OK Here goes for a FiT section...
Notes as follows from Nov 2021 when I researched the info and it could have changed reader beware : -
Some basic considerations for those on the FiT scheme.
To be paid you have to have a meter that clocks what you generate.
The meter has to be approved and you use a FiT supplier to pay you - e.g. Octopus, EoN etc...
You can change FiT supplier but everyone pays you the same amount so not much point unless you feel the service / the way they manage the process is poor.
So if you install a battery you must be wary that it does not interfere with that process.
There are now Hybrid Inverters that will work with AC and DC and you can charge the battery attached off the Inverter with Solar DC and save it for use later. Or take AC from the mains and save it for use later.
Option 1 is the easy option keep your existing Solar Inverter and just add a new AC Inverter somewhere in the house off a spare way on the consumer board and attach a battery to that new inverter.
Simple but you now have two inverters in the house and any solar you are not using will have to be converted to AC and then back to DC to be stored and then back to AC when you need to use it.
Option 2 replace the existing Solar inverter with a Hybrid and store the solar straight from DC into the battery and then you only lose out once in the conversion to AC when you need it. But you must ensure that you have recorded the collection of that solar on the meter to be paid for it, even though you are dumping it straight into the battery.
This is the option I investigated and to see if it really was possible. The other benefit is that you only have one inverter to maintain and save some space.
So I have been exploring if I can achieve option 2 and spun up this thread is to see if it is possible and what if any compromises there are.
I have been investigating GivEnergy systems and after some digging and asking the GivEnergy team they responded briefly: -
Our hybrid can be used for FIT yes, however must be used with one of our meters known as a 418 meter however we strongly suggest you contact your FIT provider to check that they are OK with you installing a DC coupled system.
I have found some schematics showing the use of the 418 meter and I am not sure if it will solve the two possible problems with this approach.
You have to accurately record the Solar generated on a meter.
You can not double dip - i.e. Take energy off the Grid store it in the battery and then have that falsely shown on the Solar Meter as generated. - So energy coming from the battery that was Grid created can not be recorded as solar energy.
Ultimately the deal breaker is that you have to get your FiT provider to sign off on the new meter and they just are not helpful in doing that. So often easier to just leave the FiT meter well alone and get another inverter. The added benefit of that is that when you are generating Solar and using battery you have a higher house capacity that helps reduce the need for mains when you need a high house load, such as kettle.
Diagrams from GivEnergy
https://zerohomebills.com/wp-content/uploads/GivEnergy-Solar-Battery-Technical-Data-Schematics.pdf
Video explaining FiT and batteries the different options and a good overview with graphics
https://youtu.be/vc5_hbnhL4A
