Battery Day. One week on…

Click for larger view. The graph shows daily electricity drawn from the grid (kWh). Before the solar panels were installed average usage was 10.9 kWh/day. After the solar panels were installed this fell by a couple of kWh/day. After an increase over Christmas when our son returned, we were back to normal. In the last month the solar electricity generated on the lengthening days have reduced the electricity drawn from the grid. Since the battery was installed a week ago, we have not drawn any electricity from the grid.

Wow. 

My wife and I have both been slightly gob-smacked by the Tesla Powerwall.

Since installation last week, without changing our daily habits, we have been off-grid, using only electricity which has been generated on our own roof.

In all likelihood, we will probably be substantially off-grid for the next six months.

The combination of the solar panels and the battery is transformational.

Solar Panels

Click for larger view. The graph shows daily electricity generated by the solar panels (kWh/day) since installation in November 2020. The solid green line shows my guess for what to expect and the yellow dots show the expected monthly average based on an EU website with historical data. The green dots show actual monthly averages and the pink line shows a 5-day running average.

Broadly, as the graph above shows, things are proceeding as I had foreseen.

Battery

We are currently using the battery in its default mode. In this mode it does not try to predict tomorrow’s sunshine ‘harvest’ and does not top-itself-up with night-rate electricity.

This seems to be fine with our relatively low usage and the increasing solar generation as we head into summer.

We will need to change mode to include off-peak top-ups when we head into winter, and possibly even in the summer after we scrap our gas boiler and install a heat pump for our domestic hot water requirements. But for now, it seems to work very straightforwardly.

I have – obviously – been analysing  the data and I will write about that in due course. But here I will just note two curiosities that make the analysis slightly tricky.

The first is that the battery itself consumes electricity, but the amount of power it uses is not declared. Using some secondary indicators I estimate that – as installed at the moment – it appears to be drawing about 50 W i.e. 1.2 kWh a day.

Click for larger view. On the App screen, energy flows between the grid, our home, the solar panels and the battery all appear equivalent.

A second curiosity  is that – as reported by the App – it looks like the energy flows in all directions are all equivalent. But this is not the case

  • For example, when the app reports 1 kW flowing into the battery for one hour one might expect the battery’s state of charge to increase by 1 kWh. But because of the conversion losses, the battery probably only stores about 0.95 kWh.
  • Similarly, on discharge, if the app reports drawing 1 kW from the battery for one hour one might expect the battery’s state of charge to decrease by 1 kWh. But because of the conversion losses, the state of charge probably falls by about 1.05 kWh.

The actual state of charge of the battery is indicated as a percentage on the app, but this data is not exported.

Altogether this makes, the state of charge of the battery – how many kWh of electricity are stored in it – quite tricky to estimate. But I am working on it.

Overall

Overall, I have the sensation of floating.

The thought that the combination of a battery and some solar panels is enough to disconnect from the grid for the next six months is truly transformational. Both from a carbon emission perspective, and from a financial perspective.

Wow!

Click for larger view. The graph shows daily electricity drawn from the grid (kWh). Before the solar panels were installed average usage was 10.9 kWh/day. After the solar panels were installed this fell by a couple of kWh/day. After an increase over Christmas when our son returned, we were back to normal. In the last month the solar electricity generated on the lengthening days have reduced the electricity drawn from the grid. Since the battery was installed a week ago, we have not drawn any electricity from the grid.

6 Responses to “Battery Day. One week on…”

  1. 171indianroad Says:

    Wow – first off – I am impressed with your blog writing energy.

    Second – transformational. The powerwall. The capacity to make amazing change is beyond the belief of a lot of people.

    I am convinced that a more hybridized approach to electricity generation, storage and distribution would work marvels.

    Not every house on a block need have a powerwall (or similar) and solar in order for every house to benefit. A nice midpoint between conventional centralized electric generation and isolated “go it alone” approaches.

    Imagine a group of ten houses – all with solar and small storage sharing everything as a pod.

    • protonsforbreakfast Says:

      Bruce

      Thank you for your kind words. And I agree wholeheartedly that building shared local resources – solar panels and storage – makes the scale of investment required more manageable. And also builds communities. The difficulty is that in suburbia it is not clear what social entity would organise that sharing.

      I actually wrote another paragraph (which I deleted because I couldn’t get the tone right) which pointed out that there are many people in Teddington and neighbouring locales with much more spare cash than me – and much bigger roofs! If the will was there we could make a difference relatively quickly and from the grass roots up.

      Best wishes

      Michael

  2. abc Says:

    Just WOW!

  3. Ross Mason Says:

    I think I am as staggered as you. Impressive beginning. On a related note I am having “discussions” with wifey regarding placement of heat pump outlet inside. We want (need?) 1 for each end of the house. I DO like the floor mounted as they take the coldest air in the room and heat it thus diminishing the puddle of foot cooling air. Kris wants it up on the wall ‘cos that’s where the salesman said it should go….
    Any arguments for or agin would be useful.

    • protonsforbreakfast Says:

      Ross – Thanks.

      I think in terms of physics there is not much in it. For all the air cooling/heating units blow air and so create forced-convection in a room. I don’t think it makes much difference whether you start at the top or bottom of the room. The wall space nearer the floor is generally more valuable and so the units placed high up on the wall seem to make efficient use of wall space.

      I think in terms of the psycho-dynamics of human pair-bonding, that you should do what your wife wants.

      Best wishes

      Michael

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