7.3 kW Bi Facial Rooftop Panels: ~7 year ROI: Fact OR fiction?

Martyn1981

Screwdriva said:

I have but estimated the annual earnings difference between a 5kW inverter and my 3.68kW to be < £40. (~3kWh per day X 18p per kWh X 60 days per year).

Am I right in thinking that's simply the 'loss' due to clipping? As the smaller inverter may be boosting generation the rest of the year due to higher efficiency at low(er) generation levels? So the net difference might be smaller.

Screwdriva

Martyn1981 said:

Am I right in thinking that's simply the 'loss' due to clipping? As the smaller inverter may be boosting generation the rest of the year due to higher efficiency at low(er) generation levels? So the net difference might be smaller.

Spot on. Sadly, it's quite hard to calculate efficiency gains but in reality the difference would be negligible relative to the price of a new inverter (minus value of the existing unit).

Nick_Dr1

Depends on how fussed you are, but I bet the 3.68kW limit is an AC limit, not a DC limit. If your current inverter happens to be hybrid then you can add a battery and anything over 3.68kW can be used to charge the battery up to the total limit of either the inverter or the charge rate of the battery. If you regularly have the potential to generate more than 3.68kW then this additional power can be significant.
Of course if your inverter is not hybrid then its probably a waste of time!

Screwdriva

Nick_Dr1 said:

Depends on how fussed you are, but I bet the 3.68kW limit is an AC limit, not a DC limit. If your current inverter happens to be hybrid then you can add a battery and anything over 3.68kW can be used to charge the battery up to the total limit of either the inverter or the charge rate of the battery. If you regularly have the potential to generate more than 3.68kW then this additional power can be significant.
Of course if your inverter is not hybrid then its probably a waste of time!

That's correct. And yes, it is a hybrid inverter. We had considered adding a DC coupled battery to the system as an option but our import/ consumption is really low so we may never recover the cost.  

NedS

Screwdriva said:

I have but estimated the annual earnings difference between a 5kW inverter and my 3.68kW to be < £40. (~3kWh per day X 18p per kWh X 60 days per year).

Great, thank you.

Screwdriva said:

Martyn1981 said:

Am I right in thinking that's simply the 'loss' due to clipping? As the smaller inverter may be boosting generation the rest of the year due to higher efficiency at low(er) generation levels? So the net difference might be smaller.

Spot on. Sadly, it's quite hard to calculate efficiency gains but in reality the difference would be negligible relative to the price of a new inverter (minus value of the existing unit).

and as you say, based on those figures may take 20 years to break even so you are almost certainly (financially) better off just packing the roof with as many panels as you can physically fit.

Solarchaser

Screwdriva said:

Nick_Dr1 said:

Depends on how fussed you are, but I bet the 3.68kW limit is an AC limit, not a DC limit. If your current inverter happens to be hybrid then you can add a battery and anything over 3.68kW can be used to charge the battery up to the total limit of either the inverter or the charge rate of the battery. If you regularly have the potential to generate more than 3.68kW then this additional power can be significant.
Of course if your inverter is not hybrid then its probably a waste of time!

That's correct. And yes, it is a hybrid inverter. We had considered adding a DC coupled battery to the system as an option but our import/ consumption is really low so we may never recover the cost.  

I wonder if it's worth running the figures?

I guess you would have to decide on a battery system, and I assume from previous comments, you would go for the Tesla system which is pretty far from price competitive in terms of the many other makes out there, however if you consider all that you export at various times of the day due to originally being oversold the solar, and upping that again with your expansion, and think instead about moving a large proportion of it to export at the optimum flux period rather than wherever during the day it happens to fall, and then in the winter, charging the battery with off peak and again exporting at the optimum flux period, I do wonder what the calculations would work out at.

So to be clear, not talking about self use, more about optimising your payback through exporting at the time which makes most money.
You have the potential of 7kw of panels producing more than they currently are when inverter is throttling, so it could almost be seen as free solar, since you can't use it just now.
Thinking along the lines of every day exporting 10kwh through the 3 hour window and getting what £3.50 odds is £1200 a year. More to it than that obviously. 

And you would be against using a more price competitive product, which would again change your calculations for the worse, but still worth running the figures.

I can't do flux as I'm a high electric consumption household and their off peak offering is too short and too expensive, but when you have such a small usage, a battery would easily help you maximise the returns, unless you go for a very high priced battery

End of the day, octopus are trying to get people to put more power into the grid between 4 and 7pm as that's when it's dirtiest, so it would be a virtuous endeavour, and may well end up more profitable even after roi for the battery

Screwdriva

Solarchaser said:

I wonder if it's worth running the figures?

I guess you would have to decide on a battery system, and I assume from previous comments, you would go for the Tesla system which is pretty far from price competitive in terms of the many other makes out there, however if you consider all that you export at various times of the day due to originally being oversold the solar, and upping that again with your expansion, and think instead about moving a large proportion of it to export at the optimum flux period rather than wherever during the day it happens to fall, and then in the winter, charging the battery with off peak and again exporting at the optimum flux period, I do wonder what the calculations would work out at.

Absolutely. I have yet to find a single "calculator" that has proven helpful with Intelligent Flux (IOF). That said, I've come up with the following rough estimate so far:

Assuming ~6500 kWh estimated total annual generation 

- 5500 kWh will be exported via IOF X .24p = ~£1320 earned 

(Octopus will pull from the PW3 during high grid demand times but not all of it will export during the peak window - hence the 24p blended rate per kWh)

2) That leaves the remaining 1000 kWh for self consumption:

- 1000 kWh X .23p = ~£230 saved

In total we are looking an annual earnings + savings of £1550 with a PW3 on IOF.

vs.

As things stand today with "Standard" Flux with no battery.

~6500kWh X .18p =~ £1150

Conclusion:

£1550 -£1150 = ~£400 incremental earnings on the PW3.

£7K installed cost of a PW3 = 17.5 year payback period. 

Sadly, it would appear that less is still more. That said, I am on the hunt for a cared for 5kW SolarEdge inverter which I hope to swap at no cost with my 3.68kW like for like.

Solarchaser

Yeah I think you will continue to struggle for a calculator that would handle the conversion,  as its really a day by day calculation and depends how much solar you make on that day, it's quite a "unique to you" thing.

I tried running them a few different ways and came up with roughly similar numbers, within £100 either way of what you have suggested.

You have over 7kw of solar, and so changing to a 5kw inverter does make sense imo, and could make a difference to the calculations as you would then be able to export 15kwh instead of just over 10 in that 3 hour period.

At £400 difference a £7k power wall just can't make sense. 
A £1500 seplos could though, and there are several forum users using them now, for around a 4 year payback

Screwdriva

Solarchaser said:

You have over 7kw of solar, and so changing to a 5kw inverter does make sense imo, and could make a difference to the calculations as you would then be able to export 15kwh instead of just over 10 in that 3 hour period.

Agree but it is split across 3 aspects, resulting in a broader, less peaky curve across the day. I suspect the real amount lost due to clipping sits between 3-5kWh between 11-2pm on a clear summer day from late May to early August only. 

Solarchaser said:

At £400 difference a £7k power wall just can't make sense. 
A £1500 seplos could though, and there are several forum users using them now, for around a 4 year payback

I wondered when that suggestion was going to make an appearance!

Nick_Dr1

Why not slap on a small second hand Pylontech just for the hell of it? You are right that the true excess is likely to be not great so maybe even a single 2.4kW battery would be not too expensive and it might be worth it just for the entertainment value if you are a data nerd!