Is that export only with PV or do you mean using the batteries with octopus flux?It's interesting if you look at the value in export only at 15ppkWh, using the figure from the other page of about £860 per kWp installed, then you only need to export 5700kWh to pay for each kWp installed.
For a full battery system my ROI is something in the region of 15 years from my man maths calculation (also not counting flux - just standard Vs eco7/EV tariff)
Assuming 6kwh/day and excluding standing charge (which is my use case)...
10kwh battery system + inverter
Price capped tariff = £1.40/day
Eco7 EV tariff @ 7p/day = 42p/day (full off-peak battery charge)
So difference of 90p/day and assuming £5.5k installation = 6100 days to break even
if i got a 5kwh battery system + inverter
price capped tariff = £1.40/day
eco7 EV tariff @ 31p peak and 7p off peak = 73p/day (a full battery charge won't last the day so estimate 1kwh use at peak time/day)
difference of 70p/day and assuming £4k installation = 5700 days to break even