Inverter Sizing

Ed050458

As the original poster, I appreciate all of this prompt and expert input. As the system is installed and paid for, I think the inverter is unlikely to be swapped out by the installer for nothing if it's within the limits of normal practice.

A few moments ago I noted that the inverter was showing 3.2kW which is the Ever Solar TL 3200's top whack I assume and presumably is it's input not output which as I say above is 3.08kW Max AC power (2.8kW Rated Power). The next model up TL 4000 is 4.00kW Max AC power (3.6 kW Rated Power).

On some list prices I saw the next model was £144 more.

EricMears

Ed050458 wrote: »

As the original poster, I appreciate all of this prompt and expert input. As the system is installed and paid for, I think the inverter is unlikely to be swapped out by the installer for nothing if it's within the limits of normal practice.

A few moments ago I noted that the inverter was showing 3.2kW which is the Ever Solar TL 3200's top whack I assume and presumably is it's input not output which as I say above is 3.08kW Max AC power (2.8kW Rated Power). The next model up TL 4000 is 4.00kW Max AC power (3.6 kW Rated Power).

On some list prices I saw the next model was £144 more.

Could be even worse than that ! If installer has supplied what he quoted for he'd not be at all keen on part-exchanging it for a better one. You shouldn't therefore be looking at 'list price difference' but maybe at (full cost of new inverter) - (secondhand value of original one).

And of course the increased output probably wouldn't justify such a charge.

But worth bearing the point in mind if the inverter does ever fail - which some prophets of doom claim will definitely happen in ten years time although optimists might hope for much longer before some minor part replacement is needed.

zeupater

EricMears wrote: »

Could be even worse than that ! If installer has supplied what he quoted for he'd not be at all keen on part-exchanging it for a better one. You shouldn't therefore be looking at 'list price difference' but maybe at (full cost of new inverter) - (secondhand value of original one).

And of course the increased output probably wouldn't justify such a charge.

But worth bearing the point in mind if the inverter does ever fail - which some prophets of doom claim will definitely happen in ten years time although optimists might hope for much longer before some minor part replacement is needed.

Hi Eric

I agree, for somewhere around ~£10/ year (estimated ~2% generation loss) at current FiT rates it wouldn't be worth the investment/hassle.

Z

Martyn1981

Not sure if this is just boring waffle, but some info and facts I've found that seemed interesting to me.

I chatted (via e-mail) with SMA in early 2012, about inverter sizing and one response troubled me, in which they said that they don't recommend under-sizing. I asked 'not even by 1W, that doesn't seem to match Sunny Design?'.

They responded that installs should be within the Sunny Design parameters. I think something had got lost in translation! So they clarified that under-sizing (and presumably over-sizing) meant outside of the parameters.

So the parameters (copied directly from Sunny Design (SD)) are:

"The nominal power ratio (max. DC power of the inverter divided by the peak power) is within the recommended range (83% - 120%)."

On SD I've slapped in an example of 16 250W panels and a SB 3600TL-21 inverter. It has a max DC of 3.88kW and a max AC of 3.68kW. SD rates the combo as having a nominal power ratio of 97% (3.88kW/4kWp).

If (and I'm only saying if) Eversolar apply a similar ratio, then in Ed's case (lifting data from their datasheet)

http://www.pvpro.co.uk/download/G83_inverter/EverSolar-3200TL-4000TL-4600TL-5400TL.pdf

we get max DC rating of 3.2kW / 3.5kWp = 91.4%. So nearer the bottom end, but still well within the parameters (albeit, another companies parameters).

Does this help Ed ...... no ...... sorry! But I thought it worth reporting out of interest.

Bit more info, decided to experiment further, as I had chosen Swansea UK as the location (don't ask!). Tried again, this time choosing Madrid Spain, and the recommended range then became 90% - 120%. Helsinki matched UK, but Stockholm was a tad wider with 82% - 120%. So clearly location/solar levels are important to the design spec, as caveated by Energetic some time back & Roger today.

Mart.

Kernel_Sanders

rogerblack wrote: »

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.

I see no reason why panel capacity should be limited as it is the inverter which curtails the volume of PV electricity converging on the sub-station. However, there is another practical snag to adding, say, an extra 3kWp of panels to an existing 4kWp system. Unless the existing array is on a huge roof, the extra panels would be on a different slope, and therefore on a separate string. My installer said my 4kWp inverter only had two strings, and that each one was limited to something like 12 or 13 amps.

Ed050458

I appreciate all this stuff guys.

In the brief justification of their reasoning for allowing a smaller inverter, they included a screen shot that gave me insight into the way they had been designing their systems. It's a site called

// everplan.zeversolar.com (I cant include full links)

With a simple registration you can apply criteria across many panel and inverter manufacturers and get a nice looking report out.

It helpfully gives a warning as you progress.

Eg for the TL3200: (Rated power ratio 113.7%)
Warning
Open-Current Voltage of the PV-modules in the string is higher than Max. DC Input Voltage which may leads to system damage.
Solution
Decrease the quantity of PV-modules in the string.

and for the TL4000: (Rated power ratio 86.7%)
Warning
The capacity of solar modules is less than 90% of the capacity of inverter,which may leads to power loss."
Solution:
1. There is no need to change it if the rated power ratio has been set artificially based on experience and demonstrate good capacity of power generation.
2. Increase the quantity of PV-modules.

Based on this advice, neither inverter is ideal for a 3.5kW system but at least I now have something tangible to use that they will understand.

Hope the site above is news to you and helps you in good work elsewhere.

Ed

Martyn1981

Ed050458 wrote: »

Eg for the TL3200: (Rated power ratio 113.7%)
Warning
Open-Current Voltage of the PV-modules in the string is higher than Max. DC Input Voltage which may leads to system damage.
Solution
Decrease the quantity of PV-modules in the string.

Ed

Hiya Ed, you've got a problem!

You are absolutely right to act on this asap. Different inverters and different panels have varying voltages, and therefore need to be matched correctly.

For instance I have 3 different types of panels with approx open-current voltages of Sharp 235 37V, Sharp 185 30V and Jetion 250 62V. The difference between the Sharps and the Jetion's is huge, and the inverter voltage range needs to be matched.

I note the Eversolar is a single tracker and single input, so doesn't look like the panels can be set up in 2 strings of 7 (I assume you have 14 panels?) thereby halving the voltage. I might be missing something, but the tech sheet does say "number of DC connections - 1".

What type of panels do you have, it'll be easy to check the open-current voltage if you post the model number.

The next question is what make and model of inverter do you fancy, but will depend on the range of voltages and the number of strings that each inverter offers.

Good catch.

Mart.

Martyn1981

Hiya Ed, sorry me again. Just been nose'ing at some inverter data sheets, there's no shortage of cheap and expensive models available, but I notice that a lot of them have similar voltage ranges to the Eversolar at around 500V to 600V. Not a problem as they offer 2 inputs, so 2 strings of 7, but I'm guessing your installer won't want to have to go on the roof and change the wiring around unless they have to, so the options are less if they want to just swap inverters and retain a single string.

Why not post up 'their' suggestions for thoughts and comments, when (if) they give them to you? As this might be an opportunity for you to bag a more efficient inverter (higher Euro efficiency rating)

Mart.

Ed050458

Hi Mart,

Yes 14 panels (Caymax 260-60M). Optimum operating voltage 30.3V. Open Circuit Voltage 37.6V.
Looking more carefully I think the software is quibbling about

"Voc at -10°C [V] = 582.96"

Which is higher than the Max DC Voltage of 580V but not by much, and only at -10°C!

Martyn1981

Ed050458 wrote: »

Hi Mart,

Yes 14 panels (Caymax 260-60M). Optimum operating voltage 30.3V. Open Circuit Voltage 37.6V.
Looking more carefully I think the software is quibbling about

"Voc at -10°C [V] = 582.96"

Which is higher than the Max DC Voltage of 580V but not by much, and only at -10°C!

Hiya Ed, the plot thickens!

Did you mean to say 260 (or 250)? If they are 260W panels, then you are up to 3.64kWp, and that inverter is getting smaller and smaller with every post. What kWp size is stated on your MCS certificate?

I'll freely admit that I'm well out of my depth here, in fact I was up to my nostrils several posts back, but I think you've got enough genuine concerns to at least ask the installers to re-consider the inverter choice.

I think you still need some qualified advice, so I'd still suggest you put a short post on Energetic's thread:

https://forums.moneysavingexpert.com/discussion/4326665

and ask if he'd be kind enough to have a quick look at this thread, assuming he's still around. It's always nice to get a little peace of mind, especially if someone knowledgeable is willing to give you a piece of their mind (in a nice way)!

Mart.