Solar PVs - worth the investment?

Martyn1981 · 19 August 2016 at 5:08PM

purple_hamster wrote: »

Is it best to have mono or poly-crystaline panels? Or does it depend on other factors?

Can anyone answer the question about my outbuilding, above?

People have been asking about mono v's poly since day one. I did the same, but have never found a definitive answer. It may be that mono are slightly better, but poly slightly cheaper, so don't worry too much either way.

Rooves and planning. If the panels don't follow the roof pitch of the house, then you do need PP. However I don't know if that applies to outbuildings. You do need PP for ground mounts that exceed 9m2, and it may be that an outbuilding roof comes under that category.

Would it be possible/economical to change the pitch of the outbuilding roof? Simplest option would be two strong uprights, bolted to the brickwork at the northern end, poking up so that a PV rail could be attached to them, with another rail attached across the roof at the southern end. This might be cheaper (and structurally safer) than weighted tubs, that panels sit in for some flat roof installs.

Are the measurements correct, as you'd only get 2 panels on there, and they'd have to overhang. Or would just fit at 46d pitch (1.6m panel sitting over a 1.2m base), I think.

I've got two panels mounted on galvanised iron brackets, acting as a canopy over the back door.

Mart.

purple_hamster · 22 August 2016 at 7:53AM

Thanks Mart.

On a typical, overcast, drizzly summer's day, how much elec do you generate? Would I still be able to run my washing machine? (can't always save the washing for sunny days!)

Martyn1981 · 22 August 2016 at 8:27AM

purple_hamster wrote: »

Thanks Mart.

On a typical, overcast, drizzly summer's day, how much elec do you generate? Would I still be able to run my washing machine? (can't always save the washing for sunny days!)

Hi, the last 3 days have been really bad here, so average generation of 16kWh for mid August (for me) actually came out as 8.1, 9.2 & 11.4.

The summer months are actually pretty good, generation is reasonably good/consistent, and predictable. It's the winter days that are a pain, can be close to zero.

We had the washing machine and tumbly going all day(time) yesterday. Though you do try to pick your fights, waiting for better sunshine periods, and one item at a time.

My system isn't very efficient compared to a south facing install, but in poor weather the brawn over brains kicks in, and sheer generating capacity helps - the poorer the weather, the less important orientation is.

Mart.

purple_hamster · 22 August 2016 at 10:20AM

16kw per day seems amazing. Even 8 does! We're using about 30kw per week at the moment. Obviously I realise some of that will be out of daylight. With batteries we'd probably be more than self sufficient in the summer. Probably not so much in winter, though! I can't quite believe, but then again I can, just how exciting this all is!

Martyn1981 · 22 August 2016 at 10:42AM

Do remember though that batts can only do so much. For the summer months you don't get too many really bad days in a row, and even the bad ones aren't too bad.

You are probably right that batts would reduce your summer import to near zero, but in the winter, if you generate nothing, you won't cover baseload, if you only generate enough to cover baseload and some daytime consumption, then you won't have any spare to charge batts.

So it's all a balance. Play with PVGIS, see what the monthly generation for your location is, and compare that to demand (current import).

For myself, summer import is about 2.5kWh per day, with far more export, so that's covered, with savings of 2.5kWh per day.

In the winter, generation is far below daily consumption, so whilst batts would help redirect any export, savings would be small, perhaps 1kWh per day.

My best months would be spring and autumn, where import is higher at 3-5kWh per day, but export would cover this, so savings of around 4kWh per day.

But even that only comes to around 1,000kWh pa, perhaps £120. That would breakeven over 10 years if the (4-5kWh) cost £1,200. So still a long way to go.

Just to repeat, play with PVGIS. Whilst my system is only about 80% efficient compared to a south facing system, most of the 'losses' are in the winter, so my summer gen will be high (due to size) in the summer months. A good 4kWp system, mid England, might be nearer 13kWh/day on average in August.

Mart.

purple_hamster · 23 August 2016 at 11:37AM

Both companies I've had round have recommended Enphase microinverters. How does these compare with Solaredge?

SavingFish · 23 August 2016 at 1:18PM

purple_hamster wrote: »

Both companies I've had round have recommended Enphase microinverters. How does these compare with Solaredge?

Enphase and SolarEdge both deal with shade in a similar way, by controlling the output of each panel individually. The difference is that Enphase places more of the electronics on the roof (there's no inverter in the house at all).

The selling point for SolarEdge is that the parts supposedly most likely to go wrong are still easily accessible inside the house (scaffolding is expensive). Most information on failure rates is anecdotal however, and no system of either sort has been installed long enough to know how they will last over 20 years.

Just to baffle you with even more information however, many single unit inverters have some variant of full scan tracking (SMA calls it global peak, ABB just calls it an MPP scan). This, combined with the bypass diodes that exist in pretty much all solar panels, allows the inverter to bypass shaded sections of panels by adjusting the voltage of the entire array.

My own installation suffers winter shading. Given the additional outlay for a SolarEdge system, and my own personal concerns about maintenance, I opted for this latter approach. Viewing realtime figures for output, I can effectively see panel segments switching off as the shade encroaches. SolarEdge may have allowed me to recover some additional output from these panels, but the rest of the string is happily unaffected.

(On the other hand, SolarEdge/Enphase will also help with varying panel quality, e.g. one of your 285 watt panels may actually be slightly better than another. These systems will allow you to take advantage of these minor differences.)

Many many swings and roundabouts :P

Exiled_Tyke · 23 August 2016 at 7:57PM

SavingFish wrote: »

Enphase and SolarEdge both deal with shade in a similar way, by controlling the output of each panel individually. The difference is that Enphase places more of the electronics on the roof (there's no inverter in the house at all).

The selling point for SolarEdge is that the parts supposedly most likely to go wrong are still easily accessible inside the house (scaffolding is expensive). Most information on failure rates is anecdotal however, and no system of either sort has been installed long enough to know how they will last over 20 years.

Just to baffle you with even more information however, many single unit inverters have some variant of full scan tracking (SMA calls it global peak, ABB just calls it an MPP scan). This, combined with the bypass diodes that exist in pretty much all solar panels, allows the inverter to bypass shaded sections of panels by adjusting the voltage of the entire array.

My own installation suffers winter shading. Given the additional outlay for a SolarEdge system, and my own personal concerns about maintenance, I opted for this latter approach. Viewing realtime figures for output, I can effectively see panel segments switching off as the shade encroaches. SolarEdge may have allowed me to recover some additional output from these panels, but the rest of the string is happily unaffected.

(On the other hand, SolarEdge/Enphase will also help with varying panel quality, e.g. one of your 285 watt panels may actually be slightly better than another. These systems will allow you to take advantage of these minor differences.)

Many many swings and roundabouts :P

Excellent summary of issues. Just to add, my circumstances (which I admit are unusual), I essentially have three sets of panels - one facing south, two facing west but on different pitches of roof. If my understanding is correct most inverters cope with two strings - i.e. two sets of panels can be set up. All the panels within the string must experience very similar light conditions in order to maximise output. The SE inverter offers more flexibility not just for the odd chimney stack but for far more complicated set ups (like mine) - each individual panel can be subject to its own light conditions. Obviously micro inverters achieve the same thing.

purple_hamster · 24 August 2016 at 7:11AM

Thanks for those replies.

Solar PVs - worth the investment?

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