solar panel predicted vs actual generation / efficiency

selsdon101

I'm thinking of getting solar panels but my SW facing roof is only 17m2 total so i wouldn't be able to fit a particularly large set of panels, leaving me wondering if its worthwhile. I had a quote from ikea for their 12.8% efficiency panels, which are supposedly better at low light performance than rival crystalline panels.

The consensus on here and the assumption in government calculations seems to be that only the kWp figure is important - the peak power output of the panel. Like buying a car based on its advertised mpg and ignoring the reality that some will perform much closer to the advertised fuel consumption than others in the real world.

They've said their 1.5kW panel install will produce 1488kW per year, but this is using the government calculations which ignore how well a panel does in low light. What experience have others had for predicted vs actual performance over 1 year? Please give the efficiency spec and panel type as well.

lisyloo

Forecast: 2876.80 (this includes 20% for shading)
Actual: 3929

We have solar edge optimisers.


BenQ Solar - AU Optronics CorporationSunForte PM318B00 325Wp 325 W

Location: Lyneham
Climate Data Record: Lyneham (1986-2005)
PV Output: 3.90 kWp
Gross/Active PV Surface Area: 19.57 / 19.59 m²
PV Array Irradiation: 21,427 kWh
Energy Produced by PV Array (AC): 3,528.7 kWh
Grid Feed-in: 3,528.7 kWh
System Efficiency: 16.4 %
Performance Ratio: 82.4 %
Inverter Efficiency: 94.2 %
Specific Annual Yield: 900.8 kWh/kWp
CO2 Emissions Avoided: 3,113 kg/a

pinnks

This all depends on where you live (latitude) and the angle of your roof, shading and a host more variables like average weather conditions.

My systems are delivering about 5% more than predicted over 2 and a bit years but maybe we have just had 2 good years?

The efficient rating isn't really a measure of performance in the way 0-60 or mpg tells you about a car. It's more about how well the panel converts light into leccy per unit area of roof. I spent hours thinking/worrying about it before I bought my first system only to have the penny drop as it were once I was up and running with what I have.

12.8% efficiency panels would be fine if you have enough roof space to pack in enough to get your 3.68kW max production under the DNO rules, or 4kWp under the FIT scheme but if space is limited then higher efficiency panels simply give you more leccy per square meter of roof.

So, if you can pack, say, 10 x 120Wp panels on your roof you get a 1.2kWp system which would cover most of your base load but not any high power stuff like kettle, washing, dish washing or driers etc, whereas if you went for 333Wp panels you would get a 3.33kWp system - nearly three times the size for the same area (assuming the panels are broadly the same dimensions) and probably only about 50% more expensive as the only additional cost would be the panels themselves - scaffolding, labour, wiring, inverter etc. would be pretty much the same. Generation would then cover those high demand appliances, at least from about lunchtime to late afternoon depending on the time of year for a SW facing system.

Do you not have a second roof to put some more panels, or would that be too north-facing?

selsdon101

A 3.33 kWp system isn't going to be almost 3x more powerful than a 1.2kWp system if it uses panels which work quite badly in cloudy weather compared to a panel with a lower peak rating but better low light performance.

I live in a terrace and the other side of the roof is NE facing. But if the cost of an extra 1.5kW of the 'better low light performance' panels fitted to the NE side is reasonable - giving a 3kW system - then that might work out better compared to fitting a (say) 2kW system of the more expensive panels on just the SW facing side for a similar total price.

What I want to know is how the 'peak' ratings of different panel technologies translate to real world performance. The ikea panels with a peak efficiency of 12.8% will be less than 1% when averaged out over a year, and so will the more expensive 15-20% panels. The question is is there a significant difference in the real world? I need stats!

pinnks

To be honest I doubt the Ikea panels perform any better in low light than any other panels. Others may have more experience/knowledge than me but it sounds like sales hype.

It just seems to me that a generally higher spec, more technologically advanced product is likely to provide good performance across the range of light conditions. So a 333W panel is likely generally to be good.

There is a lot of discussion on here about panel efficiency and quality/ performance etc and the conclusions seem to be that wattage is wattage is wattage and most panels perform similarly across the range. But whatever the truth in that, 2kWp or 120W panels spear over a SW/NE set-up will never perform as well as 3kWp on the SW roof.

ed110220

I've seen very little evidence for this supposed greater low light performance. I suspect it is a case of at best a modest improvement in conditions when light levels are already so low as to make little difference. I'm especially suspicious of it being used as a reason to use low efficiency panels on domestic roofs where they result in unnecessarily small (low kWp) systems.

The important thing is the kWp of the system... For practical purposes a kilowatt of one type of panel will yield the same as a kilowatt of another. Being told something like 2.5kw of some special panel that has special low light performance will give more yield than 4kW of another manufacturers is the type of misleading sales pitch we sometimes hear.

Ed

selsdon101

ed110220 wrote: »

I've seen very little evidence for this supposed greater low light performance. I suspect it is a case of at best a modest improvement in conditions when light levels are already so low as to make little difference. I'm especially suspicious of it being used as a reason to use low efficiency panels on domestic roofs where they result in unnecessarily small (low kWp) systems.

The important thing is the kWp of the system... For practical purposes a kilowatt of one type of panel will yield the same as a kilowatt of another. Being told something like 2.5kw of some special panel that has special low light performance will give more yield than 4kW of another manufacturers is the type of misleading sales pitch we sometimes hear.

Ed

This seems to be the general opinion here, but we really need to get stats from people who have installed the panels to see. I doubt a 2.5kW low-light optimised panel will give more yield than a 4kW one, but a 3kW one might give more than a similar priced 2kW one, even if half of it is facing the wrong way. Thats whats relevant to my situation.

ed110220

selsdon101 wrote: »

This seems to be the general opinion here, but we really need to get stats from people who have installed the panels to see. I doubt a 2.5kW low-light optimised panel will give more yield than a 4kW one, but a 3kW one might give more than a similar priced 2kW one, even if half of it is facing the wrong way. Thats whats relevant to my situation.

If you're interested, there is a large field test of solar modules being carried out at Aachen in Germany and you can see the latest yearly results here: http://www.photon.info/upload/YM_Annual_Results_2014_5302.pdf

Unfortunately even though its a large trial, given the number of panels available it doesn't cover a lot of them.

The best achieved 1178 kWh/kWp while the worst was 916. 138 out of 174 modules achieved more than 1100 kWh so I think it's fair to say the differences are fairly small.

Ed

selsdon101

Thanks, I had a look at that before but they only seem to be testing crystalline panels, rather than the thin film type which hanergy would install. I also found pvoutput.org which shows real world installs, but no easy way to analyse thin film vs crystalline.

zeupater

selsdon101 wrote: »

This seems to be the general opinion here, but we really need to get stats from people who have installed the panels to see. I doubt a 2.5kW low-light optimised panel will give more yield than a 4kW one, but a 3kW one might give more than a similar priced 2kW one, even if half of it is facing the wrong way. Thats whats relevant to my situation.

Hi

A quick suggestion ... set a benchmark before you get too deep into the relative performances between panels. Simply get a price for the panels you're talking about, then divide this by the installed capacity to get a price/watt then compare this with the cost of a larger system using standard or (expensive) high efficiency panels ...

As others have mentioned, crystalline panels generally perform the same on a kWh/kWp basis. Monocrystalline usually perform slightly better than polycrystalline in summer heat, some panels have variations on some kind of rear reflective surface to improve efficiency (ie area related) and thin film panels (the type you're looking at) are usually much less efficient in collecting energy per unit area,

Claims that thin film panels are 'more efficient' in low light conditions are a little misleading. What is meant is that they have a performance curve where generation in low light conditions (in terms of W/Wp) should be better than wafer silicon ... as a simplified working example - in low light conditions a 4kWp system using high efficiency panels could be generating 200W where a ~2.2kWp thin-film system with the same footprint could be generating 140W, this according to a salesman could represent that the panels are 27% (140/((200/4)*2.2)) more efficient in low light conditions, whereas the logical reality is that they are generating 30% (1-(140/200)) less than the more efficient panels .... and when the sun comes out, considerably less.

Anyway, to add another twist, you also have hybrid panels which combine silicon wafers and amorphous thin-film in order to increase efficiency ... these a have marginally better low light performance curve than standard wafer panels, however, the main advantage is the energy returned from a limited roof area resulting from their relatively high efficiencies ....

HTH
Z

zeupater

selsdon101 wrote: »

Thanks, I had a look at that before but they only seem to be testing crystalline panels, rather than the thin film type which hanergy would install. I also found pvoutput.org which shows real world installs, but no easy way to analyse thin film vs crystalline.

Hi

Here's a direct comparison (PVOutput) between a 2.5kWp Hanergy system facing SW (no shade) and a standard 4kWp system a few miles from it facing West (low shade) .... don't look at the relative efficiencies, just energy ... ( http://pvoutput.org/compare.jsp?sid=32131&sid1=9860&t=m&c=1 )

HTH
Z