Help thinking through solar and battery options

sunnykent · 25 May 2019 at 5:42PM

Hi all, have been a short time lurker and now thinking about a solar installation and was hoping you could all help think through the options. Sorry that this is a very long post!

Following an extension build I'm now finally in a position to go for solar. Main reason is to insulate myself from energy price rises (plus being a bit more green). I had wanted the installation to pay back over a reasonable period (10-12 years), although having got some figures I am increasingly thinking about it as a % yield on the investment made, compared to the alternative option of using the money to reduce my mortgage (interest rate currently 2.24% but variable).

Property is in South East England, and I have 2 suitable (and connected) roof spaces - a S facing sloping roof and a reasonably large flat roof that opens out to both S and E directions. No EV, but the household uses a lot of electricity (9,500 kwh/year)

Finally, I have a tree on my S border - having been very geeky with observing the position of the shade over the day and calculating angles, I think it will shade the S sloping roof by 7% and the flat roof by 24% (combined is around 15% shading). Cutting down the tree would probably be £1-2k (so not ideal - plus it's not my tree...)

I've asked a couple of local installers to quote, have one back and waiting on the other. Options so far are:
1) LG 340wp panels with enphase microinverters. 9 panels on the S sloping roof (arranged quite densely with both horizontal and vertical orientations, but mostly hidden from the road and completely hidden from the back garden) and 9 in an E/W configuration on the flat roof (total - 6.12kwp. Installer says 5,943kwh/year but I reckon from PVGIS and my shading calcs it's more like 4,860kwh/year)
Price is around £8,500 inc VAT
Using my generation figures, I get roughly a 15 year payback or a RoI of 8.1% assuming electricity costs go up by 5% a year and I get guaranteed income for 20 years
My model suggests I'd use 58% of the generated power

2) JA Solar 310wp panels also with enphase. Same configuration (9+9), i.e. 5.58kwp. Installer says 5,420kwh/year, my model guestimate is 4,431kwh/year
Price is around £6,600 inc VAT
That gives a 12 year payback or RoI of 10.5% assuming income for 20 years. (If I reduce to 15 to cover the reduced JA guarantee, RoI goes down to 7.7%
My model suggests I'll use 63% of the generated power

3) Sunpower 390wp panels, this time with 7 on the S roof (much neater installation) and 12 on the flat roof (using more of the shady space than the other installer suggested), and using a solar edge inverter. I fear the price is going to be very high (will get the price soon), and not compensated by the additional kWh (of which model suggests I'd use up 50%)
I've tried to create a worse case scenario with the panel cost being an extra £100 each over the LGs, which blows out the payback to 17 years with an RoI over an extended 25 years (assuming new inverter in year 15) of 6%

So would love any ideas and suggestions, particularly around:
- which would you go for, with the different combinations of guarantees and RoI/payback periods?
- how much better is Sunpower. Is it worth paying for the better guarantee and the extra power that I may use one day (see also battery below)?
- Is cheap and cheerful best - how much risk should I take on the JA Solar panels failing/deteriorating
- Am I being too conservative on my generation figures (sourced from PVGIS) - do you ever make the amount the installer says the system will make?

Now for the battery....
2nd installer also sells Tesla Powerwall 2 batteries. Right now waiting for the price, but have been thinking about the economics of using a time-of-use tariff (like Octopus Go) with the battery and solar, by
- charging the battery on the 5p night cheap rate (which is cheaper than the export rate, either Octopus output agile or the fixed output at 5.5p/kWh
- charge the battery again if needed during the day with the solar (expecting it to be discharged in the evenings) and exporting the rest of the unused PV output
Doing this I *think* would give additional savings of £445/year against my current Avro (13.125p/kWh) tariff, which may just about give an RoI of 6.5% if electricity prices go up 5% a year on average, assuming the battery comes in at around £6,800

Some questions on this:
- I get it - the economics on batteries are still iffy. But I like the idea of reducing my exposure to future electricity price increases!
- Has anyone else done something similar. Is my thought process sound or suspect?

Many thanks in advance for all ideas!

Hexane · 25 May 2019 at 10:09PM

sunnykent wrote: »

a RoI of 8.1% assuming electricity costs go up by 5% a year and I get guaranteed income for 20 years

Who is providing this guarantee? Feed in tariff income is no longer available.

sunnykent · 25 May 2019 at 11:28PM

I simply mean that the LG panels come with a warranty as does the solaredge inverter, Hence the returns from reduced electricity import plus the current SEG-like export tariffs (e.g. through Octopus) are effectively guaranteed (albeit warranted is probably a better adjective)

(On a separate note I also realise there is no guarantee that Octopus will continue it's export tariff, or that the government will get itself in gear to launch SEG propertly, or that import/export tariffs won't move in a way that reduces the income)

Martyn1981 · 26 May 2019 at 7:25AM

Just some quick thoughts.

With the removal of the FiT subsidy, solar is now a tough option for many. However your high leccy consumption will mean you can get better value from it than many, as you can expect to use more generation, and thereby remove more import units.

Panel size obviously is important, but as you've noticed/suggested there is a point where the costs rise dramatically. I have to admit to not knowing where that point is now, but suspect it's around the 340Wp you've mentioned, and would guess that the 390's are quite a lot more expensive per Wp. But, a small increase in price/Wp is acceptable as a lot of the other costs are fixed.

From your post, it seems that 9 panels will fit on the south facing roof and 12 on the flat roof. It might be cheaper to go with more panels of a lower Wp, when considering options.

If the system is larger than 3.68kW, then you will need prior permission from your DNO first, rather than automatic post install registering. The 3.68kW is on the potential output of the inverter(s) not the kWp of the panels. If you went with the biggest option listed of 7.41kWp and ran it through a 5 or 6kW Solaredge inverter, then permission for 5 or 6kW would be needed.

Speaking of Solaredge, my personal experience is that both my systems, which I'd guesstimate have approx 10% shading ESE and 20% shading WNW both perform roughly at the whole PVGIS figure, with annual totals over the last 8yrs at around 95% to 105%.

I can see how a battery improves things for you, but I've no idea how the economics of the additional expenditure will work out for you, too many guesses.

Edit - I'm not sure a +5% figure for leccy is reasonable. Going forward, cost increases above inflation are looking less and less likely, so something closer to 'normal' inflation may be a better guess - but what that is (2 .. 3 .. ?) I'm not sure.

sunnykent · 26 May 2019 at 9:51AM

Thanks Martyn. I'll find out the SunPower costs this week, so all will be revealed in time.

I'm hoping the new Sunpower panels promoted by the installer aren't as expensive as feared (at least compared to LG) - there is an article from Forbes (can't link as new forum member) which suggests the cost per watt should be proportionately lower than other SP panels...lets see!

"SunPower has been scaling up production of its next-generation technology panels, which offer high efficiency at commodity-panel costs. The panels are likely to offer conversion efficiencies of around 23% (roughly in line with SunPower’s top-of-the-line X-Series panels), although costs are likely to be between 30% to 40% lower."

The quote I had so far had the DNO approval built in (i.e. managed by the installer). Is there any experience of how long UKPN take on this? I see all these sunny days passing me by (maybe I'm getting a bit too serious about all the PV generation...)

I'll post back once the numbers are in :j

1961Nick · 26 May 2019 at 10:51AM

All of the options provide plenty of 'peak' midday power. The key to getting the most out of the system is to lengthen the generating period as far as possible. For this you'll need micro inverters to overcome the shading & as much generation East & West as you can fit. The South facing panels could be cheaper/lower output if that'll free up budget for the other roof.

A longer generating day will mean that battery storage won't be so costly & will become viable sooner - you don't want to be discharging batteries when the sun is still shining!

Martyn1981 · 26 May 2019 at 12:09PM

sunnykent wrote: »

Thanks Martyn. I'll find out the SunPower costs this week, so all will be revealed in time.

I'm hoping the new Sunpower panels promoted by the installer aren't as expensive as feared (at least compared to LG) - there is an article from Forbes (can't link as new forum member) which suggests the cost per watt should be proportionately lower than other SP panels...lets see!

Yep, a few restrictions till you build up your post count.

I think this is the article:

SunPower Q4 Preview: NGT Panel Rollout In Focus

I found this datasheet on Sunpower 390's, but this is the 'old' Maxeon technology, not the new NGT panels.

Reed_Richards · 27 May 2019 at 8:00AM

I have had a solar PV plus battery system since March. Despite the fact that I generate more power than I can use, I still use mains power, about 1 kWh per day. I am not completely sure why this is but one factor is that the system does not respond immediately to a surge in demand. So if I boil a kettle at noon the battery takes some few seconds to ramp up the output to meet demand although the PV + battery can meet the demand. When the kettle switches off, the battery sends power to the grid for a few seconds whilst it ramps itself down.

If I boil a kettle at night, the battery on its own cannot output enough power to meet the full demand.

These are issues you should consider when working out what equipment to use and how much you can save.

Martyn1981 · 27 May 2019 at 8:20AM

Reed_Richards wrote: »

I have had a solar PV plus battery system since March. Despite the fact that I generate more power than I can use, I still use mains power, about 1 kWh per day. I am not completely sure why this is but one factor is that the system does not respond immediately to a surge in demand. So if I boil a kettle at noon the battery takes some few seconds to ramp up the output to meet demand although the PV + battery can meet the demand. When the kettle switches off, the battery sends power to the grid for a few seconds whilst it ramps itself down.

If I boil a kettle at night, the battery on its own cannot output enough power to meet the full demand.

These are issues you should consider when working out what equipment to use and how much you can save.

A friend with a Powervault system has described exactly the same. He was told the ramp up, and ramp down is designed in to reduce wear and tear on the battery, and lengthen life cycles. I don't know if this is true, but it sounds fair(?)

sunnykent · 27 May 2019 at 9:55AM

Interesting, thanks. I have around 0.5kw background load (lots of internet/networking gear plus a big fridge/freezer plus an extra freezer). If the battery is servicing this load, and the load increases (e.g. turn on electric oven) - does the battery adjust immediately or also with a delay?

1961Nick · 27 May 2019 at 10:55AM

sunnykent wrote: »

Interesting, thanks. I have around 0.5kw background load (lots of internet/networking gear plus a big fridge/freezer plus an extra freezer). If the battery is servicing this load, and the load increases (e.g. turn on electric oven) - does the battery adjust immediately or also with a delay?

If the battery is already discharging, the ramp up time is negligible - a couple of seconds. If the inverter has to flip from charge to discharge, it can take 20 - 30 seconds depending on the load.

Help thinking through solar and battery options

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