Confused about 3.6KW limit

[PengEF]

Hi all, great forum been looking for something on Solar Pv for days now. Been reading here and saw mentioned a couple of times that inverters are capped at 3.6KW feed in?

My question is if I have been quoted for a 4KW (a touch under 4KW actually) system but wouldn't the difference be wasted and I would be better getting a panel less to take it closer to 3.6KW or am I misunderstanding things (highly likely)?

Also I have been quoted for a couple of different panels one of which is a sunpower one wish seems to have quite a bit higher wattage than others and therefore needs less panels but costs a fair bit more to the tune of £2,000 and wondering if they are worth it or would any reasonable 4KW system give the same output regardless? Can find very little info on sunpower panels and there are precious few on pvoutput as well.

Thanks for any help you can give me on this.

PS. any of you Solar PV guys on here in the North West and fit in Liverpool?

 

[Earthstore]

Most 4kw systems will get capped at 3.68kw, this is good practice for two reasons, it makes the install easy because the DNO(grid owners) rules are more straight forward, also with cable loses it is very rare for a 4kw system to peak at 4kw.

 

[whinmoor]

The difference would potentially be "wasted" on a cool, bright, sunny, day at midday if you have a perfectly south-facing roof and no shading. Such days only occur rarely. More often we have cloudy weather or even in the last few days, 4kWp systems have been underperforming due to the loss in efficiency as panels get hot. There are also transmission losses in the cables depending on the distances involved.

We often quote for a 4kWp system with an inverter limited to 3.6. Smaller inverters are a bit cheaper and more efficient with less than optimum generation (i.e. 95% of the time). In addition, your installer should apply in advance for permission to fit a larger inverter whereas retrospective permission for smaller system is allowed.

There certainly a few guys on here from your region who may be interested in assisting. I'm sure they'll be along in the next day or two.

 

[FB.]

Most of the time, solar arrays run below their maximum potential - typically because the panels don't face perfectly South, or have a strange roof pitch, of when the sun isn't out, or the weather is hot, or hazy, or the sun is not shining perfectly straight at the panels (maybe off to the side, or a bit above or below).

In the recent sunny days, most 3.5-4kW arrays have only been delivering about 3-3.25kW of power to the inverter due to the panels losing efficiency when the temperature rises.

Whether the 4kW is 20 x 200W panels or 16 x 250W panels doesn't matter. But what might matter a bit is such things as the temperature coefficient or the quality of construction.
It is normal for Sanyo/Panasonic panels to outperform others by 10-15%. However, the performance needs to be balanced against the cost and the presumed reliability/quality of the components.

 

[babba]

FB do you believe this ?

It is normal for Sanyo/Panasonic panels to outperform others by 10-15%. However, the performance needs to be balanced against the cost and the presumed reliability/quality of the components.

As the jury is out with me

 

[FB.]

FB do you believe this ?

It is normal for Sanyo/Panasonic panels to outperform others by 10-15%. However, the performance needs to be balanced against the cost and the presumed reliability/quality of the components.

As the jury is out with me

In this area, the Sanyo systems seem to outperform the others by 10-15% and others around the country seem to have noticed something similar.
But now Sanyo systems cost 50-75% more than most others, I can't see much point in spending a lot more money for only a little extra performance.

When FiT rates were more generous, and when other companies also charged high prices, Sanyo were worth considering. But with Sanyo prices remaining sky-high while others have dropped, the Sanyo's now look like an expensive luxury. It's very difficult to make the sums add up for Sanyo with soon-to-be-16p FiT, and marginal for the current 21p FiT.

However, my earlier posting was to draw a that not all 250W panels are likely to be equal in performance, reliability, or the company surviving long enough to honour any warranty.

 

[babba]

I dont believe they are all they are cracked out to be. I would need to see a 4KW sanyo system throw out 4600 to 5000 KWH in a year to be a believer as most decient set ups, perfect south, south east England with a decient roof pitch will give 4000 to 4400KWH plus 15% for the sanyo.
But Im always open to new facts.

 

[Gavin A]

FB do you believe this ?

It is normal for Sanyo/Panasonic panels to outperform others by 10-15%. However, the performance needs to be balanced against the cost and the presumed reliability/quality of the components.

As the jury is out with me

more like 5% IMO.

 

[PaulyCow]

My question is if I have been quoted for a 4KW (a touch under 4KW actually) system but wouldn't the difference be wasted and I would be better getting a panel less to take it closer to 3.6KW or am I misunderstanding things (highly likely)?

Hi PengEF,

This is a (good) question that comes up time and again. There are 2 main unrelated aspects to the first part of your question:
1) Ease of Connection to Distribution Network
2) Inverter Part load Efficiency

The 3.68kW cap relates to the 16A 'limit' that stems from G83 (16A*230V=3680W).
For AC output up to and including 16A (per phase) at nominal voltage, under G83 you do not normally need to seek permission from DNO in advance of installation. You can install your system and post-notify the relevant DNO as to what you have done. For >16A, or if you are doing multiple systems in close proximity, then you would need to request permission to connect before you install. This can take anything from a few days to 3-4 weeks to get a response. Frequently if the system is a little above 16A, or if it is the only system in an area with good grid, then the DNO will allow the connection under G83 rules anyway (we and others have done several single phase >10kW under G83). BUT, there is a chance that the DNO will not permit under G83 and require you to install under the more onerous G59, which aside from the time delay, has a whole load of cost implications attached.
As far as I am aware, there is no other technical reason for the cap.

As to coupling a 4kW PV system to a 3.68 capped inverter, it used to be very common practice to undersize inverters by 10 to 20% in the UK, because inverters have poor efficiency at very low part-load (check out e.g. SMA load Vs efficiency curve). We get a lot of overcast days in UK, plus every day the PV has large periods (early morning, late evening) when it is only producing a fraction of its rated potential due to the sub-optimal solar angle.
Unless you are off-grid, you are probably most interested in the energy the system produces (kWh) rather than the maximum peak output of the system, as the FIT pays you for kWh not W. Provided your inverter can cope with the maximum output - and especially the max voltage - of the oversized array (designers job), your 4kW of panels will certainly result in more kWh per year than say 3.68kW of panels. Yes, capping your inverter at 3.68kW will mean you lose some of the 4kW panels potential in peak sunshine, but for the remainder of the time the bigger array means the inverter is likely to be operating closer to its peak power rating than with a smaller PV array, so the yield is optimised.

Modern inverters do operate better at part load than back in the day when we routinely undersized by 20% so there is not so much to choose now in terms of overall annual yield (total kWh) between say 4kW array + 4kW inverter and 4kW array and 3.6kW inverter, but the smaller inverter will generally be cheaper so your return / payback is likely to be marginally better. And you don't have any uncertainty regarding G83.

Hope this is helpful.
Cheers

 

[PengEF]

Thanks all. Thought I had a hold of the wrong end of the stick. Least I know what and why now. great forum learning a lot about this although not sure this isn't a 2 edged sword as the more I learn the harder it seems to be to make a decision...