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Discussion Starter #1
I know that the sun is low in the sky. I know that my panels don’t face due south. But should I be a bit concerned about 20% variation between the power generated by the best panel and the worst panel? All the panels are aligned. There is a little variation in shading, but I don’t think this can be the explanation due the pattern of variation (see the layout diagram).

My installation was in early December, so all the sunlight has been at quite a severe angle so far and I’m trying to be patient before jumping to conclusions.

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I'd be patient with it until you get more data from spring/summer. I guess that's how they're physically laid out? South facing roof with no shading?

For reference, my 3kWp East + 3kWp West system (Sheffield area) produced 180kWh from Decemeber - End of Jan. It was installed in September and was generating 20kWh+ some days, so like you I'm waiting to see how it performs once we have more daylight hours..
 

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Discussion Starter #3
Yep - the layout reflects real life. The panels face south east, so I’m open to slight variations in angle leading to different production per panel when the sun is as low as it is at the moment. Fingers crossed!
 

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I know that the sun is low in the sky. I know that my panels don’t face due south. But should I be a bit concerned about 20% variation between the power generated by the best panel and the worst panel? All the panels are aligned. There is a little variation in shading, but I don’t think this can be the explanation due the pattern of variation (see the layout diagram).

My installation was in early December, so all the sunlight has been at quite a severe angle so far and I’m trying to be patient before jumping to conclusions.

View attachment 111376
December generates 1-2% of your annual yield so a variation of 20% is a 0.2% ish yield variation for the month but that’s only on the panels that are varying so it’s actually less than that.

Typically year to year overall generation will vary by upto 10% just based on weather so generally nothing to worry about.

Also if you look at measured variations between panels as they come out of the factory they could vary by upto 5% and be within tolerance.

That said is there any shade on the panels as would have thought they should be fairly consistent. What panels are they?


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Any bird droppings? In the middle of summer, cooling differences will cause power variations, but would not be a factor in the winter

How are you measuring the individual panel outputs?
 

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Discussion Starter #8
They’re a decent panel so no worries from that side, variation is likely tolerance on manufacturing or maybe shade then


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Not top of the range, but I have a big roof so I didn’t need one of the higher-output panels. I’m happy with them so far.
 

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Discussion Starter #9 (Edited)
Any bird droppings? In the middle of summer, cooling differences will cause power variations, but would not be a factor in the winter

How are you measuring the individual panel outputs?
I don’t think there are droppings, but I’ll take a quick look. They were only fitted at the beginning of December, so I don’t think the geese have done any bombing runs on them. I just think I need to wait a couple of months for stronger, more direct sunlight to see. Peak output is currently under 50% of the panel rating.

I’m measuring the individual panels with the Solaredge website.

Edit: no droppings.
 

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That looks to be very well within normal expected tolerance to me. We've had 25 panels working in our roof for nearly five years now, and with no shading at all there is a big variation when the sun is low in the sky. It helps a bit if you have panels with an anti-reflective front face (ours have this as a planning consent requirement) but the downside with the anti-reflective glass is that it gets dirty more quickly, so the panels need to be cleaned once a year or so.

As mentioned above, winter generation is a very small part of your overall generation. We tend to get slightly better winter performance on good days, because our roof pitch is 45 deg and faces 20 deg west of south, but even so there are plenty of days in December and January when there is next to sod all generation (I suspect today will be one of them).

We generate around 6,000 kWh per year, so an average of about 500 kWh per month. Looking at the December figures we generated a total of 88 kWh, and in January we generated 110 kWh. Looked at in percentage terms we generated around 1.46% of the annual total in December and 1.83% of the annual total in January.

In general, PV generation tends to just fall off a cliff in October and stays at a low level until about March, when it picks up again. We don't bank on getting any useful excess energy generation through these months, but we do get the occasional bright, clear, cold day that can be very good. These days tend to be rare, though, we might get a handful of them through the winter.
 

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Discussion Starter #11
I’ve just checked the February variation and it’s down to 11% following two out of three days being relatively high generation. I assume the variation will be less noticeable as generation increases.
 

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There will always be variation, it's normal and within installation/manufacturing tolerances. IIRC the allowable manufacturing tolerance alone can be at least 10% or so between one panel and the next. Manufacturers tend to supply panels that are slightly better than the stated specification to get around the normal tolerance issue, and ensure that delivered panels start off always exceeding the minimum output by a small margin.
 

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That looks to be very well within normal expected tolerance to me. We've had 25 panels working in our roof for nearly five years now, and with no shading at all there is a big variation when the sun is low in the sky. It helps a bit if you have panels with an anti-reflective front face (ours have this as a planning consent requirement) but the downside with the anti-reflective glass is that it gets dirty more quickly, so the panels need to be cleaned once a year or so.

As mentioned above, winter generation is a very small part of your overall generation. We tend to get slightly better winter performance on good days, because our roof pitch is 45 deg and faces 20 deg west of south, but even so there are plenty of days in December and January when there is next to sod all generation (I suspect today will be one of them).

We generate around 6,000 kWh per year, so an average of about 500 kWh per month. Looking at the December figures we generated a total of 88 kWh, and in January we generated 110 kWh. Looked at in percentage terms we generated around 1.46% of the annual total in December and 1.83% of the annual total in January.

In general, PV generation tends to just fall off a cliff in October and stays at a low level until about March, when it picks up again. We don't bank on getting any useful excess energy generation through these months, but we do get the occasional bright, clear, cold day that can be very good. These days tend to be rare, though, we might get a handful of them through the winter.
Wow that’s a lot of hours, take it your a 6kW system then, yes main generation is 2nd week in Mar through 2nd week in Oct. It’s the point that the sun reaches 17 degrees (roughly) above the horizon at midday. You’ll also notice that’s the point the weather seems to feel warmer / cooler as well. It’s to do with the angle of sun through the atmosphere and incidence angle to panels.

If you’ve ever got any questions fire away I’ve installed over 20MW and monitor over 25MW so there’s rarely a question I haven’t seen or a problem we haven’t already come across.


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It's a 6.25 kWp system built in to the roof, using the GSE Intégration system. The system performs pretty much exactly as PVGIS predicted, when averaged out over a few years. From around March onwards we generate more energy than we use; since I built the house it has used around 10 MWh (it's all-electric) and the PV has generated around 30 MWh.

We run our hot water from a phase change thermal battery (heated by a mix of E7 off peak in winter and excess PV generation via a diverter in summer). Heating is from an ASHP, also run from off-peak in winter, to "charge" our passive slab, that then acts like a storage heater to keep the house warm through the day and evening.

The car can either fast charge overnight at E7 off-peak rate, or can charge at a variable rate if the EVSE is set to use variable excess PV generation (2nd priority after charging the phase change battery). 9.6 kWh of battery storage that will be operational in the next few weeks should mean that we can run the house entirely from either off-peak electricity in winter or PV generation in summer.
 

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It's a 6.25 kWp system built in to the roof, using the GSE Intégration system. The system performs pretty much exactly as PVGIS predicted, when averaged out over a few years. From around March onwards we generate more energy than we use; since I built the house it has used around 10 MWh (it's all-electric) and the PV has generated around 30 MWh.

We run our hot water from a phase change thermal battery (heated by a mix of E7 off peak in winter and excess PV generation via a diverter in summer). Heating is from an ASHP, also run from off-peak in winter, to "charge" our passive slab, that then acts like a storage heater to keep the house warm through the day and evening.

The car can either fast charge overnight at E7 off-peak rate, or can charge at a variable rate if the EVSE is set to use variable excess PV generation (2nd priority after charging the phase change battery). 9.6 kWh of battery storage that will be operational in the next few weeks should mean that we can run the house entirely from either off-peak electricity in winter or PV generation in summer.
That’s a cool system, pretty much what I’m doing at mine, have 4kW PV at moment just about to add another 4kW but limit the export still to 3.68kW as DNO won’t allow me more, add a Leaf as a battery with bidirectional should mean will be off grid most of time with 8kE of PV.

I’m in the process of buying a solar thermal company and was thinking about adding thermal with a heat store hadn’t decided between big tank vs Sunamp storage. Also have a screed in kitchen but not sure the volume will be enough there’s only about 4m3 in screed

May also add In a Tesla PW2 with backup gateway as we are an Tesla partner but want to see what the balance on use / consumption looks like first


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I looked at solar thermal briefly, but I've experience of fixing broken systems (replacing pumps, controllers, sensors etc) and frankly didn't want the hassle. I'm using a Sunamp UniQ eHW 9 phase change thermal battery, and before that we had a Sunamp PV. The Sunamp PV worked very well indeed, the Sunamp Uniq not so great, they need to do some work on the controller to make it work efficiently when charged from excess PV generation. I've designed a solution to the controller problem, but am reluctant to implement it whilst the unit is still under warranty, and I'm hoping that Sunamp come up with a fix before too long (they are aware and are looking into it).

We had a thermal store initially, but the heat losses from that, even with an additional 50mm layer of insulation foam around it, were just too high. Our services room was reaching 40 deg C in summer, and the adjacent bedroom was over 30 deg C, so the thermal store had to go. The losses from the Sunamp are tiny in comparison, which both alleviates the over-heating problem and increases efficiency a lot.

We were lucky in that we found an "unknown" 95mm² 3 phase Wavecon cable under our plot, which didn't have any wayleave or easement to be there. The DNO moved it, to a location where it wasn't causing us a problem, and where it ran underneath the external meter box kiosk I installed. That meant our incomer is barely 2m long. When I asked for G59/3-1 consent the DNO just gave me "up to 12 kWp on any single phase" permission. It turns out that the 95mm² Wavecon only supplies one other house...

I can't get the sums to add up for the PW2; it's just too costly at the moment to recover the capital outlay from through life savings. Even the much cheaper battery system I'm putting together will struggle to recover the capital investment, but as it has a UPS function, and as we get half a dozen power cuts every year, I'm prepared to pay a modest premium to keep essential circuits working.
 

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I looked at solar thermal briefly, but I've experience of fixing broken systems (replacing pumps, controllers, sensors etc) and frankly didn't want the hassle. I'm using a Sunamp UniQ eHW 9 phase change thermal battery, and before that we had a Sunamp PV. The Sunamp PV worked very well indeed, the Sunamp Uniq not so great, they need to do some work on the controller to make it work efficiently when charged from excess PV generation. I've designed a solution to the controller problem, but am reluctant to implement it whilst the unit is still under warranty, and I'm hoping that Sunamp come up with a fix before too long (they are aware and are looking into it).

We had a thermal store initially, but the heat losses from that, even with an additional 50mm layer of insulation foam around it, were just too high. Our services room was reaching 40 deg C in summer, and the adjacent bedroom was over 30 deg C, so the thermal store had to go. The losses from the Sunamp are tiny in comparison, which both alleviates the over-heating problem and increases efficiency a lot.

We were lucky in that we found an "unknown" 95mm² 3 phase Wavecon cable under our plot, which didn't have any wayleave or easement to be there. The DNO moved it, to a location where it wasn't causing us a problem, and where it ran underneath the external meter box kiosk I installed. That meant our incomer is barely 2m long. When I asked for G59/3-1 consent the DNO just gave me "up to 12 kWp on any single phase" permission. It turns out that the 95mm² Wavecon only supplies one other house...

I can't get the sums to add up for the PW2; it's just too costly at the moment to recover the capital outlay from through life savings. Even the much cheaper battery system I'm putting together will struggle to recover the capital investment, but as it has a UPS function, and as we get half a dozen power cuts every year, I'm prepared to pay a modest premium to keep essential circuits working.
That’s an interesting point about the heat loss, not really considered that although yes it’s an obvious issue as we see same thing from commercial inverters when we’ve 200kW in one room essentially it’s a 5kW heater!

PW2 should give about a 10-12 yr payback but know what you mean, less if you don’t have FiTs, that’s the reason for wanting an old leaf 24 with bidirectional as it’s a cheap battery that you happen to get a spare car with. The PW2’s do backup from next month you just need the new gateway 2, essentially it’s a seamless backup unit (but not advertised as that). How you doing the grid compliance on your unit, presume a G59 changeover unit, did the DNO want much info on fault current etc?

The waveform was a lucky find! :)


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That’s an interesting point about the heat loss, not really considered that although yes it’s an obvious issue as we see same thing from commercial inverters when we’ve 200kW in one room essentially it’s a 5kW heater!

PW2 should give about a 10-12 yr payback but know what you mean, less if you don’t have FiTs, that’s the reason for wanting an old leaf 24 with bidirectional as it’s a cheap battery that you happen to get a spare car with. The PW2’s do backup from next month you just need the new gateway 2, essentially it’s a seamless backup unit (but not advertised as that). How you doing the grid compliance on your unit, presume a G59 changeover unit, did the DNO want much info on fault current etc?

The waveform was a lucky find! :)


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Best I can manage with the PW2 is a return on initial capital investment of around 18 years, not including capital appreciation over that time. Mind you, our house is a passive house, with a low overall energy demand through the year, which doesn't help with battery pay-back, as for most of the year we can't utilise more than a small part of the battery capacity, we can only really make full use of it in winter.

G59/3-1 compliance is a combination of two methods. We are authorised to export up to 12 kWp per phase, so as the inverter hard limit output is well under that we're OK on the export power limitation side. Also, the inverter non-essential supply output will have G59 approval by the time I get the thing installed, although that's a moot point in our case because the inverter could never come close to our approved export limit anyway.

The UPS side runs a separate set of circuits in the house on a non-grid connected island, so there's no need for a G59 changeover relay. I designed the electrical installation from the outset to be able to be split into essential and non-essential circuits, each with their own DB, so it's just a matter of disconnecting the grid link to the essential circuit DB and connecting up the supply from the inverter always-on UPS output (in effect it's two inverters in one box, with a common DC connection to the battery pack).
 

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Discussion Starter #19
Best I can manage with the PW2 is a return on initial capital investment of around 18 years, not including capital appreciation over that time.
My supplier estimated that the Powerwall would only take a few months longer to pay for itself than the panels. It’s way too early for me to judge the accuracy of the estimates yet, but over 70% of the benefits so far have been due to the battery. Obviously the winter benefits are relatively small, but I haven’t seen the summer benefits yet.
 

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It's any easy calculation to make, if you can accurately estimate the value of the stored usable energy (which is around 85% to 90% of the charge energy). Taking the PW2 as an example, it has a usable energy of 13.5 kWh, and takes a bit over 15 kWh of charge to get that usable capacity. Unless you have a large PV array, then the PW2 won't charge from PV for a fair bit of the year. For example, our 6.25 kWp array is averaging about 3 kWh per day of excess generation right now, perhaps 20% of the excess generation capacity needed to charge a PW2. That means we'd only charge a PW2 up to capacity once every five days on average at this time of the year.

After 5 days of charging from excess PV generation we'd get about £2.20 worth of usable energy back out of the PW2. This would improve markedly as the summer approaches, but our electricity demand also drops a great deal at the same time. By April we'd not be able to use more than about 1/3rd of a PW2 battery capacity, despite it being charged pretty much every day, simply because our electricity consumption during the night would be lower, and it makes more sense to utilise excess PV generation during the day for loads that we'd otherwise put on at night (saves the battery round-trip losses).

To just recover the capital investment of £6.5k in a PW2 installation, you would need to be able to use about 40 MWh of stored excess PV generation electricity from it at current prices before the PW2 dies of old age. We've used a total of 10 MWh in the last 5 years, so even if we could use all of the capacity in a PW2 then it would still take 20 years just to recover the capital investment. I doubt that the calendar life of the cells would extend more than about 15 years at the very most, and my experience with my 9 year old electric motorcycle battery pack is that calendar life ageing has been significantly greater than cycle life ageing, as that pack is now down to about 50% of it's original capacity, after around 600 cycles.

If you are able to use close to the 13.5 kWh capacity of the PW2 every day, and more importantly, are able to charge it from excess PV generation to full capacity every day (which probably needs a PV array of 20 kWp or more at a guess), then yes, it will probably recover the original capital investment within about 12 years or so, around the same as the payback for a PV installation, perhaps.
 
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