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Micro-inverters for an established 240v grid-tie back to powerwall
Welcome to my build thread. 
I'm hoping this thread will be a little different to the norm and majority of other threads here due to the inherited system already installed.
So, in the deepest darkest countryside is a little house with a 4kw solar installation on the roof.  The system doesn't use a normal grid tie inverter, instead it uses individual micro-inverters on each of the 16 panels to produce 240v in parallel. The cabling is deep routed through the house and therefore going to be a slight pain to change. What's more, the system is already connected and metered to the grid, meaning I need to keep the 240v for my FIT payments. I must add, we own the panels and the system outright, and was installed professionally with MCS approvals and certificates.

The Details

I have Sanyo 16x HIT-H250E01 panels on the roof, each with its own enecsys 240-72 micro inverters. Amazingly all the inverters are all still working, so in order to reduce ultimate costs and keep the 240v running through the FIT meter, I would prefer to keep these if logical and sensible. 
For clarification, we get a generators payment and also a feed in payment. Under the agreement we have we receive payment for 'Generation' at one rate for 100% of what we create, but the feed in rate is calculated and paid by the energy company for 'Export' at 50% of the total created.  I am aware that should I put in storage, then the feed in would need to be metered in its own right so that the energy company pays for the exact amount of energy Exported to The Grid. 

The plan

To keep the solar array as is, although due to building work we may be needing to move some of the panels on the roof. This could allow for 3 east facing, 10 south facing and 3 west facing. 
All of them have the enecsys fitted and then 240v runs from there to the meter. There are multiple AC isolators (but no DC) on the way, including before the meter and immediately after the meter. This then links to the grid tie bar within the house which then connects to the fuse board/consumer unit. (For reference, the mains power runs in to the house, through a meter and then into the grid tie bar - see image 1). 

My plan is to take a feed after the solar generation meter, step it down to a lower DC voltage (eg, 48v) into a solar charge controller, and place a battery bank created form the shedload of 18650 cells I have. These packs will be housed approx. 3m away from the grid-tie bar and consumer unit through an external wall and in a separate external storage facility. As yet, I am unsure of the powerwall capacity or voltage. From there, I am going to run the battery bank output back into the grid-tie bar (see image 2). 

As the solar controller will have a  'dump load' when the powerwall is full, this will then also running back into the grid-tie bar via an inverter to step it back to 204v and a meter to satisfy the requirements for feed in 'Export' monitoring for FIT payment as only electricity we are not taking to storage will then run back to the grid. 
I'm not convinced this is the most efficient or cost effective way of doing this, however it's a starting point using the equipment I have in place as I'm not sure if altering the physical installation and generation system would constitute a change and require new certification, thus changing my contract and FIT agreement. 

The Questions 

I am hoping to get feedback and answers to the following during this build and hopefully get a few tips and best suggestions so as to document this slightly different build for others are as follows:-

- Should I keep the 16x micro inverters or change to one large grid tie system? If I change, how will I satisfy the whole FIT requirements for generation and export differentiation, and are the above suggestions for dump load sufficient, or do I just forfeit the Export payment?

- What size capacity packs should I build, and what nominal voltage should I make each bank of batteries (keeping in mind I will have to step up and down a number of times)?

- How should I step the voltage down from 240v (from the micro inverters after the generation meter) to xxVolts in order to run the solar charge controller?

- What solar charge controller and monitoring system should I use and why it is the maybe the best solution?

- How do I step the voltage back up form the 'dump load' in order to go though a second 240v meter?

This isn't going to be the fastest build in the world, but at least the solar installation is already in place and I can get working on the storage side of things from there.  I'm also hoping to link this thread to some videos of how it's going on my shiny new Youtube channel:

I look forward to some help and input as I go along.


Leave the existing system as it is now, and fit a Victron ESS (other systems are available) .... 

It's very simple.
So I've been looking at the victron system as mentioned above and will do some more reasearch into what appears to be the 2 different options.
But looking at solar side of things. My solar system seems to be working at reduced output because of the location of the panels and that some are now in shade. Because of this I will have to relocate some of them, but this will increase the distances from the meter and breakers.

Is it possible to mix different types of grid tie inverters together?
I.e., I continue to use 10 microiverters on the existing system that will remain in their current location.
For the other 6 panels (1.5kw) I am moving to better placement but want to gradually move off the microinverter system. Can I run those 6 into a single 2kw grid tie inverter with the same voltage and Hz output as the microinverters at the same time which then links to mains. Effectively giving one 1.5kw feed and 10x250w feed along the same mains lines to the feed in meter.
I'm presuming as the voltage is the same, it doesn't matter if the amps from each string is wildly different.
The ultimate goal is to have 1.5kw array running through one inverter at one location on the property, linking into the same mains cable as the 2.5kw array which would have its own single inverter staying on the roof.
I'll try and draw up what I mean for clarity.

Also as a side note, it's possible that a few of the panels are suffering degradation and possible damage from my builders. Because of this, it's possible only a few of the 6 that are moving might be functioning. I have access to a number of lower output (Watt) panels of around 150w each, so I might be increasing the relocated array to 10x 150w panels (hence wanting to move off the microinverters atthat locartion only).
It'll mean I keep my 4kw, and end up having spare microinverters for the roof array should they start to fail.
You can have as many as you want is the simple answer.just cross check what you are allowed to hook Up in terms of max kw and stuff like that IF you add more panels
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