Solar Payback

michaels

This is US but suggests solar panels including install costs are cheaper per unit than mains for PV charging.
SolarReviews_EVReport_05 (5) (1).pdf

QrizB

You're unfortunate that your bigger string has the worst orientation (and therefore worst performance).

• 2.19kWp string - 1638.6kWh/yr, 0.748 kWh/W/yr
• 1.46kWp string - 1248.7kWh/yr, 0.855 kWh/W/yr

Getting more watts on your roof would help (yes, I know you've been struggling with installers).

Spies

QrizB said:

You're unfortunate that your bigger string has the worst orientation (and therefore worst performance).

• 2.19kWp string - 1638.6kWh/yr, 0.748 kWh/W/yr
• 1.46kWp string - 1248.7kWh/yr, 0.855 kWh/W/yr

Getting more watts on your roof would help (yes, I know you've been struggling with installers).

Its unfeasible to get any more on the south facing roof, multiple installers have kinda confirmed that.

I do have an East facing elevation but it gets quite a lot of shading on it as the sun moves around and there is a soil stack slap bang in the middle. 

QrizB

Spies said:

I do have an East facing elevation but it gets quite a lot of shading on it as the sun moves around and there is a soil stack slap bang in the middle. 

I'm not sure a third string would really help, anyway. The problem is that your roof aspects are quite small & awkward shapes. I guess that a lot of the extra installation cost is down to having to scaffold two separate aspects. Adding a third aspect facing in a worse direction will bump up the installation cost and provide even fewer kWh per Wp installed.

By any chance do you have a garden with a south aspect where you could put a ground mounted array? Or could you put a solar canopy / conservatory / carport somewhere?

I'm clutching at straws here, as you might guess, looking for better options.

Spies said:

Indeed the main reason is to be green but its hard to ignore the ROI.

If you don't mind a degree of abstraction in your greenification(TM), you'd likely get a better ROI from investing your £4650 in eg. a wind or solar energy company and then buying green electricity from an external supplier.

QrizB

Spies said:

So I know that these days it's really not feasiable for these panels to save you a considerable amount of money now that the FiT scheme has gone and I've tried to base my calculations on realistic use.

Would you say 30 years ROI is a sensible assumption on a system costing £4650?

@Spies that spreadsheet is broken. If you add up the SEG payments they come to a lot more than £27.38.

I've used your figures and I get a total annual return of £220.58, not £156.49. This changes your paypack time from 30 years to 21 years. Still not great but better!

Spies

QrizB said:

Spies said:

So I know that these days it's really not feasiable for these panels to save you a considerable amount of money now that the FiT scheme has gone and I've tried to base my calculations on realistic use.

Would you say 30 years ROI is a sensible assumption on a system costing £4650?

@Spies that spreadsheet is broken. If you add up the SEG payments they come to a lot more than £27.38.

I've used your figures and I get a total annual return of £220.58, not £156.49. This changes your paypack time from 30 years to 21 years. Still not great but better!

With those two arrays, might it be worth saving money and going for a solis 3000 inverter as my simulation shows a peak of 2680w, or would it be better to oversize so the inverter isn't operating near capacity for longevity?

QrizB

Spies said:

With those two arrays, might it be worth saving money and going for a solis 3000 inverter as my simulation shows a peak of 2680w, or would it be better to oversize so the inverter isn't operating near capacity for longevity?

Rule of thumb is that your inverter should always be undersized. If you're confident in your simulation you might want to consider a 2500W one. Looking at the datasheet it might be a better match to your two strings.

EricMears

QrizB said:  By any chance do you have a garden with a south aspect where you could put a ground mounted array? Or could you put a solar canopy / conservatory / carport somewhere?

I'm clutching at straws here, as you might guess, looking for better options.

A ground mounted array is likely to need full planning permission - particularly if you already have roof mounted panels.  However,  there's usually no restriction on siting a new shed etc within your garden (providing it doesn't occupy more than 50% of available space and probably isn't in front of 'building line') and once you have it,  no restriction on fitting panels to its roof.

Spies

I can maybe squeeze another 365w panel on my south facing roof, giving me 1.8kwp.

Exiled_Tyke

QrizB said:

Spies said:

With those two arrays, might it be worth saving money and going for a solis 3000 inverter as my simulation shows a peak of 2680w, or would it be better to oversize so the inverter isn't operating near capacity for longevity?

Rule of thumb is that your inverter should always be undersized. If you're confident in your simulation you might want to consider a 2500W one. Looking at the datasheet it might be a better match to your two strings.

When I was considering my installation I felt I was being short-changed when  the installer told me they were going with a 3kW inverter for 3.3kW of panels, particularly as they couldn't offer a decent explanation. If only I knew what I know now - especially as I have a split roof set up.  But interestingly I was recently reading the annual report of Bluefield Solar which states that they use and inverter to panel power ratio of between 1:1.2 and 1:1.4.   From any other source other than a commercial set up I would treat this number with huge scepticism.    But there we have it. 

What is also worth mentioning (as I don't think it has come up in this thread so far) is that smaller inverters working closer to capacity tend to be more efficient than larger ones working with the same power input.