Figures on my energy card (for electricity) yesterday show :
Import 9.1 kWH
Export 13.3 kWH
Solar 21.9 kWH - with rounding I could understand this
Home 16.8 kWH
Net return 4.2 kWH - again perhaps rounding accounts for the disparity
and the self-sufficiency is then shown as 46%.
How is that right?
100 x (Home - Import) / Home
= 100 x (16.8 - 9.1) / 16.8
= 45.8%
i.e. if your Import is 0 your self sufficiency is 100%
Thanks tom_l.
So it takes no notice of what’s produced here…
That seems a little odd. I notice there’s been conversation about this metric before, did that generate this view of self-sufficiency?
Self sufficiency is is about how much of the energy your home uses is supplied by you. It has nothing to do with how much you export. That is this metric:
Good day
Bad day
Yes, that’s my thought too. The “Solar” in my original post is produced here (our solar panels). Doesn’t that count as “supplied my me”?
Yes. The more you use of that the lower the import will be in this:
100 x (Home - Import) / Home
And the higher your self sufficiency metric will be.
I wonder if I have something set up incorrectly. The “yesterday” I referred to was 29th of March, and now the display for that day seems to have disappeared. Instead I have a page headed 29-30 March which has one bar of the 29th and all the rest 30th.
So, using figures from this page the “big picture” is
Am I being foolish for not understanding? Since it shows much more produced here than imported from the grid I would expect self sufficiency to be more than 100%.
This is the point of misunderstanding.
You did produce more than you used, but you used energy in a way (likely at night or above the power available from you panels) that required your house to pull energy from the grid.
Without the grid your house would not have worked the way it did, therefore the self sufficiency is below 100%.
That’s not a bad thing. It’s just how it is.
As I keep saying, export has nothing to do with self sufficiency.
Sell sufficiency is about how much of your production you use, it has nothing to do with how much you export, you are selling that to the grid.
And as pointed out above, you had to purchase some energy (most likely at night). So your self sufficiency is less than 100%.
There is no way you could be 100% self reliant without the grid. Not without a larger battery.
Thanks to both tom_l and signaleleven for sticking with me on this one.
I thought the “how much of my production I use” was the “self consumed solar energy” metric.
I’m still somewhat bothered by this. For me this is not a definition of self-sufficiency. It’s more a measure of “complete cutoff from the rest of the world”. However, I can see that doing something here would at the very least need a name change and that looks like the thing that’s sparked lots of previous debate. (Would “Grid use” be suitable? Ok, no, forget it 
)
Thank you both for your clarification.
It is, but self sufficiency is just expressed as a percentage of your total house use.
That’s exactly what it is! How likely you can disconnect from the grid because you produce everything you need, when you need it. You have a battery so this need vs supply can be spread out a bit.
Also it’s actually a well defined industry term, see: Understanding Solar Terminology 2 - What is Self-sufficiency?-Blog
Although that’s an Australian website I’m guessing (because of the “ESS” in their name) that it’s really a Chinese company. The English is a bit odd.
However, it seems to prove my point is correct. It says
The ratio of self-consumption describes the local use of PV electricity while the self-sufficiency ratio describes how PV production can cover the needs of where it is installed.
That “self-consumption” part is what you said, but not the self-sufficiency. It doesn’t say “how much grid did you use”, it says can your production cover your use, and in these particular figures it can, by more than 100%…
I give up.
Production and use can happen at very different times.
If time wouldn’t count in, you could also make a self sufficiency diagram for a year and tell me you was more than 100% self sufficient this year just because you exported the hell during summer even that you had to buy an immens ammount during winter.
Sorry, but this makes no sense …
And the time component is respected the most easiest way by looking if you had to import energy in the observed timespan.
edit:
And about your quote:
It means that self-consumption is the ratio of how many of your produced energy you used yourself. This is very different to what the people described here as self-sufficiency.
You could be completely self-sufficient by not needing to import any energy, but still have a low self-consumption because you exported a lot energy to the grid.
Simple explanation: You would have a very large pv-system that poduces a lot more energy than your house / car can use.
edit2: Even the wording is self-explaining to be honest.
Self consumption:
How much of your solar production did you consume yourself
Self sufficiency:
“The state or condition of not needing or relying on external assistance, support, or aid.”
Oxford English Dictionary
