed is negligible for our purposes).

For the most part, there’s nothing controlling which direction energy flows (i.e., no diodes). For example, the Tesla backup gateway (which sits between the house+solar+battery and the grid) doesn’t have a way to prevent battery energy from going to the grid except by disconnecting from the grid. What it does is just directs the battery to not provide more than necessary for the Home (to the point that it would result in net energy going to the grid).

So, here’s what’s happening if we connect all the major components of an electrical energy system together (I’ll go into all of these in more detail in the next post, part two of five):

• Load Center: Where everything — PV, battery, home, and Grid all come together. It does not control the direction that energy flows.
• Photovoltaics (“PV,” or solar panels): These basically push air and add pressure to the system.
• Battery: A battery is like a balloon: it can collect surplus energy/pressure and expand. When that energy is needed, it can release — push — air/pressure back into the system.
• Grid:
  • If you use more air than is provided by your PV and battery, then you’ll use air from the Grid.
  • If you use less air than is provided by your PV (and your battery is full), then you’ll push air to the Grid.
  • If you use exactly the amount of air provided by your PV and battery, no net air is going to/from the grid
  • If you’re disconnected from the Grid:
    • If you’re using more than your PV and battery can provide, you see a voltage/PSI drop (and the system usually shuts down to prevent damage).
    • If you’re using less than your PV is producing (and your battery is full), then air is just “lost” (escapes via a relief valve).

Circuits and breakers

A “circuit” generally refers to a leg or branch of the system; a path down which electricity flows (usually from the grid) to outlets (and into appliances). Each circuit has a circuit breaker which helps protect the wires that make up the circuit/branch. If too much energy flows through, the breaker “trips,” disconnecting the branch from the panel, cutting it off from the energy source (again, usually the grid), preventing energy from continuing to flow through the circuit, which helps prevent fires and other dangers.

In the pneumatic system, a flow meter that can close a valve would be analogous to a circuit breaker. In fact, if you have natural gas, your gas meter may do this; i.e., “turn off the gas” if too much gas is flowing, indicating that there may be a leak.

Key Takeaways

1. The load center, where everything (PV, battery, home, and Grid) all comes together, is pretty much just a big splicing center and has no ability to control flow other than by cutting off circuits with excessive flow (circuit breakers).
2. Likewise, the circuits, breakers, and their wires don’t care which direction energy is flowing.
3. Each device attached is what “decides” which direction energy flow (into and/or out of it). For example, the PV will only produce — push — energy into the load center, appliances will only consume — pull — energy from the load center, and batteries and the grid can push or pull.
4. Theoretically — because you shouldn’t do this — if you had a generator and an appliance attached to the same circuit, and the generator produced exactly as much energy as that appliance needed, it would result in no net energy flowing to/from the load center.

Note: For simplicity, I use “electricity” and “energy” interchangeably even when it’s the latter that would be more accurate. My use of “power” is also colloquially for “providing energy to.”

In the next post, Understanding my Tesla Energy System, we’ll learn more about the specific components in my Tesla (PV & battery) system.