Are these options realistic? (aka 'Rate My Solar Plan' :) )

Magnitio

Brimble said:

@Magnitio - thanks for taking the time to address my individual questions. Yes our base load does seem rather high. We have one big fridge / freezer, a chest freezer, security alarm, security cameras, as well as all the obvious (internet etc) but it still seems high. I've been trying to turn things on and off and see how it impacts - will do more of that when I have more time, but already at times I've been able to reduce the base load to under 0.4kw (still high but better than it was). The fridge freezer is erratic - it increases the load by maybe 0.15-0.20kw from time to time, when it's 'buzzing' (i.e. presumably cooling). But thanks for the nudge to start with the basics.   Given your comments on my optimistic usage, I might amend my spreadsheet accordingly.

A good way to check the usage of individual appliances over an extended period (24/48 hours) is to use an energy monitoring smart plug such as this:

https://www.amazon.co.uk/TP-Link-Tapo-Monitoring-Required-P110/dp/B097YBXHTW/ref=dp_prsubs_1?pd_rd_i=B097YBXHTW&psc=1

If you monitor your fridges/freezers, you will notice that the compressor cycles on and off. It should be off far more than on. If you have an old or large fridge/freezer, you may find this is a significant user of electricity and will easily be able to justify the cost of replacing it with a new efficient one. When you've done with monitoring, the plug can then be used as a timer to switch certain things off when not in use.

Hope this helps. Good luck with whatever option you choose.

70sbudgie

I have E-W panels and for the last few months have been getting a good 12 hours of generation of 100W or more. 

I chose not to go for batteries, because I have a 41kWhr EV and although my car can't do vehicle to grid (or house) charging, the technology is there. I couldn't bring myself to spend £5 on a 3kWhr battery (or similar) when I already had a much bigger battery parked infront of the house.

I wish I had gone for an immersion diverter, but am now rectifying that. I am particularly keen to reduce our reliance on gas, which is why I like the immersion diverter, even if it isn't the most cost effective solution right now.

I think the starting point should be to fit as many panels as you can and then design the extras around your usage. If you can generate a lot and are out most of the day, go for batteries. If you have a longer generating day (due to E or W panels) and are at home when you're generating, static batteries might not be for you. If you don't drive V2G isn't really an option!

I'm quite optimistic that the electricity market review could see the price of electricity decoupled from the price of gas within the next 12 months. But then I am an optimistic tree hugger and have noticed that not many people share my optimism! (So accept that it might be misplaced).

silverwhistle

My PV installation is quite old (2013) so didn't go for batteries at the time, and wouldn't do so now for similar reasons to budgie's post. I'd also prefer panels over batteries too.

As a singleton with low usage I do find my peak production leads to a quite large proportion of export (don't worry, I'm on FITs), but I'm S facing so production is naturally peaky and were I to have an E/W split I'd still go for it.

As an Octopus customer (and observer) the decoupling of gas and electricity is something I've thought about too, but it would certainly have interesting repercussions. I've just remembered a couple of grand in a dormant account so maybe looking at Ripple would also be a good idea..

Brimble

So, quick update based on all I've learned from here and elsewhere, then another question...

I've adjusted and re-run all of my Excel models. As a particularly interesting element, I took actual production data per day for all of 2019 for a PV set-up not too far from me - not to assume that future years or my production will necessary follow the same pattern, but more to see how my assumptions would have played out on a day-to-day basis. The main one for me was as follows:

• As many of you keep pointing out on here and on various forums, you definitely can't get much useful info from annual calculations, and whilst monthly calculations are much more useful, even they don't give an accurate insight into your likely usage of energy produced, because weather isn't consistent. So for some months when I had thought "I've got this much surplus in that month, if I divide it by 30 days then it would all fit in my battery and get used each night" - obviously it doesn't work like that. Some days in that month there will be little or no surplus. Other days, there will be far more surplus than my battery can take. I know most reading this already know that, but it was interesting to go through the painstaking task of proving it to myself. I'm much happier with my spreadsheet now.

Anyway, spreadsheet details aside, I'm now relatively confident that I'd like the following:

• 16 x 400w panels on my SSE-facing roof = 6.4kw
• c.10kwh battery
• Possibly 6 x 400w panels on SWW-facing roof = 2.4kw - because I will consume a lower % of the energy from these than my 'original' 16 panel plan, they are not as profitable (i.e. longer payback), however I've been persuaded by what I've read here and elsewhere that they still 'do enough' to make sense for me, including effectively lengthening my production hours slightly.

BUT what size inverter?

Next I need to learn about and decide upon an inverter:

• I've read about over-sizing & under-sizing - all fine.
• I see lots of talk about the 3.68kw export rule. If I understand correctly, this has nothing to do with the panels, and nothing to do with what is consumed / stored locally, it is purely about what can be exported to the grid - is that correct?
• Does the DNO usually approve higher, or is it more often rejected (or approved with a significant cost)?
• With the system above, should I get a 3.68kw inverter, or a 5-6hw inverter, and if the latter (which i expect), should I get it capped at 3.68kw by my installer in order to make the DNO bit easier?
• With the system above, typically what would I lose ('clipping') from being restricted to 3.68kw?
• If I had a bigger inverter but it was restricted to 3.68kw, would that only apply to export, or would that impact my local usage of my production?
• I'm expecting the replies here to be 'get a bigger inverter and seek DNO approval' but I'm not sure I understand exactly what the benefits would be - please excuse my continued ignorance, I am here to learn!

EDIT: If relevant, I don't believe our export will be very high in the long term, as the day consumption and batteries will take a reasonable portion. We will use Outgoing Agile for the remainder while it's available, then probably use iBoost or similar to help replace gas (esp. for the wet underfloor heating) when and if that tariff is removed / significantly worsened.

Magnitio

You are right: "get a bigger inverter and seek DNO approval" is my response. I purchased exactly the same size system as you are proposing and the installers obtained DNO approval very quickly. I can now generate up to 5kW on days like today without any clipping, but it also means that the inverter will power up to 5kW of appliances without drawing from the grid. So the benefits are significant and well worth making the application.

Just to add: with the decent size battery you are proposing, I would put all the panels on the SSE roof for maximum generation. Without a battery, then it probably makes more sense to split them across the SSE and SWW facing roof.

Brimble

Magnitio said:

You are right: "get a bigger inverter and seek DNO approval" is my response. I purchased exactly the same size system as you are proposing and the installers obtained DNO approval very quickly. I can now generate up to 5kW on days like today without any clipping, but it also means that the inverter will power up to 5kW of appliances without drawing from the grid. So the benefits are significant and well worth making the application.

When you say you purchased the same size system, do you mean just the main 6.4kw on my proposal, or the full 8.8kw with the extra 6 panels on another roof? Either way, what size and type of inverter are you using please?

Boffinboy24

 Am curious how different the results were when you modelled daily?

Re: the inverter, if you really want to check you can use SMA Sunnydesign for free to put your panels on a 3D image of your roof (you have to create it based on satellite image). I think solaredge have a similar web portal you can use. You can then see how different inverters fare - though it’s just SMA models it will probably be close enough. My inverter is massively undersized - I have a 3.68 on my 5.6 array. I was initially quite annoyed when I realised this but the installer insisted I will only lose a couple of percent of total generation which will be at peak times, vs the inverter potentially firing up earlier and running later on low sun. I modelled it myself on the SMA software and it indeed only came out 3-5% lower than a 5kW inverter. that’s not to say I’d recommend you get a massively undersized inverter, but thought it was worth sharing. It does annoy me how much clipping I get on sunny days, but of course I can’t tell how much more I am getting on the shoulders/low sun days. With a battery you probably want to maximise generation, so maybe undersize 10-15% and I would guess you’d only get a bit of clipping in rare circumstances that’s made up for by generation outside peak. 

Magnitio

Brimble said:

Magnitio said:

You are right: "get a bigger inverter and seek DNO approval" is my response. I purchased exactly the same size system as you are proposing and the installers obtained DNO approval very quickly. I can now generate up to 5kW on days like today without any clipping, but it also means that the inverter will power up to 5kW of appliances without drawing from the grid. So the benefits are significant and well worth making the application.

When you say you purchased the same size system, do you mean just the main 6.4kw on my proposal, or the full 8.8kw with the extra 6 panels on another roof? Either way, what size and type of inverter are you using please?

I was suggesting the 6.4kw on SSE roof. If you went for the additional 6 panels, then you might want to consider a 6kW inverter or two seperate inverters; this will add more to the cost of course.

I have a Huawei SUN2000-5KTL hybrid inverter which is described as 5kW, but handles 5.5kW peak. This is connected to 10kWh of Huawei Luna2000 battery packs. The panels I chose are REC Alpha Pure which are quality panels manufactured in Singapore with a low carbon footprint.

Brimble

Boffinboy24 said:

 Am curious how different the results were when you modelled daily?

Right, this might be tricky but I'll try to explain as best I can (or skip to bottom for ultimate impact ):

Daytime usage:

Original: Take the smaller of the production per month and the daylight usage by month, and multiply by 80% to estimate inefficiencies caused by erratic weather.

New: Look at actual data – not exactly day by day, but grouped into 5 bands of productivity from very low to very high, model pattern of usage / storage behaviour for each band.

Result: This lowered my usage for winter months, but significantly increased it for summer months. I think that’s simply because the weather is a little bit more consistent in summer than in winter, and also our overall need is no different between summer and winter (heating is gas-fuelled underfloor) but I’m not 100% sure of that.

 

Battery storage:

Original: Take the smaller of the surplus production from above, the battery capacity total per month (obviously flawed), and the monthly usage not already covered by the above, and multiply by 90% to estimate inefficiencies caused by erratic weather. (My logic for using 80% above and 90% here was that the battery adds flexibility in relation to usage time, that’s its whole purpose).

New: As for daytime usage, look at actual data – not exactly day by day, but grouped into 5 bands of productivity from very low to very high, model pattern of usage / storage behaviour for each band. For each band, once daytime usage has been removed can use surplus to charge the battery (only up to its capacity of course).

Result: Generally this reduced how much production I can use with my battery by quite a lot, about 30% in fact. As you might expect, this is because my Original mechanics for that part made no sense, i.e. monthly is irrelevant, the battery can only store up to its capacity each day. Naturally the reduction was more severe March to September where there is more production.

Overall result:

My %s for daytime used and ‘battery used’ from my production changed from (respectively) 38% + 37% = 75% to a slightly more conservative 39% + 28% = 67%. I know many will say that still seems too high, particularly the daytime usage one, but it’s what the actual data is telling me so… ???

Right, next job over the weekend will be to investigate Sunnydesign etc, sounds like another interesting modelling adventure.

Brimble

Magnitio said:

Brimble said:

Magnitio said:

You are right: "get a bigger inverter and seek DNO approval" is my response. I purchased exactly the same size system as you are proposing and the installers obtained DNO approval very quickly. I can now generate up to 5kW on days like today without any clipping, but it also means that the inverter will power up to 5kW of appliances without drawing from the grid. So the benefits are significant and well worth making the application.

When you say you purchased the same size system, do you mean just the main 6.4kw on my proposal, or the full 8.8kw with the extra 6 panels on another roof? Either way, what size and type of inverter are you using please?

I was suggesting the 6.4kw on SSE roof. If you went for the additional 6 panels, then you might want to consider a 6kW inverter or two seperate inverters; this will add more to the cost of course.

I have a Huawei SUN2000-5KTL hybrid inverter which is described as 5kW, but handles 5.5kW peak. This is connected to 10kWh of Huawei Luna2000 battery packs. The panels I chose are REC Alpha Pure which are quality panels manufactured in Singapore with a low carbon footprint.

Are there particular pros and cons of having two separate inverters rather than one bigger one?

Also would even a 6kw inverter be enough? If the overall PV system is 8.8kw, then a 6kw inverter would be under-sized by 47% (am I calculating that correctly?), it sounds quite a lot, even with the two different arrays having different peaks... How can I estimate the costs / impacts of running a 8.8kw system with a 5kw, 6kw or 7kw inverter, is there any useful online resource for that? (Happy to try to model it myself if I can understand what variables I need to manipulate and how). EDIT: Hold on, this is presumably what Boffinboy24 has suggested I look at )