4 or 4.5 kw system?

Wh05apk

Just looking at installing a system, we have the space for an extra couple of panels, and wondered if they would be worth installing, I appreciate the output is capped at 3.68kwp, but on less than perfect days - 90% of the time? presumably I would be generating an extra 12.5%? it would seem the panels are relatively cheap in the overall cost of things?

Would this be worthwhile?

EricMears

Wh05apk wrote: »

I appreciate the output is capped at 3.68kwp

The 'cap' doesn't stop you generating more - you just need to consult your local distribution network operator to ensure that local network has capacity to absorb what you propose to generate. And you'd need to do that for a 4.00 system (or even a 3.69 one) so your 'extra' 500Wp isn't a limiting factor from that point of view. I didn't have to pay for such an application (and I don't think my contractor did and was absorbing cost) and permission was granted without undue delay.

The main reason most existing SP owners stopped at 4.00kWp is that FIT rates for systems >4.00 were considerably less than for the smaller ones. I believe the latest FIT structure has reduced that 'step' considerably.

Wh05apk

From what I can see, going onto the next tariff for the sake of a potential 500w, would wipe out any benefit? So presumably best to keep it capped under 4kwp? but using the extra panels to maximise production on less than perfect days?

Martyn1981

Wh05apk wrote: »

From what I can see, going onto the next tariff for the sake of a potential 500w, would wipe out any benefit? So presumably best to keep it capped under 4kwp? but using the extra panels to maximise production on less than perfect days?

There are two different issues here:

1. If you want to install a system that is capable of exporting more than 3.68kW, then you'll need permission from your DNO.

2. If you install more than 4,000Wp of panels, then you will get the the 4-10kWp FIT rate, regardless of the inverter, capping etc.

Mart.

Edit: Sorry, to answer your question, 4.5kWp would result in more capping, but as you have guessed this would be small. So the small(ish) extra cost of the additional 500W would be worthwhile ..... but ......

dropping from the 4kWp FIT to the 4-10kWp rate might remove all financial gains. M.

EricMears

Wh05apk wrote: »

From what I can see, going onto the next tariff for the sake of a potential 500w, would wipe out any benefit? So presumably best to keep it capped under 4kwp? but using the extra panels to maximise production on less than perfect days?

Pretty sure (but I may of course be wrong) that you have to register the maximum possible output of your system. An 'extra' couple of panels that you could add on dull days would have to be declared as part of the system or else you'd be considered to be making an unauthorised modification every time you switched the extra two on.

I'm not too sure of the latest tarriff implications but, under the rules that existed when I investigated, I thought I'd have to get up to at least 6kWp to earn more money than the 4.00kWp system on which I decided.

EricMears

Martyn1981 wrote: »

Mart.

Snap !

We could of course both be wrong but you'd need to check very carefully before proceeding on that basis.

Martyn1981

EricMears wrote: »

Pretty sure (but I may of course be wrong) that you have to register the maximum possible output of your system.

Yep, OFGEM base the FIT rates on the whole kWp of the system panels, at their rated outputs (even if they have a zero to + performance guarantee).

Technically the definition of DNC, declared net capacity, is the amount that a system can run at continuously, so that would be around 90% or less of kWp after inverter losses, panels heating up etc.

Some in the PV industry tried to argue this point, since OFGEM don't seem to be following the legal definition, but dropped out when no progress was made and the next step was to go to court, as the costs would have been high, and the difference between the two tariff rates narrowed. Also, as FITs comes down, but leccy prices and export rates go up, the difference narrows even further.

Mart.

spgsc531

Would an option be to install 4kWp now, and increase it 'in the future'?

You would have to investigate the 'time frame' to maximise advantage, if there is one.

Just a thought.

OffGridLiving

Sorry, did I understand correctly that the FIT payments fall across the board if your panels have a greater capacity than 4kw (i.e. they don't pay the full FITs for the first 4kw and then drop down the next rate for any capacity above 4kw)?

That seems to be a disincentive for people to maximise their solar generation and totally against the whole point of moving towards greater renewable energy production?

<Edit> it seems that it does reduce the whole tariff: http://www.yougen.co.uk/blog-entry/2106/Don'27t+limit+your+solar+PV+system+to+4kWp/

Martyn1981

OffGridLiving wrote: »

Sorry, did I understand correctly that the FIT payments fall across the board if your panels have a greater capacity than 4kw (i.e. they don't pay the full FITs for the first 4kw and then drop down the next rate for any capacity above 4kw)?

That seems to be a disincentive for people to maximise their solar generation and totally against the whole point of moving towards greater renewable energy production?

<Edit> it seems that it does reduce the whole tariff: http://www.yougen.co.uk/blog-entry/2106/Don'27t+limit+your+solar+PV+system+to+4kWp/

You have understood it correctly, it doesn't reduce steadily, it goes in steps. The idea being that it reflects the differing costs of install. There used to be a slightly lower rate for new builds, as compared to retro-fit.

In fairness the 4kWp step does 'kind of' match the DNO 3.68kW limit, but as people are discovering, it's not that simple, and in the UK, going a bit over might be beneficial.

The difference is still 10%, but in money terms that's only 1.5p now, and obviously export and leccy savings aren't affected, so it shouldn't really be too big of a concern anymore. But can't blame anyone for wanting to avoid a 4.1kWp install.

My 3.6 install got the upto 4kWp rate, but my 2kWp extension a year later was at the 4 to 10kWp rate (all of it, not just the last 1.6kWp).

The WNW is not a good example, since it was at the lower, of a lower rate, and despite outperforming expectations, will take about 20+ years to pay back. But will hopefully generate about 45MWh's of clean leccy in its lifetime.

Mart.

Kernel_Sanders

Martyn1981 wrote: »

Technically the definition of DNC, declared net capacity, is the amount that a system can run at continuously, so that would be around 90% or less of kWp after inverter losses, panels heating up etc.

In June and July, the 4kW inverter on my 3.92kWp sytem shows that it is quite capable of producing in excess of 4000W continuously (as opposed to continually). I've always assumed (until now ) that this display is after inverter losses and not before.