The pros and cons of an over spec PV install (ie going large!)

Zarch

I'm curious to get the thoughts of folk about getting as much PV on your roof as possible.

I've just had my annual electricity statement through and once again my yearly usage has dropped.

In the past few years my usage has dropped through 4200kWh, 4000, 3800, 3600 and the most recent one says an estimate of 3400 for next year. Not sure how realistic that last estimate is, but likely 3600 again for sure.

I imagine some of this being conscious usage decisions (lights off, closing laptops etc etc) and some just from buying more energy efficient appliances and devices.

As you may have seen i'm considering Solar and am in a lucky position that I might be able to go as high as 17 x 300w (5.1kWh) or even 5.6 with 330w panels. This install would be multiple aspects/directions and a tricker (costly) install whereas a more straightforward 11 x 330w southerly facing system could be an option.

So with a below average annual consumption around 3500kWh what are the pros and cons of

a) going with a 3.5kWh system that would go a long way to cover much of our yearly usage?

b) going with a oversized 5.5kWh system that would produce more than we are likely to currently need or use?

Obviously the cost of more panels and trickier install increasing the whole install price. But it should pay for itself in the long run despite the current paltry FIT.

Are we future future proofing ourselves? 20 years is a long time for things to change? Electric cars? Battery storage? Anything else?

Look forward to hearing your opinions.

Martyn1981

Brilliant question, and I suspect no right answer.

Firstly, sorry to bubble burst but 3,500kWh's is about average, not low. Not a criticism, just clarifying a baseline.

Regarding the amount of PV and generation I'm yet to hear anyone say that they have 'too much generation'. So consideration will be needed for each individual install.

The main reason for going bigger IMO is that doing it later will cost far, far more, so make the most of it today. Hopefully going bigger reduces the cost per kWp installed as many costs are fixed, and even if you need more scaffolding it shouldn't be twice the cost, just 'a bit more'.

However, I don't want to mislead or trick, so whilst the cost per kWp may go down, you need to get quotes and play with PVGIS to see the impact on the cost of generation, since the additional PV (in your situation) won't generate as much pa as the easier south facing option.

Also, in your case, you can hit a good amount of PV with just the south facing aspect (upto 3.6kWp), whereas for some, they may need multiple rooves to get to a decent size array.

Also consider the value of west facing panels, as teatime demand is one of the hardest to avoid, so SW to W facing PV generates higher value leccy since you are more likely to consume it than say mid day generation ..... IYSWIM.

I think future proofing is a good idea whilst you are doing the job and FiTs/export payments exist.

I would however suggest caution in simply comparing generation to household demand since a lot of the time you will generate too much and export, and the rest of the time you'll generate too little and have to import, so to reduce import significantly (especially without batts) you do need to go bigger. We generate around 4.5MWh pa, but still import half of our 3MWh of consumption .... sadly.

Edit - I should add that this isn't a criticism of small PV systems either, since consumption will be higher. EG a single panel system will be pretty expensive per kWp installed, but since you'll consume the vast majority of all generation it also provides the highest value generation. A two panel system will export a bit more, and so on, so diminishing returns per kWp installed, but at the same time diminishing costs per kWp installed. Boy - that got confusing real fast!

Heedtheadvice

Good advice there from our resident expert!


I used to buy 3.6kWh annually now have S facing array (4kW peak) that produces 3.6kWh (minimum) p.a.


Bought leccy now down to 2.6kWh so saving about one third.


As Martyn says I would love a bigger system so that in the poor months I could get more generation. In the good months generate far more than I consume but still need to buy as generation and consumption do not synchronise. Import falls to about 2kWh daily though.


So three choices:
Live with it and the realisation that better use of generation (apart from 'diverters' possibly some automation or load spreading)is not so easy and involves dimishing returns;


Get battery (or other storage) to store generation to when needed. Currently still costly (Martyn has posted chapter and verse);


Increase pv generation capability. Again fairly costly for the extra benefit. Mostly of use in poorer months (so limited time) or early and late in the day (such as M's post above) and then either limit generation by the inverter or apply for max generation dispensation.

In my opinion the latter two options are not very economically viable compared to thebalternative of the simple grid paid for import.
It does however gall when winter generation does not meet basic electricity need (i.e. ignoring high demands of kettles, cooker, heaters etc) or when I export lots for most of the year and cannot use it there and then but need to buy it shortly afterwards! Such as putting the kettle on outside peak generation periods (light cloud and around midfle of the day)!



Until storage costs reduce significantly, like most people I expect, I just live with it and try and balance consumption with generation periods as best I can.


BUT you zre right to think about it at the outset and before your system is registered for FIT as any later system changes would probably affect your FIT agreement -for the worse!!

pinnks

For me the decision to go larger was with the benefit of hindsight, i.e. the expensive way by having a second system installed, because none of the installers who quoted actually considered or even commented on the spread of generation over the day. All they focused on was the fact that over the year you might get about 5% more FiT if you split the array 50/50 across the two directions.

My main/first roof faces just south of west and I spent ages considering the impact on production of the chimney at the south end of the roof only to find once the system was installed that my pre-midday generation is pants. i.e. until the sun peeks round to the west of south I get a couple of hundred watts. Within 30 minutes of that happening I am up to max for my system. The chimney shadow is long-gone by that time and makes virtually no difference to the amount I generate during the morning.

This irritated me because nothing other than base load was covered in the mornings. No kettle, no washing machine etc etc.

My second system is on my roof at 90 degrees to the main array, so just east of south and in the full sun from early morning. Even though it is only half the size of the other array, from about 9am I can make the coffee, use a low-wattage kettle and generally cover all of our use. Washing goes on at about 11:30 as the other array comes properly on-line.

As Mart says, the benefits of the west roof come into their own in late afternoon as I generate 1kW plus until after 8pm at this time of year, so the evening peak use is pretty much covered by PV.


If I had the roof space I would put roughly equal amounts at 3kWp plus on each of east, south and west-facing roofs. That way you would pretty much cover even the washing machine from 8am right through to 8pm...

More costly of course but also maxing out on the green stuff as you would be exporting tonnes to the grid for use by your neighbours. Whether the maths quite pans out with the current FiT is a good question but once batteries become economic you could be virtually self-sufficient for a big chunk of the year - maybe 10 months with your overall use.

I know, I'm a dreamer but that is what I would do, roof and pocket depending, of course.

Martyn1981

pinnks wrote: »

If I had the roof space I would put roughly equal amounts at 3kWp plus on each of east, south and west-facing roofs. That way you would pretty much cover even the washing machine from 8am right through to 8pm...

My plans/dreams involve a small 4 panel system in the garden facing SSW and steep pitched, around 50deg. The idea here is to boost winter gen when E/W systems are very poor.

Just 1.3kWp would increase Nov, Dec & Jan generation by about 50%.

This is all part of the battery plans to reduce the VAT on install from 20% to 5%, assuming the 4 new panels can be added in to the existing ESE SolarEdge system. No FiTs, and no export but as costs come down it's all about tweaking.

NigeWick

Zarch wrote: »

Are we future future proofing ourselves? 20 years is a long time for things to change? Electric cars? Battery storage? Anything else?

I live in South Lincolnshire and had a 4kW system with a diverter to heat water fitted three years ago. I'm averaging 4,000kWh per year.

Last year, decided to get an electric car. Charged it on Economy 7. last September, I ordered a Tesla Powerwall 2 and Zappi smart charger that will use spare solar to charge the car battery. Powerwall is £5,000 plus VAT and installation. If one just buys the battery, VAT is 20%. Bought as part of a system, VAT is 5%. I therefore had 2 300W panels added to my system "for free."

I think it won't be many years before all new cars are battery electric ones. This means that if you can afford a larger system, it's what I'd go for. If you're lucky like me and can afford to be an early adopter, I believe larger system battery and BEV are the way to go.

V2G is definitely coming soon but I'd still have my separate battery for when I'm away from home and SWMBO is staying there.

Coastalwatch

Zarch wrote: »

I'm curious to get the thoughts of folk about getting as much PV on your roof as possible.

Are we future future proofing ourselves? 20 years is a long time for things to change? Electric cars? Battery storage? Anything else?

Look forward to hearing your opinions.

Hi Zarch,


With the considerable help of this group here we had a PV array installed three months ago and went for the largest layout our premises would accommodate. It is 8.85kW but in the winter months barely supplies sufficient energy for our winter consumption, circa 2.5k kWh annually, 7kWh daily. Even at this we shall need storage capacity to cover for the days of poor generation. That is once costs reduce to a sensible figure.
We shall be using another 2.5k kWh!!!8217;s annually charging our EV in the most productive months, possibly 9/12. Which leaves just under the 50% remaining for export and something close to a clear conscience for the payment received to cover it!!!8217;s generation.
As we consume around 10k kWh!!!8217;s of gas for heating then as advised here is another area where additional generation could be utilised in spring and autumn.
From our experience we have no regrets about Going Large and would do similar again with the possibility of investigating higher output panels(295W installed) if costs stacked up.
I guess it!!!8217;s just how far down the line you wish to travel and the funds available to cover the initial outlay that determines which you choose!:)

sevenhills

I have read the odd post about the weight of the panels and building regulations, is this ever a real concern?

Martyn1981

sevenhills wrote: »

I have read the odd post about the weight of the panels and building regulations, is this ever a real concern?

Some of the newer builds have rooves built that just meet the winter snow loadings, so they may need beefing up. Hopefully that's relatively simple by adding extra supports, perhaps a purling running across the roof (inside) distributing the weight to the outside block walls.

For most rooves they are probably strong enough already when you consider that the panels have very little point loading. At around around 20kg each, they will add about 300kg to the roof, but spread evenly, and the weight of the bottom and side panels will be going straight into the structural walls.

The roofers with their 70-100kg weight pointed loaded onto each foot as they walk around, will be far greater.