Inaccurate Solar reporting when charging from grid

[reef]

Is there a better way to calculate solar output on a day? At present I'm using the following:

Solar generated = (battery charge - battery discharge - consumption yield + Load yield + Feed in yield)

Previously this gave a figure approximately 5% out from the generation meter: close enough to be useful.

Now however I'm charging the batteries from the grid in the 0200-0500 Octopus Flux window and feeding out at 1600-1900 during peak time its throwing the figures off badly. In the last 24 days using the equation above I get 91kWh compared to 276kWh from the generation meter! On dull days I tend to get a negative figure on occasion which I can only assume is due to losses. Its become pretty useless unfortunately!