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

Screwdriva · 1 December 2019 at 2:36PM

Updated system size is 7.3 kW at a total installed cost of £10K. Current ROI is forecast at 6.5 years.

Hi there,

I've recently been quoted for solar panels from 3 installers referred by Solarguide. My guidance to these companies was maximum solar power yield per panel for their proposals. The only company to recommend and offer the option of LG Bi Facial 400W panels was a company called Light Renewables.

Quote:

The LG panels are dual faced and typically generate upwards of 500W peak by capturing energy from the rear, despite being rated at 400W, and are highly regarded the Solar community. They also work better than all other panels on cloudy days. That said, they're a bit more expensive than most other panels at around £250 per panel. I've been quoted £7000 all included for the following:

10 x 400w LG Bi mono panels, 25 year warranty to 86.2% efficiency + 25 year product warranty
10 x Solar optimizers. 25 Year warranty
1 x Solar Edge HD Wave single phase inverter[Battery ready inverter] 25 year product warranty
1 x Solar I Boost water heater
1 x All cable run’s isolators and fitting’s
1 x Micro generation meter
1 x System monitoring
1 x System registration
1 x Front of property scaffolding
(No batteries)

Consumption:

We consume around 3200 kWh of electricity and 5500 kWh of gas. Monthly bills for both via Bulb Energy hover around £85 per month on average.

Return on Investment

Here's where it gets interesting (and where I could use actual user experience!). Given that we will save on hot water heating (natural gas powered Worcester Bosch greenstar boiler, 20+ year old copper tank and radiators), savings from the iBoost will likely apply.

Light Renewables shared the following breakdown:

"Output; This system will generate; 4,500 kWh If you export 10% = 450 kWh of the power, the Export tariff is 0.10p you get paid = £45.00p. Bulb Energy and Octopus will offer between 5.5-10p by 01/20.

Total income of £45.00p per year

Saving’s Output; This system will generate;
4,500 kWh per year if you use 70% = 3,150 kWh of the power per year and you are paying .19p per kWh you will save £598.50p per year on your Electric bill

That leaves 20% of the power 900 kWh per year to heat your water and save you an extra £350.00 per year on your Gas Bill
.
Total income and saving of £993.50.00p per year"

Questions for the community:

1) Does the above sound achievable/ realistic? Would you tweak the proposed system in any way?

2) If not, have we been misled? Based on your actual user experience, what can we expect to achieve realistically when the above information is factored in?

Many thanks in advance!:beer:

1961Nick · 1 December 2019 at 2:51PM

Initial thoughts are that the figures are over optimistic.

Without batteries, if you use 50% of the generated energy you will be doing well.

I think you've got the decimal point in the wrong place in the DHW calculation. Savings are likely to be in the £30 - £35 range.

The amount of energy going to the iBoost is likely to be higher than 900 kWh because you'll probably use less than 70% of the generation.

Generating 4500 kWh pa from a 4kWp system might be a bit of a stretch - very few UK locations can do that & you'd need perfect location/orientation/inclination to be in with a shout.

Screwdriva · 1 December 2019 at 3:02PM

Sorry, I may have misunderstood but are you saying that the heating savings from the iBoost are only £35 per annum?

Normally, I would be inclined to agree with you on the 4500 kWh if it weren't for the incredibly yield that the Bi Facial panels are capable of generating. Have a quick search for yourself incase you haven't.

The house roof is South facing with the sun passing over the property from an East to West arc, located in South East London, totally unobstructed, 3.5 stories high at a 35 degree incline.

What would you estimate the ROI would be in a realistic scenario where we are selling unused energy to the grid during the day? Thanks again!

1961Nick · 1 December 2019 at 4:18PM

Screwdriva wrote: »

Sorry, I may have misunderstood but are you saying that the heating savings from the iBoost are only £35 per annum?

Normally, I would be inclined to agree with you on the 4500 kWh if it weren't for the incredibly yield that the Bi Facial panels are capable of generating. Have a quick search for yourself incase you haven't.

The house roof is South facing with the sun passing over the property from an East to West arc, located in South East London, totally unobstructed, 3.5 stories high at a 35 degree incline.

What would you estimate the ROI would be in a realistic scenario where we are selling unused energy to the grid during the day? Thanks again!

The iBoost is displacing 900 kWh of gas ... 900 x 3.5p = £31.50

You orientation/inclination/location is favourable, but I'd still base my calculations on 4000 kWh pa to avoid disappointment.

so...

2000 kWh self use x 19p = £380

2000 kWh export x 5.5p = £110

(If you can get 5.5p for export there's no point in diverting to your hot water cylinder.)

The cost of the system without iBoost should come down to £6500 giving a worst case ROI of 13 years.

Screwdriva · 1 December 2019 at 4:50PM

Quite far from ideal but better than some of the dismal estimates read on other's ROI. Using your method, this will increase in a better case scenario to

2200kWh for self use -> £425 @ today's rate of .19p. This will likely rise in the future but I've not included this.

2300kWh for export -> £175 @ an expected FIT of 0.075p via Bulb Energy.

£600 p.a, which translates to a 11 year ROI.

Martyn1981 · 1 December 2019 at 4:53PM

Screwdriva wrote: »

Sorry, I may have misunderstood but are you saying that the heating savings from the iBoost are only £35 per annum?

Normally, I would be inclined to agree with you on the 4500 kWh if it weren't for the incredibly yield that the Bi Facial panels are capable of generating. Have a quick search for yourself incase you haven't.

The house roof is South facing with the sun passing over the property from an East to West arc, located in South East London, totally unobstructed, 3.5 stories high at a 35 degree incline.

What would you estimate the ROI would be in a realistic scenario where we are selling unused energy to the grid during the day? Thanks again!

Hiya. I have to agree that 4,500kWh does sound a tad high, but I accept that with height and Solaredge kit, it's possible. In fact a quick check on PVGIS and a random pin in SE London gave me 4,100kWh, so doable, but I'd also suggest working the economics out on 4,000kWh and seeing any extra as a bonus.

But ........ bifacial panels? I may have gotten the wrong end of the stick on these, but I thought they primarily benefited free standing panels - those on ground mounts - which also get some reflection from behind, plus NE + NW generation in the mornings/evenings in the Summer. I didn't think they gave much of a boost to roof mounted PV. I may be wrong.

Cost wise, do you only have space for 10 panels? If you have more space, then you'll most likely find that more panels, but less efficient, is cheaper. Basically, at a certain point you find that panels get disproportionately more expensive, for example:

Made up example:
100Wp panel cost £50
200Wp panel cost £100
300Wp panel cost £150
400Wp panel cost £300

I've exaggerated the figures here, but the principle stands, in that the cost per Wp stays roughly comparable until somewhere in the low 300's, then it ramps up quickly.

If you've got enough space for say 14 panels, then a ~5kWp system of ~350Wp panels would work well, and if there are restrictions on export (typically 3.68kW), then a capped inverter would still work well.

Consumption of PV - Our leccy and gas was similar to you, around 3,000 / 7,000 before PV. We manage to consume around 1,500kWh out of 4,500kWh of generation, so that might be a rough guide for you, though we don't have a water tank nor diverter.

PS - Sorry, hello and welcome, and keep on chatting for tips and advice. M.

Screwdriva · 1 December 2019 at 5:14PM

Martyn1981 wrote: »

bifacial panels? I may have gotten the wrong end of the stick on these, but I thought they primarily benefited free standing panels - those on ground mounts - which also get some reflection from behind, plus NE + NW generation in the mornings/evenings in the Summer. I didn't think they gave much of a boost to roof mounted PV. I may be wrong.

LG recommends painting the roof area directly behind the translucent panels white, to maximize reflection and yes, they can work on rooftop installations if this is done. That said, I haven't come across many who have forked out the extra cash for these on their rooftops.

It is entirely possible that we could exceed 4500 kWh with these panels though we haven't used this in our estimates. (I dropped a pin at my exact location and listed a 4kW system and got a 4400 kWh estimate).

While we could use the remaining 40% roof area, I'm keen to save as much real estate as possible for a future expansion of panels, as technology progresses.

GreatApe · 1 December 2019 at 5:43PM

It doesn't have to be a financial calculation you can get them just because you want them. The only exception would be if you are truely in difficult financial situation but the solution to that isn't solar but to get a higher paying job or invert income somehow

Do it because you find it fun or interesting not because it might save five quid here or there

Screwdriva · 1 December 2019 at 7:38PM

GreatApe wrote: »

It doesn't have to be a financial calculation you can get them just because you want them

As with nearly every financial decision we make in our household, we prioritize economic risk/ reward and a focus on the environment. For example, our cars are made pre-1997, consume little fuel, even by modern standards, and are reliable enough to last for many more decades with proper care. They also appreciate over time, get loads of praise on London roads and are exempt from all taxes and testing. In other words, exceptions to the planned obsolescence that is endemic in modern culture.

This approach is a remnant from when our family didn't have much, and has been passed down over the past 3 generations. Even though we are definitely comfortable today, we try not to deviate from it. So while we can easily afford these panels upfront, a < 10 year ROI is difficult to argue against.

Screwdriva · 1 December 2019 at 7:40PM

Response from Light Renewables after I went back to them following comments on this thread below. In a way, reassuring that they respond even on a Sunday.

"The export tariff I quoted is based on the export rate from 1st January 2020 when the SEG is Rolled out. The only company paying a export rate at this time is Octopus energy. They are paying 5.35p per kWh but in January all the renewable energy companies will be competing for your custom and you can negotiate your export rate.

You normally use half of the energy you produce with your system that's 2250 to 2500 kWh per year but your energy usage will be alot higher than normal. The Gas savings is based on you using 3 kWh per day per year."

Cardew · 1 December 2019 at 8:29PM

Leaving aside the(IMO) optimistic estimate of income, I would take issue with your definition of Return on Investment(ROI).

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

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