So brown leccie would still be paid for because they can't prove what generated it (and everything would be behind a MCS certified system even if one built a large self build battery and a second dc-dc solar pv array to top it up)4 Payment Method
4.1 Your monthly bill will show both the import consumption data under your Import Tariff and the export data under your Export Tariff.
4.2 You will pay the net difference if the cost of Energy you consume under your Import Tariff exceeds the payment that we owe you for the Energy you are exporting under your Export Tariff for the same period.
4.3 If the total payment due to you for the Energy you are exporting under your Export Tariff is greater than the cost of Energy you are consuming under your Import Tariff, this will show as credit on your online account.
4.4 Provided your meter readings are up to date, you can request a cash refund if your account goes into credit. This option will appear on your online account.
4.5 If you do not maintain your direct debit payment on your Import Tariff or do not pay your Import Tariff bill on time, in addition to the consequences outlined in our Import T&Cs (in “Non-Payment of Charges”), we may stop applying export credit under your Export Tariff to your account until all outstanding payments on your Import Tariff have been made.
4.5.1 We are not obligated to make Export Tariff payments to you:
(i) for any ‘Brown Export’ – i.e. exported electricity not generated by an eligible generation asset (including standby generators, batteries and technologies not supported by the Smart Export Guarantee; or
That tax liability is only realised once the money is drawn out of the account but as it is offset even against a gas bill it feels like this tariff had alot going for it: sell 1kwh during peak allows the purchase of over 3kwh of gas whenever you need it, Export in summer to cover winter costs, and with a large enough battery can cover arbitrage between cheap and peak slots without running into HMRC.
I'll probably move to it once it get a smart meter fitted.