Solar + battery project thoughts

Introduction

I want to go solar, if not for economical reasons, at least to try and preserve our environment. I now have the proper authorisations from the local authorities for a 20m² solar array (about 3kWp). I have been putting a lot of thinking in that project so I am going to share some here.

Standard system without batteries

First of all, this is a bit daft. Yes I know batteries are expensive, but how much my almost 4MWh (1300 x 3kWp) system will I actually use without storage? Also, if I get some cheap 2.88kWh batteries from eBay at 923€, then I am not so bad off.

Simple inverter or MPPT charger with AC-coupled batteries

AC-coupled batteries system are not cheap  Whether you go for Tesla Powerwall 2 (7000€+), a Sunny Boy Storage (around 1100€) with a high voltage (> 100V) battery or a Sunny Island 3.0M (starts at 2000€) inverter with 48V batteries. The cheapest seems to use a Victron Multiplus charger/inverter (around 1500€ + 330€ for the power meter), but still: buying a storage-ready “hybrid” inverter always comes cheaper (not taking into account the product life time).

Grid-tie hybrid inverter

There are quite a few grid-tie hybrid inverters these days. The cheapest I found here in France is the SolaX X-Hybrid SK-SU3000, which suits my needs at 1750€ and supports 48V batteries. This will of course not provide much backup in case of power failure: only 2kVA via the specific EPS output, but that’s not bad, especially as it uses both battery and solar. A good point is that its MPPT solar charger accepts up to 530V, so only one string of solar panels is needed in my case which means a maximum current of 9A (under 360V) and thus 4mm² cables.

Off-grid hybrid inverter

Why go off-grid when you are in France with a reliable grid? So you can have big backup and grid independence. There is cheap inverter out there that can do just that. Call it Voltronic Power Axpert MKS 5kVA, WKS, MPP Solar PIP-MS, or some other brand (I found it under like 5 or 6 brands, usually in grey but also in red or green), this inverter is cheap (less than 900€) and can deliver 4kVA directly to your home (or more if you buy several and put them in parallel), switching back to grid when necessary (more power needed for example!).

Note that this is not a grid-tie inverter: it either inverts the DC from the solar array and/or batteries or connects the grid directly to the loads. It can charge the batteries from the grid though, if that is required.

One issue I have with it is that, unlike the SolaX, its solar MPPT input is only 115V, so the solar panels need to be arranged in 4 strings of 3 panels, and the current will be high (up to 36A under 90V) so the cables needed will be quite big (minimum 10mm²). I saw some guys on Youtube add an external MidNite Classic MPPT along it but they did not say why.

I should add that, as this inverter is off-grid and not grid-tie, you don’t need any authorisation from your local grid provider, as you are not injecting any current into their system. On the other hand, it really needs to be installed very close to your grid supply…

Edit: there seems to be a new “Heavy Duty” (WKS 5kVA 48V HD, 990€) version of this which cannot be put in parallel, but supports a panel array voltage of 120 to 450V. On top of this, it claims to run the loads from a mix of solar/battery and grid, which would be great, but does it do without injecting? Needs more research…

Conclusion

It’s hard to draw a clear conclusion as there is no clear winner. I also have some issues with space as these inverters are big and require proper ventilation, so having the choice of where to put the inverter is a plus. I shall write again when I have chosen my solution…

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