Help on Solar Panel Setup

QrizB

QrizB said:

Planeteer said:

QrizB said:

A row near the ridge might still be worth it, although I'm not convinced in the current market.

Could you explain this comment? I thought in the current market, any electricity that I feed in would pay back well?

Sure.

Your panels need to get enough sum that they generate a worthwhile amount of electricity. Your west aspect has so much shade that this isn't necessarily going to happen.

I've got a bit more time now so I'll give a longer version of the explanation.

Solar panels only generate electricity when light is falling on them, and the amount of electricity they generate is proportional to the amount of light. Put a panel under full sun at noon and it will generate far more than it does under clouds or in the shade, and (for fixed installations in the northern hemisphere) pointing it south will produce more electricity than pointing it east or west will.

Your roof has two aspects, east and west. The eastern aspect seems to be essentially unshaded, and being at a 35 degree pitch it will get sunlight from dawn until sometime after noon. In the summer this might be as late as 3pm (I haven't done the geometry) but in winter it could lose the sunlight by 1pm. This will reduce your generation capacity considerably; modelling a 1kWp system near Oxford in PVGIS, a south aspect and 35 degree pitch gives 990 kWh/yr but changing to an east aspect reduces that to 795 kWh/yr.

The western aspect, if unshaded, would suffer a similar reduction. The shade however makes it much worse. Looking at your photos, the lower part of your western pitch will spend most of the afternoon (from 2-3pm) in shade even in midsummer, and the upper section is going to suffer for much of the day during the shoulder months and winter. You might only manage to generate 500 kWh/yr from 1kWp of panel on the best part of the western pitch.

Why is this important? It''s because solar panels aren't free and the returns depend on them generating electricity.

Planeteer said:

I thought in the current market, any electricity that I feed in would pay back well?

Let's consider the eastern aspect. Putting 1kWp of panel on the roof might cost £1200, and in a year it might generate 795kWh. If you value those at 15p/kWh (and no-one knows what electricity prices will be in the future), that's £119 a year and a 10 year payback period. Over 25 years your effective return is about 8.7%.

The western aspect is less effective due to shading. The panels will cost the same amount but might only generate 500kWh/yr. At 15p/kWh that's £75 a year, a 16-year payback period and a 25-year return of about 3.8%. That's less than half the return you would get from the same panels on the eastern aspect.

Both those rates of return ignore the costs of eg. replacing the inverter during the life of the system.

Now, you might be lucky and electricity prices might stay high; at 20p/kWh, you're looking at a 6% return from the western aspect and 12% from the eastern one. But you might also find that some time in the next 25 years prices fall back to where they were in 2020 (only two years ago) and the value is only 10p/kWh. At 10p/kWh the 25-year return from the western panelswould be barely above zero.

Screwdriva

QrizB said:

Let's consider the eastern aspect. Putting 1kWp of panel on the roof might cost £1200, and in a year it might generate 795kWh. If you value those at 15p/kWh (and no-one knows what electricity prices will be in the future), that's £119 a year and a 10 year payback period. Over 25 years your effective return is about 8.7%.

The western aspect is less effective due to shading. The panels will cost the same amount but might only generate 500kWh/yr. At 15p/kWh that's £75 a year, a 16-year payback period and a 25-year return of about 3.8%. That's less than half the return you would get from the same panels on the eastern aspect.

Both those rates of return ignore the costs of eg. replacing the inverter during the life of the system.

Now, you might be lucky and electricity prices might stay high; at 20p/kWh, you're looking at a 6% return from the western aspect and 12% from the eastern one. But you might also find that some time in the next 25 years prices fall back to where they were in 2020 (only two years ago) and the value is only 10p/kWh. At 10p/kWh the 25-year return from the western panelswould be barely above zero.

Have you considered the loss of all leaves from those seasonal trees over the next 5 months? Or that a quality inverter will come with a 20+year warranty? 

I'm fairly confident a row of 5 panels arranged in landscape layout on the topmost part of OP's Western aspect will generate normally till ~2 to 3pm. Optimizers will mitigate some of the effects of shading for the the later part of the day, so I doubt the loss of generation will be as severe as you have forecasted.

QrizB

Screwdriva said:

Have you considered the loss of all leaves from those seasonal trees over the next 5 months?

Yes. Generation in the winter months is low. Run the model at PVGIS yourself if you don't believe me.

Or that a quality inverter will come with a 20+year warranty?

Considering that I haven't included any cost for inverter replacement, the length of the warranty is irrelevant. I've assumed it lasts for 25 years.

Optimizers will mitigate some of the effects of shading for the the later part of the day, so I doubt the loss of generation will be as severe as you have forecasted.

Optimisers can't create sunlight, just reduce the effects of inbalance between panels. And they will add another £50 per panel, increasing the capital cost.

I'm fairly confident a row of 5 panels arranged in landscape layout on the topmost part of OP's Western aspect will generate normally till ~2 to 3pm.

I disagree. In the OP's photo, taken at 3pm BST in mid-October (ie. 2 hrs after solar noon) the entire building is in shade.

Hopefully the OP's installer will model the system for him and give a more rigorous assessment of its potential than either of us can with two photos and a quick poke at PVGIS.

Planeteer

Just a big vote of thanks for your very generous analysis on my behalf @QrizB and @Screwdriva

It has given me a lot to think about - and the exam question therefore becomes whether the west roof is worthy of any investment or not. Time to think it through well!

So far the best analysis that I have received for the westroof is as below:

Screwdriva

QrizB said:

Yes. Generation in the winter months is low. Run the model at PVGIS yourself if you don't believe me.

It's not that I don't believe you. It's more that you're claiming low generation in winter and then penalizing the panel placement for a poor return. In the summer, when the sun's much higher, the panels will perform till 2-3pm, maybe even later, reducing that loss. While I do agree they will underperform in the Autumn till the leaves fall, the optimizers will help with branch shading once they do. 

QrizB said:

Considering that I haven't included any cost for inverter replacement, the length of the warranty is irrelevant. I've assumed it lasts for 25 years.

You said "Both those rates of return ignore the costs of eg. replacing the inverter during the life of the system". My point here is that there will not be any cost to replace the inverter should it fail, so nothing to ignore. 

I continue to encourage the OP to landscape a row of 10-12 panels across both aspects of the roof and enjoy the result for decades to come! 

QrizB

Planeteer said:

So far the best analysis that I have received for the westroof is as below:

Your installer has put some effort into that, which is a good sign. That looks like the MCS shading model.

They've suggested that a 2.075kWp array will generate 1056kWh a year. That's 509kWh/kWp, slightly more optimistic than my guess!

The decision is now yours.