MCS, SEG & VAT
Do I need a MCS or Flexi-Orb certified installer?
If you intend to sell your energy back to the grid, either a Flexi-Orb or MCS certified installer will have to do your install, register the system and issue you with a certificate, which will be required by your energy supplier if you sign up for SEG.
Make sure you fully understand the role of e.g. the MCS as in essence the MCS can ONLY investigate, arbitrate and potentially withdraw an installer's license to operate if they find it appropriate to do so.
Be aware, these organisations don't offer any financial protection if something goes wrong with your "MCS certified installer"
The MCS standards for a certified installer can be viewed here where you can get an idea of what to expect from them and certified installers.
SEG the FIT replacement
The original FIT and generation incentive payments were subsidies intended to implement a "quick fix" to a government's lagging renewable energy ambitions. It has now been discontinued and replaced with SEG, which typically pays customers feedin rates at far less than what you pay per kWh.
In March 2011 FIT offered very lucrative incentives for energy sold to the grid to entice households to install solar panels. This has since been replaced by the market driven SEG (smart export guarantee) which made headlines with initial supplier offerings of 1-2p per kWh vs the over 50p per kWh when FIT was started. Although export pricing per kWh have improved and become more competitive, you may have limited options depending on your existing household rate with your supplier and definitely have to shop around.
Each supplier now sets their own SEG tariff. Taking this into account, 100% self consumption and energy storage may make a lot more sense today if you are a heavy consumer, but make sure you fully understand how to decide, which is best or you.
PS - If you have an existing FIT agreement, take care to not impact the current metering with any upgrades of an existing system as this can see the FIT agreement being terminated.
VAT
New complete PV and storage system purchases and the installation thereof are currently VAT exempt as is the retrofitting of or storage upgrades and the installation thereof. Be aware this does not apply to e.g. online purchase. (Different terms apply to Northern Ireland)