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Micro-inverters for an established 240v grid-tie back to powerwall
#1
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:
https://www.youtube.com/channel/UCp03O1f...B39ZdDAYYA

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

Cheers

JCUK
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Micro-inverters for an established 240v grid-tie back to powerwall - by JCUK - 05-17-2019, 02:01 PM

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