Battery Day: First Results

Me and my new Tesla. This unit contains 13.5 kWh of battery storage along with a climate control system to optimise battery life. We have placed it in the porch so that (when visits are allowed again) everyone who visits will know about it!

Last September, Tesla held their ‘Battery Day‘ during which they unveiled their road map towards cheaper, better, batteries.

Not to be outdone, last Monday VW held their own ‘Battery Day‘ during which they unveiled their road map towards cheaper, better, batteries.

And last Thursday was my own battery day, when Stuart and Jozsef from The Little Green Energy Company came and installed a Tesla Powerwall 2 at Podesta Towers in Teddington. I was (and still am) ridiculously excited.

I am still evaluating it – obviously – but here are a couple of notes.

How it works

The system has two components. An intelligent ‘gateway’ that monitors loads and supplies, and a climate-controlled battery storage unit.

Click for a larger version. The left-hand graphic shows how AC power enters our house, and how DC power generated by solar panels is linked to the grid. When the Solar PV is sufficient ,power is exported to the grid. The right-hand graphic shows how the TESLA ‘gateway’ device monitors the solar PV, domestic loads and battery status and intelligently decides what to do.

The gateway (and the battery) are electrically situated between the electricity meter and all the loads and power sources in the house. So all energy enters or leaves the battery module as AC (alternating current) power.

But its internal batteries must be supplied with DC (direct current).

This makes it ideal for storing power from the AC grid, but less than ideal for storing the DC current generated by solar PV panels.

One might have expected that a device designed to store solar power might intrinsically operate using DC and indeed, some battery systems – positioned between the solar PV and the inverter – do this.

So the choice to place the Powerwall™ where it is, is a compromise between the extra functionality this location offers – it can back up the entire house – and the inefficiency of storing solar PV which is first converted to AC by the inverter, and then re-converted back to DC by the Powerwall. The support document states that the conversion from AC to DC and back to AC has 90% round-trip efficiency.

The photograph below shows the gateway installed under the stairs in our house.

Click for a larger version. The Tesla ‘Gateway’ installed in our house. The unit is positioned in between the electricity meter and all the domestic loads. The black conduit leads under the floor to the battery which is installed in the porch.


The system is controlled by an app which is – frankly – mesmerising. It shows how electrical power flows between:

  • the grid,
  • the battery,
  • our home, and
  • our solar panels

Click for a larger version. Screenshots from the app at various times yesterday.

There is less room to adjust the parameters of the system than I had anticipated. This appears to be because, in exchange for a guarantee that the battery will retain at least 80% capacity (10.8 kWh) after 10 years, one is required to relinquish detailed control to the Tesla Brain.

Through a built in network connection, the device is in constant touch with Tesla who monitor its performance and can detect if it is abused in some way. I am not sure how I feel about that – but then guaranteed long-term performance is certainly worth something.

One feature of this relinquishing of detailed control concerns ‘time-of-use’ tariffs. I anticipate that – especially in winter – I will need to charge the battery overnight on cheap rate electricity.

The system supports this mode of operation but is not yet operational. Apparently it needs to study the patterns of household use for 48 hours before being enabled.

When operational, one gives the system general instructions and then allows it to choose when, and by how much, to charge. There is for example no way to force the battery to charge to 100% on command.

In practice I suspect it will be fine, but at the moment it still feels a little weird.

Performance on Day#1

The simplest way to show how the Powerwall™ works is by looking at the data which the ‘App’ makes available.

The first graph shows the household demand through the day. It’s fascinating to look at this data which has 5 minute and 0.1 kW resolution. The metrologist in me would like more – but in honesty, this is enough to understand what is happening.

Click for a larger graph. See text for details.

Now we can look to see how that demand was met. Overnight, we relied mainly on the grid.

Click for a larger graph. See text for details.

The battery could have supplied this overnight electricity, but it had been set to hold a reserve of 16% of its capacity (~2 kWh) in case we required backup after a power cut. We have lowered that setting now because, thankfully, power cuts are rare in Teddington. The battery drew power from the grid overnight in two short periods to maintain this reserve.

Additionally, at the end of a sunny day in which the solar PV filled the battery, there was brief period where we returned electricity to the grid.

During the day – which was very sunny 🙂 – the household electricity demand was met by the electricity from the solar panels.

Click for a larger graph. See text for details.

Without the battery, most of this 16.91 kWh of electricity would have been sent to the grid. But now only a tiny fraction was returned to the grid, most of it being captured by the battery – see below.

Click for a larger graph. See text for details.

The graph above shows the battery maintaining its reserve charge at night, and then charging from the solar PV during the day. At peaks of household demand, the charging is paused. At around 16:00, the battery was briefly full, and shortly thereafter it began discharging to meet household demand.

As I write this at 1:00 p.m. on the day after the day shown (a rather dull day 😦 ), the battery is 56% full and charging.

The graph below shows all the above curves together.

Click for a larger graph. See text for details.


The Powerwall system is an object of wonder. It is beautifully engineered and miraculous in its simplicity.

It transforms the utility of the solar PV allowing me (rather than electricity companies) to benefit from the investments I have made.

I will post more about the performance in terms of cost, electricity and carbon dioxide when I have more data.

But for the moment I will just thank Jozsef and Stuart from The Little Green Energy Company for their professionalism and attention to detail. And ‘No’. I am not being paid to say that – quite the opposite!

Stuart and Jozsef from The Little Green Energy Company. You can’t see it, but they assure me they were both smiling. Click for a larger version


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5 Responses to “Battery Day: First Results”

  1. Simon Says:

    A fascinating read – thanks, Michael!
    I couldn’t resist plotting energy rather than power. With apologies, my inner calorimetrist has taken over …

    Although they contain the equivalent information, I have often found that the sheer noise in power data can hide a lot of understandable behaviour which is more obvious in its time integral. Or rather, in the time integral of deviations of the power from some model-based prediction of the power.
    To clarify, it was “often” the case that I was trying to make improvements to my model of what a particular calorimeter was doing. I would naturally look at discrepancies between my model and reality, and if expressed in terms of power (of heat transfers, etc) they were usually as noisy as can be, but if I looked at energy (~temperature) some structure or other would emerge from the noise.
    For your battery, I’m thinking that its stored energy (what can be got out, rather than what went in) could be plotted, along with household consumption (time integral of demand), solar energy harvested (time integral of power from those PV panels) and energy supplied by the grid. The energy available from the battery is, of course, always limited. But, if I’m thinking straight it is also the sum of the other three (up to losses in the battery), which individually just grow, roughly linearly, with time.
    Perhaps give it a try? I’m genuinely curious…
    Best wishes

    • protonsforbreakfast Says:


      Good Morning. In fact I had already tried integrating the data and initially concluded that at the resolution available [5 minutes (i.e. 0.083 hours) and 0.1 kW] the data was not quite good enough.

      I concluded this because the integrated charge came to 14.5 kWh which exceeds the battery capacity of 13.5 kWh. However I was wrong.

      The battery consumes power itself and so I can introduce a ‘self-consumption’ term of about 60 W which does two interesting things to the integrated data.

      Firstly, it makes the battery state of charge top at close to the rated 13.5 kWh. Secondly overnight the battery drew current in two pulses to maintain its back up states of 16% of charge.

      On the integral this made it look as though the battery was charging slowly – which it wasn’t – it was simply maintaining charge. A self-use rate of 60 W compensated for the apparent increase in the state of charge.

      So, yes, integrating the data has proved useful :-). I’ll post the graph on the blog shortly.

      Have a nice afternoon:


  2. Simon Says:

    PS I would have expressed myself better if “couldn’t resist” had come out as “wouldn’t be able to resist”.

  3. Jozsef Serfozo Says:

    Hello Michael,

    Hope you’re still enjoying the use of your system.
    Would you mind if I share this blog post?
    Kind regards,

    • protonsforbreakfast Says:


      Of course. I am really grateful to you and Stuart. The battery is fantastic and the installation is neat and solid.

      Thank you very much for your attention to detail and expertise.

      We are still off-grid after almost 4 weeks – and still slightly shocked!

      Best wishes


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