Mismatched strings - south/west facing roof.

[mattg4321]

Hi all, for background I'm in the UK and looking to add a grid tied system to my house. I have only fairly limited roofspace I can use for PV panels.

The main problem I have is that I can only fit 3x relatively small panels to my south facing roof. The only panels I can find that will fit are the Perlight Delta 295w. In contrast I can and plan to fit 10x Jinko Tiger Neo 430w panels to my west facing roof.

Clearly this gives me a problem with regard to inverter choice and sizing. The 3 panel string won't generate enough voltage to switch on the MPPT on every dual MPPT inverter I've looked at that's also big enough to run the 10x panels.

To complicate things, I'm thinking of a Hybrid inverter so I can add (Pylontech?) batteries in the relatively near future. I'm looking at the Sunsynk ECCO 5kW inverter.

I've been reading on here that I could wire 2x strings in parallel and that might get me over my problem. However, I think I might be misunderstanding the other threads on here. If I have 2x strings of roughly 90v, connected in parallel to 1x MPPT, then surely my voltage at the MPPT will still only be the higher of the 2 strings and still not enough to start up the MPPT (needs 125v)?

Anyone got any clever ideas or do I just need to accept I need 2 inverters?

If I went with 2x inverters, 1x hybrid and 1x pv only, presumably the batteries would only charge from the generation from the string connected to the hybrid inverter and not the string connected to the other inverter?

Would appreciate some expert guidance as it seems an unusual problem and starting to make my head hurt trying to find the correct solution!

Thanks

 

[wattmatters]

Anyone got any clever ideas or do I just need to accept I need 2 inverters?

Micro inverters are a good solution for difficult roof facet combinations.

I've been reading on here that I could wire 2x strings in parallel and that might get me over my problem. However, I think I might be misunderstanding the other threads on here.

Strings in parallel must be of the same voltage (you will likely have code requiring that for a grid tied system, and it'll also be an inverter manufacturer's requirement) and often specified to be the same number of the same type of panel. IOW this won't solve your problem.

 

[robby]

Yes I agree, you need micro inverters.

 

[mattg4321]

Thanks guys, that confirms my thinking.

Probably I’ll just go with a second inverter as a small 0.7kw costs peanuts and micro inverters will otherwise not add much value as neither roof suffers from shading issues.

Second inverter being best bang for buck

 

[MisterB1959]

Thanks guys, that confirms my thinking.

Probably I’ll just go with a second inverter as a small 0.7kw costs peanuts and micro inverters will otherwise not add much value as neither roof suffers from shading issues.

Second inverter being best bang for buck

have you considered the Fogstar battery (15.5kw) they have just introduced to the UK market. it will work with a sunsynk inverter and be a damn sight cheaper ..... if i hadnt already bought two Seplos 280L (14.3kw) from Fogstar i would def have gone for their own battery as its self built and ready to go, with a few benefits more than the Seplos!

Fogstar Energy 15.5Kwh 48V Battery

A high-performance 15.5kWh battery designed for UK conditions, featuring intelligent heating, plug-and-play inverter compatibility, an intuitive LCD touchscreen interface, premium Grade A EVE cells, scalable energy storage,

www.fogstar.co.uk

 

[wattmatters]

Probably I’ll just go with a second inverter as a small 0.7kw costs peanuts and micro inverters will otherwise not add much value as neither roof suffers from shading issues.

Shade tolerance is the least of reasons to use microinverters. What GT inverter is going to work with such a low voltage string?

 

[mattg4321]

Shade tolerance is the least of reasons to use microinverters. What GT inverter is going to work with such a low voltage string?

Solis do a 0.7kW one which isn’t expensive

Solis S6-GR1P0.7K-M-DC Inverter 0.7kW

<p>Since 2005, Ginlong Solis have been at the forefront of developments in the solar inverter industry. The company are internationally renowned for their reliable, innovative and cost-effective energy management solutions: tailoring their inverters to the demands of regional markets. Ginlong...

www.tradesparky.com

I’m not sold on microinverters I must admit. In my ignorance it seems I’d be paying a 4 figure sum on top of just going for strings, for very little benefit in my situation

 

[DIYrich]

Put the small string on an mppt charger. Power goes to battery. Works on or off grid.

You can parallel different inverters when on grid. grid down, you need to make sure they frequency shift. Easier if just one inverter has batteries.

Each string needs to meet min voltage of mppt. Otherwise that string may be ignored, even if other strings are above minimum.

Might be able to wire in series with one panel on another roof to boost the string's voltage. Not optimal, but may be cheapest fix.

 

[mattg4321]

have you considered the Fogstar battery (15.5kw) they have just introduced to the UK market. it will work with a sunsynk inverter and be a damn sight cheaper ..... if i hadnt already bought two Seplos 280L (14.3kw) from Fogstar i would def have gone for their own battery as its self built and ready to go, with a few benefits more than the Seplos!

Fogstar Energy 15.5Kwh 48V Battery

A high-performance 15.5kWh battery designed for UK conditions, featuring intelligent heating, plug-and-play inverter compatibility, an intuitive LCD touchscreen interface, premium Grade A EVE cells, scalable energy storage,

www.fogstar.co.uk

Thanks, cheaper than the Pylontech batteries do you mean?

Batteries will probably be a year or two away for me unfortunately

 

[mattg4321]

Put the small string on an mppt charger. Power goes to battery.

That’s worth thinking about, thanks. Do you have a charger recommendation?

I’m not sure that 2x chargers could power the same batteries though? One from the small string and one from the hybrid inverter on the large string?

 

[DIYrich]

That’s worth thinking about, thanks. Do you have a charger recommendation?

I’m not sure that 2x chargers could power the same batteries though? One from the small string and one from the hybrid inverter on the large string?

SmartSolar MPPT 75/10, 75/15, 100/15 & 100/20 - Victron Energy

Maximize the energy-harvest, driving it intelligently to achieve full charge in the shortest possible time using the SmartSolar MPPT. Learn more now.

www.victronenergy.com

OK to put multiple chargers on one battery as long as you don't exceed the max charge rate of the battery.

 

[zanydroid]

HM-1500 microinverter costs the same as that string inverter, if I apply a straight currency conversion from USD.

Actually HMS-800 might be able to handle those ancient/tiny 295W panels wired in parallel. HMS has higher input current than HM- series.

If you need monitoring or export limiter the price will likely 3x though with that ecosystem because you need two extra boxes of $300 each

 

[MisterB1959]

Thanks, cheaper than the Pylontech batteries do you mean?

Batteries will probably be a year or two away for me unfortunately

Yes a lot cheaper £2500 for 15.5kw ....

 

[SeaGal]

If I went with 2x inverters, 1x hybrid and 1x pv only, presumably the batteries would only charge from the generation from the string connected to the hybrid inverter and not the string connected to the other inverter?

The UK-approved Solis Hybrid inverters (like S5-EH1P) will charge batteries from a secondary microgenerator (in my case, a second Solis). So, if the PV only inverter (whatever make) is producing more power than the house load, then the surplus will be used by the hybrid Solis to charge its batteries, simultaneously with it's own PV production. Not sure if this is common to most hybrids or whether the Sunsynk supports that mode of operation, but it works with the Solis.

Solis do a 0.7kW one which isn’t expensive

I’m not sure that 2x chargers could power the same batteries though?

We had a separate thread on that a month or two ago. I'd be wary of just connecting two inverters, particularly two different makes, to the same battery connection unless you know that the battery connection is truly floating. My Solis hybrids have a voltage of nearly -200V with respect to ground on their battery negative lead!

Even it it were possible, there would be issues, IMHO, with battery to inverter comms. Without a MITM controller the battery would instruct both inverters to charge or discharge at a given current, which would then effectively be doubled if you have two inverters trying to achieve that charge or discharge rate. Far simpler to use AC coupling.

In your situation, I'd prefer to use a small string inverter, like the small Solis you mentioned, for your 3 panel array, rather than micro-inverters.

 

[SeaGal]

if i hadnt already bought two Seplos 280L (14.3kw) from Fogstar i would def have gone for their own battery as its self built and ready to go, with a few benefits more than the Seplos!

oy!... this is a DIY forum

But, seriously, yes better choice than Pylontech, IMO.

 

[robby]

Micro inverters are a good solution for difficult roof facet combinations.


Strings in parallel must be of the same voltage (you will likely have code requiring that for a grid tied system, and it'll also be an inverter manufacturer's requirement) and often specified to be the same number of the same type of panel. IOW this won't solve your problem.

Well we tried.
If there was ever a clear cut usage case for micro Inverters this is it.
I am not even a fan of Micro Inverters but I know when strings are not the solution and you just have to go with Micro's.

 

[wattmatters]

Well we tried.
If there was ever a clear cut usage case for micro Inverters this is it.

I guess a 700 W string inverter is near enough to a microinverter.

 

[zanydroid]

I guess a 700 W string inverter is near enough to a microinverter.

At this point you can buy a 2000W (not just VA) microinverter with 4 inputs

So who is the micro now? lol.

 

[DIYrich]

Fyi: in the UK I belive there is a 3.6kw magic number where you don't need permission . If you get an inverter that can limit export to 3.6kw per phase, and store the rest of the power in batteries, you might be able to avoid one level of review.

 

[wattmatters]

Fyi: in the UK I belive there is a 3.6kw magic number where you don't need permission . If you get an inverter that can limit export to 3.6kw per phase, and store the rest of the power in batteries, you might be able to avoid one level of review.

That doesn't resolve the small south string issue.