Here is how it works for domestic users in UK.
The panels and invertor must be installed by a registered installer in order to qualify for the so called "Feed in Tariff". This is a government subsidised payment administered by the normal energy supply companies.
The output of the invertor is metered and is connected into the house panel via a standard MCB. This meter is read quarterly and passed to the energy supply company with which it is registered. (This does not have to be the same supplier as your normal supply, though I can see no advantage in having them different).
The Feed in tariff is paid to the householder shortly after submitting the generation meter reading. It comprises two elements, a basic payment per unit generated plus an allowance for the amount uploaded to the grid. This latter amount is assumed to be 50% of what is generated. Obviously there is no way that either the consumer or supplier can know what this amount actually is.
The normal supply meter is read and billed as normal. It cannot run backwards, being the modern electronic type, though not a smart type in my case, but just shows "ER" (for error) when power is being exported.
The overall economics are quite attractive at present, though the FiT has been reduced as installation costs fall, and there are threats to make more drastic cuts soon. Existing contracts are locked in for 20 years from commissioning.
My 3.5kW system cost about £4,500 and I estimate it is worth at least £700 per year to me in cash income plus savings on my supply bill.
