Solar PV inverter ratio - What happens when I add more AC capacity (DC/AC < 1)?

eldaniel

I am going through process of finilising my quote. Panels will be in two separate orientation 5x385W (1.9kW) facing South and 7x385W (2.7 kW) facing south east. We have decided to go with two separate strings therefore I need two inverters. I have been quoted for 2.2kW and 4kW to go with those panels. Company offering this to me so far have been brilliant and very understanding despite me requesting amendment to the quote few times and at no point I felt pressured to buy. With this in mind I kind of trust their judgement even more, but in this respect I am reading that ideal DC/AC ratio is around 1.1-1.25. My one will be well below 1.0. First of all would it matter please? Would the only outcome of that be potentially paying for too powerful inverter? I have read few articles warning not to go over 1.25 etc but barely anything mentioning what happens at ratio below 1.0. Would that influence energy production negatively in any way? I have asked the sales rep and he said changing for a smaller inverter would save me £50 or so - in his opinion not worth it. Would appreciate feedback. Many thanks 

gefnew

One inverter with dual mppt  capalbility

QrizB

There is no benefit from an oversized inverter unless you have plans to increase the size of the array (the number of panels) in the short-to-medium term. The downsides are that the larger inverter may have a higher start-up threshold, larger internal losses and (if you go too large) might not MPPT power track optimally at low outputs.

As gefnew says, a single inverter with dual-MPPT inputs would seem worth considering unless your two strings are so dissimilar that this isn't going to work.

I'd probably be looking for a 1.5-2kW inverter for the smaller string and a 2.5-3kW one for the larger string, or a 4kW dual-MPPT.

Solarchaser

I'd agree 4kw dual string inverter seems like what you need.

Spies

As others have said, one inverter, dual MPPT.

eldaniel

That is strange because that is exactly what I suggested to them after reading up on it and he said that I still need two inverters for some reason... 
I know I will not have any benefit of having more powerful inverter but can't find definitive answer anywhere about this cons that QrizB mentioned. Is it because the loses will be minimal?
Thanks 

gefnew

A bit of a long read but maybe helpful.
Calculating Solar PV String Size - A Step-By-Step Guide - SolarDesignGuide

Martyn1981

eldaniel said:

That is strange because that is exactly what I suggested to them after reading up on it and he said that I still need two inverters for some reason... 
I know I will not have any benefit of having more powerful inverter but can't find definitive answer anywhere about this cons that QrizB mentioned. Is it because the loses will be minimal?
Thanks 

Yep, the losses will be minimal. Once the panels start to get hot they'll lose 10% efficiency, and sitting in the sun, with warm air and little wind, say in June, will lose 20%.

Ironically, your system of 4.6kWp x 80% coincidentally gives a very interesting number 3.68kW. That's interesting because you can add a PV system with a 3.68kW capped inverter without prior permission from the DNO (local leccy company - District Network Operator). The installer just notifies them after the install, but for 4.6kWp and 6.2kW inverter (total), prior approval will be needed.

In the UK, and countries with similar weather, PV doesn't sit at max all that often, but spends a lot of time at lower generation, so maximising some of that lower generation with a proportionately smaller inverter makes sense. A rough rule of 10%-20% is typically used, but the specific conditions need to be considered. For instance a south facing system in Cornwall, with lots of cooling sea winds, you'd maybe go -5%, but an off south system in the North, might warrant -20%.  But for say Spain, where max gen is important, and low gen less so, you wouldn't want to undersize an inverter which might put strain on it (over time).

Also the fact that you have two orientations, means that the system will never 'max' out at 4.6kW, since the panels won't all be perfectly aligned at the same time, so that allows for a smaller inverter too. My 5.58kWp systems can spike to about 4.9kW in the right conditions for seconds, but can only hold 4.2kW sustained. However my panels are 180d apart, yours would be impacted less with a 90d difference.

Would I put 4.6kWp on a 3.68kW inverter ...... maybe, maybe not with me leaning towards a dual MPPT 4kW inverter. But if the DNO said no to going bigger, then it would be a totally acceptable solution.

Would I put 4.6kWp "split orientation" on a 3.68kW inverter by choice..... hmm .... probably but 4kW would also be fine.


Edit - Sorry, should have said, the solution offered to you of two inverters, both oversized is not just strange, but really bad. I'd be quite concerned about those installers, or at least the member of staff who came up with that. Perhaps they have those inverters in stock (returned etc) and just want to shift them?

Spies

My 5 x 395w south facing and 6x 395w west facing system does see 3.6kw briefly when the sun pops out from behind cloud, so at least in my case, my inverter is correctly sized.

ispookie666

Maybe consider enphase micro inverters 

MikeHEQA

My 5.2 kW array split two strings on roofs East and South West orientation using large double garage (so far Generated over 1 Mega Watts since middle September 2021 install, so not seen first Summer of Generation) 2 Strings x   7 panels  x 380w = 2660w JA 380 panels, with 3.6kw Solax X1 Boost dual MPPT (Solar only not the Hybrid version Solar and Battery) is split on power when the Sun moves from East to West constant energy generation throughout the entire day. 

I intend to favour AC coupling for my Battery system in the future... Solax AC 3.6 inverter for that project.  Batteries could be 3000C Pylontech or the Solax 5.8 version better DOD