Panel efficiency: Why does it matter?

[Mark42]

I’ve been comparing various makes and models of PV panels for an 8kW project.

Sales people keep on and on at me about this or that panel being ‘more efficient’ than some other, so is obviously worth the extra cost.

My understanding is that the efficiency of a panel is the rate at which it converts solar energy into useful power, PER UNIT AREA. Is that right?

So an ‘inefficient’ 250W panel would simply be BIGGER than a super-modern, new chemistry expensive one of the same, 250W, output?

So my reckoning is that it doesn’t matter a monkey’s ---- whether the panels are more or less efficient (within reason), unless you are short of roof space and need to cram in as much generation capacity as possible per square metre.

Therefore, if you have space, as I do, buying a cheap, big array is more sensible than an expensive small one of the same output.

Am I right?

It seems to me that ‘efficiency’ is a nice comfy word which the uneducated public likes to hear, so it’s in general no more than a sales gimmick. Sign now! It's is a special price for this day only and I’m only allowed to give it to one house in this street! (Does anyone really fall for that rubbish?)

 

[finchy]

It seems a lot of the public fall for such offers as the REAL assurance scheme let's everyone and their dog sell solar even double glazed window companies next the local used car sales man would be giving it a go !

Correct it comes down to how much space you have, it's up to the specialist solar installers to assess this and offer you the various options available.

Ps under REAL no installer should use pressure selling techniques so report any crooks ! :furious3:

 

[jupestar]

Exactly. efficiency in Solar terms is basically W/sqm. If you take panels size say 1580x808 which is 190W. you just do 190/(1.580x0.808) = 14.88. divide by 1000 which is the w/sqm of test conditions and you get 0.1488 or 14.88%. Some manufacturers will call this 14.9 some will call it 15 and some will call it 14.88.

Watch out for sales guys who quote cell efficiency instead of module (they get shown the door). and look out for panels with large negative tolerances (although these are quite rare now). But basically yes efficiency is just about getting as much out the space as possible..

 

[screwdriver]

What you really want to be looking at is the specific annual yield per kwp of panel, I have seen a chart somewhere.......... basically it is down to how good the panels are working under fairly low light conditions, I am sure someone knows where the link is! some manufacturers claim better yearly yields than others.

 

[Mark42]

What you really want to be looking at is the specific annual yield per kwp of panel, I have seen a chart somewhere.......... basically it is down to how good the panels are working under fairly low light conditions, I am sure someone knows where the link is! some manufacturers claim better yearly yields than others.

Yup, exactly, there's probably a technical term for that parameter, ie maximum conversion per annum, but I don't know what it is. But I do know it's VERY different to Watts/unit area 'efficiency', and is almost certainly geographically specific.

An array engineered for the peak possible extraction of energy in the Sahara should be very different to one engineered for Scotland.

 

[screwdriver]

little excerpt from pvsol site
Why is the specific annual yield for amorphous modules better than for monocrystalline modules, even when the systems are the same size?The decisive factor for the yield is the part load efficiency. In this area, amorphous modules usually have a better efficiency than monocrystalline modules. The part load efficiency is presented in the efficiency characteristic curve which can be displayed under Databases > PV Module > 'U/I Char. - Part Load' by clicking the 'Efficiency Char. Curve' button.

 

[screwdriver]

ahh part load efficiency, must be a better word than that to use, sounds like setting up a fuel injection system......

 

[FB.]

Sanyo panels are about 20% smaller per kW output, so can be useful to help get more kW from a given roof space.
Sanyo are also superior in less-than-ideal operating conditions, such as dull days and hot days (when other panels struggle).
It would be hoped - but it's not certain - that Sanyo panels would have a longer life.

Personally, unless one of the above situations applies, I can't see how the benefits match the additional cost. This is especially true if the trend of lower prices continues (as it does for household electronics); install cheap panels now and then replace them in 10 years time with high-spec Sanyo panels for a fraction of what Sanyo panels cost today.

I have no problems with roof space on my house, so I have cheap-ish, heavy and bulky Chinese panels. They don't seem to be significantly worse than anyone else's panels in my region, although, as suggested earlier; on these short, dull winter days a few arrays with Sanyo panels may be consistently generating a little more.

 

[SolarCity]

By part load efficiency, are we effectively talking about a panel's performance in low light conditions?

 

[Mark42]

… Personally, unless one of the above situations applies, I can't see how the benefits match the additional cost. This is especially true if the trend of lower prices continues (as it does for household electronics); install cheap panels now and then replace them in 10 years time with high-spec Sanyo panels for a fraction of what Sanyo panels cost today.

That sounds like very wise advice, and like you, FB, is exactly what I am doing.

I’ve just ordered 8kW of HJ-Solar 250W panels, at £5662.40 the lot, which I think counts as ‘cheap’.

… on these short, dull winter days a few arrays with Sanyo panels may be consistently generating a little more.

Agreed. But so what? It would have cost thousands of pounds more for a Sanyo array, and I suspect you’d never get it back.

By part load efficiency, are we effectively talking about a panel's performance in low light conditions?

Yes, exactly Biggs. This is something I don’t understand (perhaps because I haven’t bothered to find out), but no doubt it’s something to do with the chemistry and physical arrangement/cell mix in the more expensive panels.

Obviously important in temperate climes where solar irradiance varies wildly during the year, and even during the day. But my research does show that the economic argument wins. The ‘best’ panels are (currently) too much more money than the slightly less ‘clever’, but very much cheaper, ordinary ones.

As FB says, wait a few years and see. I suspect paying not much short of a pound per watt will very soon seem outrageously expensive!

Now here’s a question: what do commercial installers recommend? I envisage wild differences in pricing between the panels used, and when taking into consideration my point about running the DC back to the house from distant arrays, hugely saving on copper.

The normal client probably doesn’t understand, or even care, about how it’s done, so there are obvious opportunities for a contractor to both undercut his rivals, and make more profit, with what I would consider an economically intelligently-designed system.

Or is it just spout the well-known Japanese names and let a high price stand on that? I appreciate this might be a touchy subject. But touchy subjects always make for interesting conversations.

I recently got a quote from an irritating cold-call salesman (I don’t mind wasting cold-callers’ time) who said their panels were the best, most ‘efficient’, and ‘Made in England’. I wonder if it’s true. I didn’t know there were silicon wafer plants in the UK doing this stuff. Are there? I suspect it’s ‘last manufacturing operation,’ maybe assembly into the frames, final wiring, or something like that. Does anyone know?

 

[mdovey]

As most have pointed out the efficiency of a panel is only important if you are trying to squeeze as much power out of a particular area as possible. Sanyo (seem to be rebranded Panasonic who is the parent company) and SunPower are the most efficient, although SunPower have decided to go for massive 320/327W panels which may limit their applicability on awkward roofs.

In terms of performance, the figure to look for is the NOCT rating. The headline power of a panel is its performance at when the panel is 25C and under 1 kW/m2 (i.e. full sunlight). In real-life, a panel in that sort of sun light will be far hotter than 25C (unless you have active cooling on the panels!) and its power output reduced accordingly. The NOCT rating is measured at 800 W/m2, ambient air temperature of 20C, wind velocity of 1m/s (light breeze) with open backed mounting. This is a little more realistic in "normal" conditions. The ratio of the NOCT rating to the headline rating is a good rule of thumb for determining better performing panels (the higher the better).

Even this is not the whole story. Sanyo/Panasonic will come out good in terms of the NOCT/headline rating since they have a low thermal co-efficient (rate at which the power degrades as the panel gets hotter) compared to most others. However, they don't always come top when compared with other via the PVSol simulations - there's a thread recently claiming that this is because the Sanyo/Panasonic are thicker than standard panels and so cool down more slowly that others. However, NOCT/headline rating is as good a measure as any.

As others have said, the other considerations with a panel are their tolerance ratings - most are now guaranteed to be at least their headline rating although there are a few which may be 3-5% below this on delivery; how confident you feel the manufacturer will be around in 10-20 years time to honour their manufacturing or production warranties; cost of the panels (per watt); and increasingly whether the frame is silver or black.

 

[SolarCity]

Did they mention the manufaturer of this panel?

 

[SolarCity]

there's a thread recently claiming that this is because the Sanyo/Panasonic are thicker than standard panels and so cool down more slowly that others.

That's interesting. I wasn't aware that the panel was any thicker to be honest.

 

[mdovey]

That's interesting. I wasn't aware that the panel was any thicker to be honest.

I was referring to this post - http://www.electriciansforums.net/p...ual-yield-figures-pvgis-pvsol.html#post322495

There seems some debate in the thread whether the figures from pv sol's dynamic simulation indicate smaller difference in yield between sanyo and other makes because the sanyo's coo slower, or because pvsol uses generic statistics for the sanyo panels.

 

[Mark42]

Did they mention the manufaturer of this panel?

I just looked it up: 'Ultima Solar.' The promotional material is headed by some Pop Star I've never heard of.

Apparently he had these panels put on his house, so they must be good.

It doesn't say what the pop star's electrical engineering qualifications are.

(They do look like perfectly good panels, but at a grand each fitted they bloody-well should be )

 

[MCS Renewables]

We have two similar systems with identical roofs and identical inverters and wired exactly the same way even the lengths of cable and routes are similar .



On the left hand side are Moser Baer modules and on the right hand side are Sanyo modules.

The MB project has been in since May 2010 and just about exceeded our annual solar prediction.
The Sanyo project has been in since October 2011.
The Sanyo beats the Moser every day, sometimes doubling the generation figures.

The Moser system was a lot cheaper installation but the additional FIT returns and import savings with the Sanyo will easily justify the extra cost.

Also Moser is an unknown manufacturer whose long term position is unknown as opposed to the Sanyo corporation.

The Moser Baer customer wishes he had Sanyo modules now.

 

[SolarCity]

Excellent comparison, thanks for the info.

Getting such an accurate example to judge performance is rare so they're extremely helpful.

Our Sanyo installs also all appear to be outperforming the other panel installs.

 

[pwboston]

thats ideal conditions for comparison being next to each other, that proves it better than any program can come up with in reality as you can physically see it

 

[FB.]

We have two similar systems with identical roofs and identical inverters and wired exactly the same way even the lengths of cable and routes are similar .

View attachment 10428

On the left hand side are Moser Baer modules and on the right hand side are Sanyo modules.

The MB project has been in since May 2010 and just about exceeded our annual solar prediction.
The Sanyo project has been in since October 2011.
The Sanyo beats the Moser every day, sometimes doubling the generation figures.

The Moser system was a lot cheaper installation but the additional FIT returns and import savings with the Sanyo will easily justify the extra cost.

Also Moser is an unknown manufacturer whose long term position is unknown as opposed to the Sanyo corporation.

The Moser Baer customer wishes he had Sanyo modules now.

The comparison is valuable - and appreciated.

Could the Sanyo's extra performance mostly be due to the dull winter days, where its extra technology would be expected to give it an edge.
It would certainly be interesting to see how much different the two systems are, after a whole year - in terms of total output (especially seasonally-analysed) and also the output relative to the cost of the system.

 

[Chris Nunn]

Some further thoughts on comparing modules using PV*SOL; Here’s a nice tip to look at when comparing modules or inverters. The trick is to combine the PV*Sol graph function and a spreadsheet. Here’s one I’ve made to show the difference in module part load efficiencies.



You could then include comparisons at different temperatures or for modules with a different temperature response. PV*Sol has a lot of hidden depths which the German translation doesn’t always immediately reveal. The manufacturers load the raw data up into the PV*Sol database to make it easier to analyse. If a module even has one letter or number designation different then there is a separate entry for it.