“Off-Grid” or Not “Off-Grid”? That is the question.

Friends, it’s May, and slightly later than last year, our house is now ‘Off-Grid’.

Click on image for a larger version. Graph shows the daily household consumption of electricity (kWh) since the March 2021. Also shown is the amount of electricity drawn from the grid (kWh). Both quantities are averaged over ± 1 week.

By ‘Off-Grid‘ I mean that the combination of…

  • 15 kWh/day of solar electricity, and
  • 13.5 kWh of battery storage in our Powerwall,

…is enough to allow us to…

  • consume 10 ± 2 kWh/day of electricity and
  • export on average 6 kWh/day

…without having to draw any electricity from the grid. Almost

Almost?

On the graph above it looks like in this happy situation, we draw zero electricity from the grid.

However, if one looks closely one can see that electricity usage is low, but not quite exactly zero.

The figure below shows half-hourly electricity usage (watts) over 4 consecutive days in May. This data was acquired using the highly pleasing Powershaper software.

Click on image for a larger version. Chart showing electricity used by the Tesla Powerwall throughout 4 consecutive days in May 2022. Averaged over half-hour periods through the day, the Powerwall draws around 1 watt, but occasionally draws as much as 20 watts. Over a day it adds up to less than 0.1 kWh costing between 1 and 2 pence.

I don’t know why the Powerwall is doing with this: it typically has a battery filled which electricity which it could use!

My guess is that it uses the grid to help it meet transient demands. I suspect that if we could look more closely then we would see that rather than consuming 1 watt continuously, it would instead be consuming no power most of the time, but then occasionally it would a draw a kilowatt or so for just a second or so.

The software that controls the battery does have a ‘True’ Off-grid mode which apparently would isolate the house completely from the grid. But in honesty, I have been too scared to push the button!

Being ‘practically off-grid is enough of an adventure for me.

Update: 26 May 2022

The 9-minute video below explains the effect I am describing with admirable simplicity.

6 Responses to ““Off-Grid” or Not “Off-Grid”? That is the question.”

  1. leedalton1979 Says:

    That’s pretty cool. Any theories on why it’s been later this year, just a delay in the longer days of sunshine, or has your usage increased compared to last year ?

    We had a solax battery and inverter fitted in march, and we find that it seems to over and undershoot on power output with fluctuating loads, like hobs and oven that pull a lot, but pulse as they get up to temp. So whilst we have been largely self sufficient with our energy generation, we have still used around 13kwh over the whole month, for the occasions the inverter didn’t ramp up quick enough to suit demand.

    • protonsforbreakfast Says:

      Hi. The delay in going ‘Off-Grid’ this year as opposed to last is mainly because we are heating with a heat pump and last year we had gas central heating. We only turned the space heating off at the end of April, and even then we had to turn it back on for a few days at the start of May. And we still use around 1 kWh/day to top up the domestic hot water.

      I don’t have any experience with the Solax system, but I imagine this response to transient demands is at the heart of this residual usage.

      Best wishes: Michael

  2. MikeGraham Says:

    Love your blog!!

    I recently got a Solis and PylonTech combo for battery storage, and found exactly as you did, it still pulls a little from the grid. I raised a support ticket with Solis, and one of the issues for me was a bug in firmware, in that it would flip between charge and discharge, when the battery was almost empty, this has since been fixed, but the undershoot was still very apparent. Luckily they had built into the new firmware offset values, which allowed me to specify a +60W offset to the output of the Solis inverter, and this has brought down usage to around 0.4kWh per day on a ‘Grid Free’ day. For a small time I tried a +100W offset, and whilst this made grid import practically zero, it was an over compensation which ended up draining the battery prematurely, I suspect I could probably add another 5 or 10W to the offset, but the import reading tends to be almost 0W majority of the time, so i’ve left it for now.

    The other issue is the way these things read (not sure about the Tesla), using the CT clamp, i found that it just wasn’t polling quickly enough to match the demand quickly, so there was always a bit of leaky import, and then when the load switched off, a leaky export. This was another thing which was resolved in the firmware upgrade for me, they now poll the CT clamp much more often, which has helped massively, but theres still always will be a slight delay, the signal has to be received, processed and acted upon, it can never be a 0ms reaction time.

    Cheers
    Mike

    • protonsforbreakfast Says:

      Good Morning:

      The subtlety of the effects you describe are why I selected teh POwerwall over the other combinations that allow more granular control – the complexity can be overwhelming!

      Anyway – it looks like your on top of it: do you publish your data anywhere or have a description of your set up?

      All the best

      Michael

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