What is the 4kW limit for FIT?

rogerblack

Is it the inverter?

I came to wondering, after noticing on a local PV installations instantaneous generation graph how little time it's close to its maximum.

 *    orig  new  ideal  %
  1 1911 1911 1911 100
1.2 1911 2285 2293 99
1.4 1911 2628 2675 98
1.6 1911 2932 3057 95
1.8 1911 3200 3440 93
  2 1911 3441 3822 90
  3 1911 4399 5733 76
  4 1911 5102 7644 66

This was a simple calculation based on computing the 10 minute interval instantaneous outputs of a local 4kW installation with larger panels (for about 5 months)

Columns are:
Excess installed capacity - so the '2' row is for 8kW solar with a 4kW inverter.
Original output in kWh over the period.
New output in kWh over the period.
Output if there was a properly sized inverter installed.
Efficiency (how much you get relative to a 'properly' sized system.

If it was legal to install more panels, then for this period, for this system, quite large increments in panel size seem to make sense.
For example, a 7kW set of panels with a 4kW inverter generate 90% the output of a 7kW inverter.

This assumes that the inverter only draws 4kW from the panels, and does not switch off - if the panels voltage is too high, it will switch off - the install would need to be properly designed.

Martyn1981

Roger, you've spotted a good one here. If you take a look at inverter prices such as SMA you'll see price to size falls rapidly.

The reasons for the 4kWp limits are various.

Most domestic houses would struggle to fit more panels than that (unless E and W).

The DNO's require prior approval of any system that will exceed 17A export, some state 3,680w. This I think is a coincidence, but after system losses and background consumption, that might be close to cold panel peaks from a 4kWp system. However most inverters can now be set to G83 stage 1 compliance, which means they'll limit output to stay within the 17A rule.

Although the 70A to 80A import supply can cope with higher output, there is the risk that if there aren't enough properties to mop up excess at peak, then the sub stations will need hardening to avoid damage as they can't run backwards.

Lastly, I think, not sure, the govt set the upto 4 and 4 to 10 rates to reflect falling costs due to economies of scale.

Does that help?

Oh, also, I struggle to follow the electrical engineering side of this, but I've read many comments similar to yours that oversizing the system, or undersizing the inverter, can lead to an increase in overall production, as the inverters are running at optimal levels for a greater percentage of the time, so effectively lower system losses.

But, as I say, just a curious onlooker, can't really grasp the technical stuff.

Mart.

jamesingram

Roger , yes the 4 kW is the max ac output of your inverter for the purpose of the FITs payment bands , and it's called the DNC, declared net capacity , which DECC define as the output of the system after plant losses .
Though there is debate as to the definitions, the other important figure is TIC, total installed capacity .
Many people ( I believe ofgem included) consider the total wattage of the panels to be the 4kW FITS band limit, but looking further into (see navitron forum) its the DNC thats the important figure.

It's suggested in the UK that we size the inverter at 80% of the total wattage of the panels.
the 16A output limit is a seperate issue with the DNO ( district network operator ) and not to do with the FITs payment bands , though as Martyn suggest the rate band was probably set at 4kW to enable simpler installs for the domestic market up to the 16A limit .
It concerns management of small scale generation input into the local grid and whether the local infastructure is up to the job. Below 16A ( 3.68kW DNC) connection is allowed without pre-informing the DNO , above 16A the DNO needs to be contacted prior to installation, so as they can check the network is up to the job. So keeping the DNC ( max ac output of inverter ) below 3.68kW make life simpler for all involved. No reason why you cant have say 4.2Kw of panels on the inverter ( as you mentioned as long as paramiters allow ) and yes it will maxims outputs and returns.

Cardew

So, who can give a definitive answer?

If it is the DNC, as opposed to TNC, and the fact that panel prices are dropping like a stone, a, say, 6kWp array with a 4kW inverter would make a lot of sense. It could give an annual kWh output some 30%?? higher over a year.

jamesingram

Inverter max voltages , DC inputs etc. limit the total array size relative to the inverter max ac output , so 6kW would be taking it a bit far, but yes , there are systems out there registered with the DNC < 4kW and array size > 4kW . ( edit. got my arrow round the wrong way! )
From discussion on the navitron forum ofgem apparently weren't keen to clarify , but if you look at the DECC definitions it seems TIC = DNC just to muddy the water more.
MCS online job registration only asks for DNC as do SSE on the owners FITs application form , though other FITs suppliers ask for TIC.
I think its currently down to what your installer thinks DNC means , so no doubt problems for the future.
Currently from info. from others and if I remember correctly , I think its about 99+% of system are registered TIC=DNC on the ofgem records (would need to check this ) so no one seems bothered if your installer takes the preferred definition,

Tell your friends !

Martyn1981

Cardew wrote: »

So, who can give a definitive answer?

If it is the DNC, as opposed to TNC, and the fact that panel prices are dropping like a stone, a, say, 6kWp array with a 4kW inverter would make a lot of sense. It could give an annual kWh output some 30%?? higher over a year.

I don't think anyone wants to risk going up against DECC & OFGEM and missing out on the higher tariff rate. Not only do they have to defend their DNC argument now, but against any possible reviews over the next 25 years. The deal is pretty sweet, so maybe best not to rock the boat too much.

In fact the common sense of a larger system that you suggest and the economies of scale, kind of lead to the reduced tariff for 4 -10kWp systems.

As James says there is a point where the inverter may fail or lose life expectancy. However, there is an interesting scenario. I believe the SMA 4000TL and Aurora 3600 MPPT's could cope with 3kWp on both strings if East and West configured. That way you'd get economies of scale regarding larger system, but only need 1 inverter.

However, this would only give about 25% more generation than a 4kWp south system, but hopefully the cost ratio would be similar. Plus longer and lower generation throughout the day (6am to 8.30pm summer, and 9am to 3.30pm winter) allowing for higher consumption.

Nobody will do this however till subsidies are gone, and even then, panels will have to be getting very, very cheap.

Mart.

jamesingram

As things stand it seems it's DECC and OFGEM that are going up against each other re. definition of DNC , though I not sure if they're aware of it !

jamesingram

From the DECC doc.
“Declared Net Capacity” - means the maximum capacity at which the installation can be operated for a sustained period without causing damage to it (assuming the source of power used by it to generate electricity was available to it without interruption) less the amount of electricity that is consumed by the plant.

“Total Installed Capacity”

means the maximum capacity at which an Eligible

Installation could be operated for a sustained period

without causing damage to it (assuming the Eligible Lowcarbon
Energy Source was available to it without
interruption), a declaration of which is submitted as part
of the processes of ROO-FIT Accreditation and MCScertified Registration;

Both can only refer to the output of the inverter

"the Eligible Lowcarbon Energy Source" in this case would be the PV panels

Martyn1981

James, not just them. You've also got to add in confusion with the DNO. Some staff will claim that any system over 3,680w needs approval, and may incur hardening costs.

The next member of staff to answer the phone will ask, is it G83/1 compliant, if yes, then why you bothering us with these silly questions!

Varies by member of staff, and DNO as they seem to have different ways of interpreting the 16A rule.

I mustn't be unfair however, this is a very young scheme, and I guess like everything, it takes time to settle in. But I've heard of several people having systems near 4kWp turned down flat by DNO's on the phone. Luckily I don't think any such decisions have stood once a written explanation for the decision is requested - perhaps bob 1 asks bob 2 for advice when challenged in writing?

Mart.

jamesingram

Mart , the DNO are only intersted in DNC , yes there is some debate as the 16A limit , local voltage could be considered rather than the "official" 230V ( 230 x 16= 3680 ) as you say under this G83/1, over this G83/2 or G59 application , but this is really an electrical engineering detail.
The unfortunate confusion is really over the seperate issue of what the 4kW FITS tariff band refers to.
If you take "total installed panel wattage, the Eligible Lowcarbon Energy Source", (which is a max. figure the system will never achieve in a month of sunny sundays) to be DNC or even TIC then installation are missing out on potential improved efficiency and returns, where space is available for additional panels .
As mentioned when you register a job on the MCS site as an installer they only ask for DNC , SSE on the FITs application only ask for DNC . So if that's what they ask for, then tell them, no need to mention you've got ,say 4.5kW of panels on your roof
cheers Jim

Martyn1981

James, I'm in total agreement regarding efficiency.

In fact a quick scan of this thread appears to show that every contributor has suggested ways of improving efficiency.

It's really the PR not PV side of this scheme that is causing problems. As FITs rates and install rates move apart, accusations of greed move in. Hopefully this will now be better managed by DECC with faster FITs reviews.

Clarification of DNC and TIC would be excellent, as from previous posts it's now obvious how personalisation of each system could bring genuine efficiency rewards. I know one shouldn't laugh too hard at their own jokes, and similarly shouldn't promote their own thoughts, but since thinking of that 6kWp E/W system, I can see clear cost benefits and efficiencies. But will the public go with it till subsidies are further reduced or ended. Interesting.

Mart.