A look at the production and finances of my Tesla Energy system

Posted on July 5, 2023 by Robert

This is part four of five in a series of posts about my Tesla Energy system and my understanding of how the system works. In the first three posts I cover how the system generally works. In this post, I share details of the performance of my system.

There are so many factors that affect how a solar energy system will perform and behave, including latitude and longitude, house construction, house orientation, roof shape, trees, neighboring buildings, nearby mountains, HOA, utility rates and regulations, family size and habits, whether you drive an electric car and how much you intend to charge at home, and so much more. Your system and factors will be different, and there’s no way I can provide information that covers all cases. What I can do — and will focus on — is sharing details of my setup. Hopefully that’s enough for you to figure out how that applies to you!

About our system and the data analyzed

  • Time period analyzed is 2021-11-03 – 2022-10-29, the first whole billing year that I have data for.
  • Our PV system is rated for 9.41 kW, but effective output maxes out at about 6 kW.
  • Our solar production during that period was 9,555 kWh (average 26.5 kWh / day)
  • Our consumption was 8,194 kWh (average 22.7 kWh / day, or 950W average instantaneous use)
  • We have one Powerwall.

Continue reading →

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FAQs about Tesla Energy systems

Posted on July 5, 2023 by Robert

This is part three of five in a series of posts about my Tesla Energy system and
my understanding of how the system works.

In this post, I try to answer some questions you might have about what a system is capable of and how it can be configured as you evaluate whether solar or solar and a battery are right for you.

I suggest you read the first two parts, first:

  1. Home Electricity Fundamentals, since it covers some fundamentals about how electrical energy flows, which contributes to understanding the answers to the question here.
  2. Understanding my Tesla Energy System, since it covers the basics of the roles of each component in a Tesla Energy System.

Terminology

Here’s a quick reference of concepts/terms covered in earlier posts that are important to answering some questions:

Term Quick Definition My Setup
Backup Load, “inside,” critical The load that remains connected to solar and battery when the system is disconnected from the grid. Everything but my garage (where the car charges).
Non-backup Load, “outside,” non-critical The load that is not connected to the “inside” system when it disconnects from the grid.
MSP-attached, non-backup load All non-backup loads should be attached to the Backup Gateway, but sometimes some circuits may remain attached to the Main Service Panel, or “MSP.” I have one outdoor outlet next to the MSP which is rarely used, so we left it on the MSP.

Frequently Asked Questions about Tesla Energy systems

Q: Does the battery provide power to the non-backup load when connected to the grid?

Yes, if it’s attached to the Non-Backup Load of the Backup Gateway. The Backup Gateway watches how much energy is being consumed by the Non-backup Load and tells the Battery to push enough to power it (as well as the Backup Load).

However, if there’s a circuit attached to the MSP (rather than the Backup Gateway), what I refer to as an “MSP-attached, non-backup load,” then the battery does not provide power it. The Backup Gateway Backup Gateway is not aware of its draw and thus is not able to tell the Battery to push enough power for it, too. This can mean:

  • If there is excess solar (more than the Load Center loads — both the Backup Load and Non-backup Load — and the battery is not taking excess), then the solar would power such loads. It would not be reported in the Tesla app.
  • If the loads are higher than solar production and the battery is supplying the remaining load, the battery would do so for the Non-backup Load attached to the Backup Gateway, but the MSP-attached non-backup load would be supplied by the grid!

Q: If the power goes out, can PV provide power to the non-backup load?

No. Because the gateway disconnects the “inside” from the “outside,” no power from the inside (whether PV or battery) can reach the non-backup load on the outside.

Q: What happens if I disconnect from the grid while the grid is still on?

If you disconnect from the grid while the grid is still on, the Backup Load can be powered by solar and/or battery (unless the load is too high, in which case the system shuts down).

The MSP-attached Non-backup Load will use grid energy.

Q: Do I need to upgrade my Main Service Panel?

Not necessarily, but that’s highly dependent on your home’s needs, your utilities policies, and more.

I didn’t need to because I had no new need to pull more from the grid than I was pulling before. That is, Solar + Battery didn’t change my needs.

If you believe your electricity needs will grow (e.g., to charge two EVs, or replacing gas appliances with electric), then while installing Solar/Battery would be a good time to also upgrade the MSP, especially if there are incentives that help cover the those (e.g., High-Efficiency Electric Home Rebate Act)

Q: Can I charge my battery from the grid?

In general, I cannot due to utility policy. I don’t know enough about policies elsewhere to give any more useful information.

Q: Can I push electricity to the grid from my battery?

I don’t know When and whether the battery may pull from the grid or push to the grid can be subject to policy complexities. For example:

  • Tesla: “When Powerwall is installed with solar, it will not be able to charge from the grid,” and if I try to enable it in the app, it says “Grid Charging may have tax implications and may be restricted by your utility. Confirm with a tax professional and your installer before enabling.
  • PG&E: “Home battery storage systems […] you can store power generated by your home rooftop solar system — or from the grid when electricity prices are lower — to be used at a later time.”
  • Alameda Municipal Power: “A battery storage system must be paired with a renewable energy system, like a solar power system, in order to qualify for interconnection to the electric grid. The battery storage system must not send energy back to Alameda Municipal Power’s distribution grid.

I think one of the main motivations behind restrictions for charging a battery from the grid or pushing to the grid from a battery is to prevent customers from engaging in arbitrage (i.e., pulling and storing electricity from the grid, and selling it back to the grid later at a profit) when it’s a benefit to the customer but a detriment to the grid (and its other customers).

Q: What happens if I use more electricity than the Powerwall(s) are capable of supplying when disconnected from the grid?

The Powerwall will shut itself off.

Using more energy than the battery(ies) are capable of supplying means that they can’t maintain nominal voltage, so voltage drops. In the pneumatic analogy (from the first post), this is like air leaving the system faster than it can be pushed in, so the pressure in the system drops.

The Powerwall(s) shut off to protect both the Powerall and appliances (some may be damaged by operating below nominal voltage).

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Understanding my Tesla Energy System

Posted on July 4, 2023 by Robert

This is part two of five in a series of posts about my Tesla Energy system and my understanding of how the system works.

In this post, I share my understanding of the components of my system, what they do, and how they work together.

I am not an electrician nor a solar/energy professional. Take my information with a grain of salt. Much of it is based on internet research, opening up electrical boxes and tracing wiring, and some experimentation. There are likely things that I’m wrong about (if you know better, please let me know)!

Before you read on, I suggest you read part one, Home Electricity Fundamentals, as I think it will be very helpful to understand some fundamentals about how electrical energy flows. I also refer here to an analogical pneumatic system that I establish there.

Component Details

Load Center

This is where everything comes together: PV, Battery, Home, and Grid (via the Tesla Gateway).

Using the pneumatic system analogy (of the previous post), this is simply a chamber where everything is connected so air moves freely and pressure is equal throughout the system.

Continue reading →

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Home Electricity Fundamentals

Posted on July 4, 2023 by Robert

This is part one of a five in a series of posts about my Tesla Energy system and my understanding of how the system works.

Before we get into how my Tesla system with solar and battery works and what it can and can’t do, it will be very helpful to understand some fundamentals about how electrical energy flows. That’s what this first post will focus on.

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.
Continue reading →

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Use ‘%%’ to indicate a percentage change of a percentage?

Posted on November 9, 2021 by Robert

“40% of people clicked through when the button was blue. When we changed it to green, the click-through rate increased by 20%!”

We all know that the above statement is ambiguous; we can’t be sure whether the writer intends to let us know that the final percentage is 60% (40% + 20%) or 48% (40% × 1.2).

Percentage Points

‘Percentage Points’ is a well-established way to unambiguously communicate the first type of difference. That is, “When we changed it to green, the click-through rate increased by 20 points,” is clear that the final percentage is 60%.

Percentage change in Percentage

However, I can’t find a well-established way to unambiguously communicate the latter type of difference, the percentage change of a percentage.

Can we use ‘%%’ and/or maybe ‘p/p’ (“percent-of-percentage”)?
Continue reading →

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Reader-augmented Writing

Posted on November 8, 2021 by Robert

Words gave us the ability to effectively share ideas and knowledge, and writing allowed those words to spread further and be carried across time. The printing press accelerated that spread, and electronic mediums allowed even more people to share ideas more quickly.

Yet articles and books — even e-books — are still largely static and one-way. We read only what the author knows at the time of writing. Technology will change these from one-way, static content into the basis for conversation, bringing out more ideas and critical discussion, magnifying what we get out of them.

I’m looking forward to that day.
Continue reading →

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Containerized Passenger Transportation

Posted on February 26, 2017 by Robert

Getting someone from point-A to point-B will often — even in the distant future — involve a variety of methods of transport. For example, getting someone from their home in San Francisco to an office in Los Angeles might primarily (the majority of the distance) be served by something like high-speed rail or Hyperloop. However, there’s still the need to get the person from their home to the train station in SF, and from the train station in LA to the office. Further, less common routes might involve transfers; for example, from regional rail to inter-regional hyperloop.

Containerization will allow someone to enter into the transportation module — a pod — at the beginning of their journey, and not have to exit it until they’ve reached their final destination. This is especially important when a passenger also has luggage.

I’m certainly not the first one to think of or write about this. I mostly just wanted to write about this for the fun of it; predicting some of the details of how some aspects will work, including ownership of the various parts, and billing (namely from the customer perspective). Maybe it could be a tiny bit useful to take into account when designing something like Hyperloop, or embarking on creating pods.

Why this matters now

  1. We are at the cusp of transportation automation. Self-driving cars are quite literally around the corner.
  2. Just as self-driving cars have becoming reasonable due to current technology (e.g., fast and portable computers, advanced software, efficient and lightweight batteries), the technology to support containerized passenger transport is also reasonable. That is, all of the ingredients are available and close to becoming economically, socially, and politically ready to deploy, including:
    • Robotics to transfer a pod from one vehicle to another
    • Self-driving vehicles that can bring a pod to anywhere a car can.
    • Technology allowing real-time matching of a passenger with all the parties necessary to transport them, and billing.
  3. We are beginning to re-invest significantly in transportation infrastructure. For examples:
    • In California: High-speed rail.
    • Hyperloop

We should start designing and standardizing passenger transportation pods so that we are poised to start taking advantage of them as we get closer to major transportation infrastructure investments that are expected to have 50+-year lifespan.

What we can do now

  • Begin to think about pod standardization
  • … and much more… <TODO>

My vision

Pods

Passenger pods — what the passengers actually sit in — are the staple of this vision, as it’s what will most define their experience.

Also, as the one constant throughout the trip, it is also main channel through which billing must occur. That is, as the passenger’s pod embarks with, say, a train, the train will essentially bill the pod for the trip (including for, possibly, providing power to the pod).

Ownership

While individuals could own pods, they probably won’t.

  1. People will likely want to use a variety of pods. One might be optimized for their daily commute (expected to take 45 minutes), while another might be optimized for long-distance travel (e.g., 6 hours).
  2. Just as we’re seeing with “ride-sharing” services (like Lyft), and as many are predicting with self-driving cars, not-owning your transportation vehicle/vessel is more economical. Owning your own vessel means your paying for it even when you’re not using it (often >95% of the time). Likewise, you don’t have to find — and pay for — a place for it while it sits idle while you’re at work.

Continue reading →

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Schrödinger’s Laugh

Posted on September 25, 2014 by Robert

Schrödinger’s Laugh [shroh-ding-ers laf]

-noun

  1. The strong expression by one that paradoxically may be either laughing or wailing, this being tied to an earlier random event. It creates an awkward period during which the observer does not know which and thus how to properly react.
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HTTP Status Codes 401 Unauthorized and 403 Forbidden for Authentication and Authorization (and OAuth)

Posted on June 15, 2012 by Robert

When a client requests a resource from an HTTP server and it’s not allowed to access that resource, the client needs to know enough about why in order to present the right message or options to the user. Basically, we need to know whether the user can do something about it or not.

HTTP status codes help us differentiate these scenarios and when the reason has to with authentication (verifying who the client is) or authorization (what that client is allowed to access), the server should use the 401 and 403, respectively.

There are a couple things that complicate the use of 401 and 403:

  1. The terminology used around the 401 status code in the HTTP spec (RFC 2616), namely “unauthorized” is often misused in place of “unauthenticated,” and
  2. HTTP doesn’t provide a status code for authenticated users who aren’t allowed to use a resource, so we use 403.

The Scenarios

Let’s start by understanding the scenarios that we need to be able to differentiate. There are six outcomes of a request when viewed from an authentication or authorization perspective:

# Authentication Authorized Resource delivered HTTP Status Code Resolution
provided good
1 no n/a yes yes 2xx n/a
2 no n/a no no 401 Provide Authentication
3 no n/a no no 403 none
4 yes no n/a no 401 Provide Valid Authentication
5 yes yes no no 403 none
6 yes yes yes yes 2xx n/a
  1. The unauthenticated client is authorized to access the resource (HTTP 200-class).
  2. The unauthenticated client is perhaps authorized to access the resource if authenticated (HTTP 401).
  3. The unauthenticated client is not allowed access the resource; authentication will not help (HTTP 403).
  4. The client’s authentication credentials are incorrect, invalid, expired, or revoked (HTTP 401).
  5. The client is authenticated but cannot access the resource (use HTTP 403 Forbidden).
  6. The client is authenticated and may access the resource (HTTP 200-class).

Continue reading →

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Protected: Now Entering Germany

Posted on February 17, 2012 by Robert

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