Here’s another random example, October 5th:

How does that add up to 8.69 kWh ? You can see that throughout all the day I was below .3kwh average, and if you do .3 x 24 you get 7.2 kWh… so max it should be is below 7.2 but no, I used 8.69??

I’m in my phone so haven't totaled them - but if you total each one rather than a guestinate then how does it look?

It may still be slightly different because the half hour figures shown are shown to 2 Decimal Places whereas the total uses the full 3 DPs that are available from the meter, and then still shows it to only  2 DP.

Is all one has to do simply add up all 48 values to get the total kWh used for that day? Or is it find average and then multiply by 24

You should not try to average them - just add them all up.

You’ve just discovered your solution .. each slot is for half an hour and there are 48 of them (not 24). There may be some discrepancy due to rounding. 
The maths does work

‘Way more’? I haven’t added up your figures, but  how big is the difference?

The usage data are largely for interest’s sake (although this will be changing from next year); charges are calculated solely from the actual meter readings.

There’s another factor: the single figure for the day’s consumption is the difference between meter readings from one day to the next. The half-hourly usage data may not be timed from precisely the moment the meter readings were taken, so their sum will not always match the meter advance precisely. That said, mine are always within .001kWh of each other if I use the underlying numbers with three decimal places.

Note, too, the BST fudge. The first two figures in the tables in fact belong at its bottom, because the underlying data and the meter readings are always based on GMT. The meter readings you see are taken at 01:00 local time. 

@cypherpunk1

Being the kind (of) person that I am I have now totalled your 22 October figures from above

Simply adding each of the half hour figures up gives a Total of 10.19 kWh for the full day.

Which is a negligable 0.03 different from the 10.22 kWh that you say is the total given by OVO

That tiny difference can also be easily explained.
It’s simply the difference between me using those 2-DP figures to calculate the total, whereas OVO will have calculated the total using the full 3-DP figures. (which aren’t shown there).

I hope that helps put your mind at rest.

I don’t think so (as I wrote earlier). The total figure on the Day tab, and the (same) figure on the Month tab, is simply the difference between meter readings. OK, they are also usually to three decimal places, and the meter simply calculates Reading 1 + 48 Hh figures to get Reading 2, giving rise to a possible difference of 1 in the least-significant digit.

You can see this occasionally if readings have turned up before usage data, which happens to me about half the time. On those days, the day total is shown on the usage pages even though the Day chart hasn’t yet been populated with Hh data. 

thanks everyone I get it now. 

Hi @cypherpunk1,

It seems our forum volunteers have already got to the bottom of this for you. Hope it was helpful.

Please let us know if you have any further questions.