New Solar PV design - split system (u/floor heating + domestic supply)

[deanwilts]

Hi all.

I'm new here and I'm not an electrician - I was hoping to get a little bit of advice on a solar installation that I'm looking into.

First off, I won't be doing this installation (I will get an electrician to do it), but I would like to be involved in the design of the system and to at least understand it so that I can make informed decisions around some elements, so I'd really welcome any guidance. I do have a background in electronics so have a rough idea of the physics etc involved and know enough to leave the dangerous stuff to an expert.

I have a slightly complex domestic mains setup. We have an old 18c house which we recently refurbished. When we did, we went all electric (we're in a remote location and I'm not a fan of on premise oil). The heating is underfloor electric element and as a result of having it the electricity board required us to upgrade our supply.

So we have 3 phase coming in, out of which we are using 2 of the 3 phases. One consumer unit (phase 1) feeds the underfloor heating and 9kw hot water boiler (which in turn heats a couple of cylinders) and the other consumer unit (phase 2) powers the rest.

Ground floor heating when all switched on is about 18.5kw and upstairs is about 12kw. For the rest of the house I've never seen us go over 10kw, and typically we peak at about 8kw, depending on if the dryer/washing machine etc are running. We're all LED lighting and actually don't use much, other than the floor which is thirsty...

What I'd like to get out of a solar PV design is to get the floor and hot water as much as possible off-grid. I have no need or desire to sell electricity back to the grid and instead would like to use as much of what we generate as I can. I appreciate I need to be grid tied - or at least I think I do - in order to have the grid as my backup for when there's no sun but I'm assuming the value of that will be peanuts and I'd rather try and use what I generate than sell it.

It was always the plan to do this when we bought the place - the cost of electricity increasing significantly in a couple of months has made us want to get on with it now.

In basic logic, I am thinking of something like this:-

In winter

1. In the morning, pump all available solar PV energy into the downstairs floor.
2. Any excess, pump it into the hot water cylinders (they have immersion heating elements currently unused)
3. Any excess beyond that pump into batteries for later use
4. Any excess beyond that use for the domestic circuit
5. In the afternoon, use to offset the domestic supply (possible manual override if it's really cold and we need more heating)

Any shortage, pull in either from battery if available or grid if not.

Space wise I have, subject to planning, some ground where I can put between 24kw and 30kw of panels. The plot of ground is about 125m from the house so I am conscious of cable losses - my thinking at the moment is to set up the strings to run at about 450v DC to reduce the loss as much as possible on 10AWG cables for the long run. I'm therefore looking at inverters with an up to 500v or so max DC input (fortunately there seem lots like this to choose from). I have just started to understand and explore micro-inverters as an option, but I like the idea of high voltage DC at the moment. The ground area is totally unshaded and it's my land so I can keep it that way (this is all subject to planning permission of course, but I have high hopes and have seen similar projects granted in our region).

So, the bit I can't quite get my head around is how I switch across the two phases to try and achieve what I want - which is the ability to push all power into one consumer unit when needed but switch it out to both when not.

I've created a diagram to help explain what I want to try and achieve based on what I think I need, which is a pair of inverters that are capable of parallel operation. It's basically 2 modes - "Split" which is just one inverter feeding each of my 2 consumer units/phases and "Winter" mode which is both inverters feeding phase 2/consumer unit 2. In both cases, excess energy going into hot water and/or battery.

I would be grateful for any tire kicking/thoughts on this or even if what I'm looking to do isn't possible. Incidentally I am aware that I won't get the theoretical maximum kW out of this and in winter it could be quite low. Heating is my largest cost, so the idea is whatever energy is available in Winter, put it in the floor. Downstairs the under floor elements are encased in concrete and once heated up continue to radiate heat for quite a few hours.

Many thanks!

 

[ProDave]

I wish you had come and asked before you embarked on this route. Resistance direct electric under floor heating is going to be very expensive to run.

Instead for all electric, I would have suggested an air source heat pump driving wet under floor heating which for eacj 1kW of electricity consumed would deliver about 3kW of heat into the house, cutting your heating bills to 1/3 what you will be paying now.

Solar PV will offset some of what you use and that too would be simpler if it had stayed as a 1 phase supply, which probably would have been possible.

 

[deanwilts]

Hi Dave,

There are a bunch of reasons why things are as they are and I'm overall happy with the current setup.

As you say, the objective is to use the solar PV to offset some of the current grid supplied power for heating/water and also for the general domestic use. Yes, the 2 phase supply does make it a little more complex - hence I'm here. I'm not even sure if what I want to do (be able to switch the inverters between the phase supplies like this) is even possible. And if it is possible, what kind of kit can do this.

Thanks!

Dean

 

[binky]

there's nothing 'off the shelf' I can think of to auto switch supplies between the phases. I think with the set up you have youre kind of stuck with using a separate arrays for each phase. As you can get about 7kW max inverters for single phase, then you will need multiple arrays anyway. Batteries are debatable when you have so much you can sink energy into, if you can get planning permission for a wind turbine, this may be a better option for your money.

 

[deanwilts]

Hi Binky,

Thanks for your advice/thoughts.

You're right about the battery - it might be overkill and something I can always add later so may consider not doing it at all to start with.

there's nothing 'off the shelf' I can think of to auto switch supplies between the phases.

That fits with me not being able to find anything (I just wasn't sure if I was using the right search terms).

If I had two inverters that were able to run in parallel into a single phase (I believe some manufacturers do this so you can, eg, pair 2 x 7kw inverters for a total 14kw output), do you think I could manually switch them? I wouldn't need to do it often - maybe 3-4 times a year.

I found this guy doing this with 3 x Growatts (he's in the US but I don't believe it would be different here?):-

if you can get planning permission for a wind turbine, this may be a better option for your money.

I'd love a wind turbine. It's the height of them that makes it an issue - more about relationship with the neighbours than planning law. The PV array site is completely out of view of the neighbours but in full south facing sun all day due to the topography (it slopes slightly to the North).

 

[binky]

I don't see why you can't have a manual change over switch, we fit such items to isolate some sites from main connection eg during a power failure, so that a diesel generator can be used. One thing you do need to consider is your connection with the grid, you need to apply to the DNO for a large array, and will probably need export limitation to comply with their requirements. This means fitting a control at the consumer head that links to the inverters by cat 5 cable. Generally cat 5 is good for about 100m, so this probably means locating inverters closer to the house. You can use SWA cable for DC side to achieve this - do not try burying solar cable, it fails after about 5 years in constant damp conditions. I've rewired solar farms for this fault.

 

[King Arthur]

I can't help with the specific question about connections but my experience of big old houses means I couldn't resist the invitatio for a bit of 'tyre kicking'

About 15 years ago I retired early and moved to a rambling old listed house needing full restoration from the roof down. Among other things it had dreadful electrics - not just the wiring but things like only one socket in a room and many without. No central heating, of course, just an oil-fired AGA in the kitchen that also supplied hot water (but only to one end of the very large house) a nd only open fires in the main reception rooms. Bedrooms relied on electric blankets for survival! So basically a clean slate to work from.

Armed with my new-found enthusiasm, a background in computer/electronics design engineering and visions of eco-friendly, 'green' sustainable, lower-impact living, I set about researching and planning how to bring the house into a 21st century living standard. Long story short, after researching all the options I discovered I could get a mains gas connection for £300 and that, alone, blew away all other economic considerations. Nothing else could come close, even with PV FITs being much better than they are now. So I now have a mains gas boiler with interlinked wood-burning boiler stoves (I have loads of woodland to clear so wanted to use the free wood - the more wood I burn the less the gas boiler kicks in) driving wet underfloor heating where possible and radiators in rooms with suspecnded floors. In the first winter of operation my gas bill was a little under £100 because I burned mostly wood plus the extensive zoning means I only heat the occupied rooms (with 8 bedrooms this makes a massive difference!).

I appreciate this won't help anyone who really can't access mains gas but I mention it because it shows what a massive economic shock the country is going to face over the next 5-10 years as gas is (apparently) phased out and replaced, basically, with everything electric. Good news for sparkys but an economic nightmare for everyone else in a country where 85% of homes are heated with gas (or so I've read). In short our energy costs are going to rocket upwards and there's little or nothing that can be done about it - as we've recently seen with government pussy-footing around regarding tariff price-caps and offering £200 'free' loans over five years to houses in certain council tax bands. It's all just rearanging the deckchairs while the ship gradually sinks anyway, or rather while energy prices continually rise.

I looked at heat pumps, of course, but compared to mains gas they are useless. And not just ASHPs but the more efficient GSHPs because I have the land available and the machinery to install myself, so I could save the £20k installation cost but even that didn;t help the economics! Sure, they are energetically 300% efficient (more for GSHPs) but what does that actually mean in practice when a kWh of electricity costs 5 or 6 times more than a kWh of gas? A payback period well in excess of 15 years is a brave bet in the face of a rapidly changing environment and technologies. Too brave for me. And that's before all the issues around ASHP efficiencies falling off at lower outside temperatures etc etc.

Electricity will ALWAYS be an expensive form of energy simply because it is not a PRIME resource. It's an incredibly useful way to UTILISE energy but it is not a SOURCE of energy. It has to be generated, which means energy losses. Sure, it's technically fairly easy to generate electricity by all sorts of means, but they all involve losses and inefficiencies. Same with the much-touted hydrogen economy. H2 is not a SOURCE of energy, we have to make it, manage it, transport it etc - all of which costs.

So, 15 years on, I'm happy with my mains gas decision even if it only lasts another 5/10 years. But I'm in no way complacent and can see the writing on the wall and have also been thinking about how to migrate away from it. My experience with UFH has been very good and it's a great way to store energy, which is becoming - if not already - massively important. Batteries are all well and good, (expensive at present but I'm confident prices will fall, especially for applications like homes where size and weight are not as important as they are for vehicles) but inevitably involve inefficiences in charging and discharge plus they have limited lifetimes. I don't know if there's a limit to the number of times a 4-inch concrete (or limecrete in my case) han be heated and cooled but I'd bet it's not a limit in practice! So, direct heating of a UFH system seems pretty attractive to me, which leads me to . . . .

Solar thermal or solar PV? Because I have wet UFH installed I could go solar thermal and I've read that the efficiencies in the UK are pretty good (50% of average DHW requirements even in winter?), whereas solar PV generation can drop to around 30% of system capacity in winter. Even with wet UFH I could store solar PV energy in a thermal store (which I how I already link my boiler stoves and gas boiler into the same system) so PV would not mean ripping up floors to lay new heating wires. So perhaps a wet UFH system is more versatile than an electric one, in terms of the buried heating 'elements' at least?

Ultimately, all things (well, most things) are doable. If there is enough space for a PV array and enough money to fill it with PV panels then enough electricity can be generated to meet all your needs, even in winter, regardless of the actual heating technology. But I say it's this lure of 'free' energy with no monthly bills and the feeling of freedom and independence it brings that can blind us to the economic realities. Do the maths and consider the wider picture. Is a 20+ year payback period really a sensible investment? Is a 50% hike in your annual energy bill really the signal to spend (or borrow?!) £20k just to (hopefully) keep it level? Do you really need to walk around your home in shorts and tee-shirts during winter? I reckon my jumpers along must have saved me thousands of pounds in heating over the years! It's possible to be warm and comfy at 18C if you're dressed appropriately.

Of course, there are no definitive answers to these, and other questions - everyone has their own priorities affecting their decisions and circumstances. And that's how it should be in a free country. But I think we can all agree that the inevitable move away from essentially 'free' fossil fuels ('free' as in just lying around ready to be used) is going to mean more expensive energy in the future.

Unless of course the fusion boys ever get their Tokomaks to actually deliver on their 30 year rolling promises . . . but even if they do, don't we all remember the promise of nuclear power and electricity 'too cheap to meter'?

All in all, I'm glad I only have to worry about another 20 years or so and have a good enough pension to fund them!

 

[binky]

I can't help with the specific question about connections but my experience of big old houses means I couldn't resist the invitatio for a bit of 'tyre kicking'

About 15 years ago I retired early and moved to a rambling old listed house needing full restoration from the roof down. Among other things it had dreadful electrics - not just the wiring but things like only one socket in a room and many without. No central heating, of course, just an oil-fired AGA in the kitchen that also supplied hot water (but only to one end of the very large house) a nd only open fires in the main reception rooms. Bedrooms relied on electric blankets for survival! So basically a clean slate to work from.

Armed with my new-found enthusiasm, a background in computer/electronics design engineering and visions of eco-friendly, 'green' sustainable, lower-impact living, I set about researching and planning how to bring the house into a 21st century living standard. Long story short, after researching all the options I discovered I could get a mains gas connection for £300 and that, alone, blew away all other economic considerations. Nothing else could come close, even with PV FITs being much better than they are now. So I now have a mains gas boiler with interlinked wood-burning boiler stoves (I have loads of woodland to clear so wanted to use the free wood - the more wood I burn the less the gas boiler kicks in) driving wet underfloor heating where possible and radiators in rooms with suspecnded floors. In the first winter of operation my gas bill was a little under £100 because I burned mostly wood plus the extensive zoning means I only heat the occupied rooms (with 8 bedrooms this makes a massive difference!).

I appreciate this won't help anyone who really can't access mains gas but I mention it because it shows what a massive economic shock the country is going to face over the next 5-10 years as gas is (apparently) phased out and replaced, basically, with everything electric. Good news for sparkys but an economic nightmare for everyone else in a country where 85% of homes are heated with gas (or so I've read). In short our energy costs are going to rocket upwards and there's little or nothing that can be done about it - as we've recently seen with government pussy-footing around regarding tariff price-caps and offering £200 'free' loans over five years to houses in certain council tax bands. It's all just rearanging the deckchairs while the ship gradually sinks anyway, or rather while energy prices continually rise.

I looked at heat pumps, of course, but compared to mains gas they are useless. And not just ASHPs but the more efficient GSHPs because I have the land available and the machinery to install myself, so I could save the £20k installation cost but even that didn;t help the economics! Sure, they are energetically 300% efficient (more for GSHPs) but what does that actually mean in practice when a kWh of electricity costs 5 or 6 times more than a kWh of gas? A payback period well in excess of 15 years is a brave bet in the face of a rapidly changing environment and technologies. Too brave for me. And that's before all the issues around ASHP efficiencies falling off at lower outside temperatures etc etc.

Electricity will ALWAYS be an expensive form of energy simply because it is not a PRIME resource. It's an incredibly useful way to UTILISE energy but it is not a SOURCE of energy. It has to be generated, which means energy losses. Sure, it's technically fairly easy to generate electricity by all sorts of means, but they all involve losses and inefficiencies. Same with the much-touted hydrogen economy. H2 is not a SOURCE of energy, we have to make it, manage it, transport it etc - all of which costs.

So, 15 years on, I'm happy with my mains gas decision even if it only lasts another 5/10 years. But I'm in no way complacent and can see the writing on the wall and have also been thinking about how to migrate away from it. My experience with UFH has been very good and it's a great way to store energy, which is becoming - if not already - massively important. Batteries are all well and good, (expensive at present but I'm confident prices will fall, especially for applications like homes where size and weight are not as important as they are for vehicles) but inevitably involve inefficiences in charging and discharge plus they have limited lifetimes. I don't know if there's a limit to the number of times a 4-inch concrete (or limecrete in my case) han be heated and cooled but I'd bet it's not a limit in practice! So, direct heating of a UFH system seems pretty attractive to me, which leads me to . . . .

Solar thermal or solar PV? Because I have wet UFH installed I could go solar thermal and I've read that the efficiencies in the UK are pretty good (50% of average DHW requirements even in winter?), whereas solar PV generation can drop to around 30% of system capacity in winter. Even with wet UFH I could store solar PV energy in a thermal store (which I how I already link my boiler stoves and gas boiler into the same system) so PV would not mean ripping up floors to lay new heating wires. So perhaps a wet UFH system is more versatile than an electric one, in terms of the buried heating 'elements' at least?

Ultimately, all things (well, most things) are doable. If there is enough space for a PV array and enough money to fill it with PV panels then enough electricity can be generated to meet all your needs, even in winter, regardless of the actual heating technology. But I say it's this lure of 'free' energy with no monthly bills and the feeling of freedom and independence it brings that can blind us to the economic realities. Do the maths and consider the wider picture. Is a 20+ year payback period really a sensible investment? Is a 50% hike in your annual energy bill really the signal to spend (or borrow?!) £20k just to (hopefully) keep it level? Do you really need to walk around your home in shorts and tee-shirts during winter? I reckon my jumpers along must have saved me thousands of pounds in heating over the years! It's possible to be warm and comfy at 18C if you're dressed appropriately.

Of course, there are no definitive answers to these, and other questions - everyone has their own priorities affecting their decisions and circumstances. And that's how it should be in a free country. But I think we can all agree that the inevitable move away from essentially 'free' fossil fuels ('free' as in just lying around ready to be used) is going to mean more expensive energy in the future.

Unless of course the fusion boys ever get their Tokomaks to actually deliver on their 30 year rolling promises . . . but even if they do, don't we all remember the promise of nuclear power and electricity 'too cheap to meter'?

All in all, I'm glad I only have to worry about another 20 years or so and have a good enough pension to fund them!

you make some very valid points that I would agree with, in particular Solar Thermal - I've met customers with thermal panels in place and extolling their virtues long before PV became a thing. PV does have the advantage of being more flexible in application though. A bit of both is always good.

Jumpers, marvellous things, it's only the youngsters that have grown up with central heating .

I've had a few customers who wanted to go 'off-grid' the numbers just don't stack up unless you like living a bit like a hermit in the middle of nowhere - numbers will have improved now of course.

Ultimately, reducing energy use is the way forward, a friend has a 'passive haus', he has a 1 kw electric heater for the occasional cold weather as a back up, which he reckons only really gets used if they go away for week in winter. He has no central heating system what so ever, a major cost saving and maintenenace free of course! He does have a mechanical ventilation system for the whole house, which he switches off in warmer weather. Installing insulation can be a pain in older properties, this is part of the reason you see houses flattened and completely new houses built in their place. With VAT on a new buid being 0% in can be cost effective and you get a modern glass box if that's your thing!

Batteries are getting cheaper and do payback more than they cost due to cost reduction and improved tech, but they still aren't a 'no brainer' option.

20 year payback? 15 years ago that was probably true, however, solar panels have dropped an awful lot in price. I've just priced a simple to fit 8kW system fat £5500, when I started with solar it was £12-14k for 4kW. Not sure about other tech as I don't fit that.

 

[deanwilts]

Thanks @binky for the insight.

you need to apply to the DNO for a large array, and will probably need export limitation to comply with their requirements. This means fitting a control at the consumer head that links to the inverters by cat 5 cable.

Yes, I had assumed I would need something from DNO given the size of installation. I'm not that bothered about selling back to the grid given I'm confident I can consume almost all I make, but I need the hook up so that I can pull from the grid when I'm not making enough. I'd be happy to limit flow back to the grid completely if it's easier to do that than to limit it? Or maybe the same thing.

Generally cat 5 is good for about 100m, so this probably means locating inverters closer to the house. You can use SWA cable for DC side to achieve this - do not try burying solar cable, it fails after about 5 years in constant damp conditions.

The house topology means the solar array will be about 125m or so from an outbuilding where the inverters can go. From there it's about a 20 meter run to the house. There is a pair of existing 40amp circuits from outbuilding to house that we may be able to use for the run to the house consumer units. That saves a lot, as trenching here is trickier. The field I don't care about and a local farmer will do it for me.

I don't see why you can't have a manual change over switch

So my current thinking is now that this is the likely way to go. Manual switchover between "summer" and "winter". In summer, if I have a battery, I am fairly confident that I can generate enough myself to run the whole house and have zero requirement from the grid (based on getting planning, BNO approval etc for a 24kW array into twin inverters that I can parallel up). Possibly even late spring/early autumn. Winter I'm trying to calculate at the moment but I'm getting confused by some of the websites that let you plot predicted output based on geography.

@King Arthur :-

whereas solar PV generation can drop to around 30% of system capacity in winter.

I'd be pretty happy with that!

20 year payback? 15 years ago that was probably true, however, solar panels have dropped an awful lot in price. I've just priced a simple to fit 8kW system fat £5500, when I started with solar it was £12-14k for 4kW. Not sure about other tech as I don't fit that.

I'm still running the numbers but my ROI is looking more like 5-6 years and a very solid investment. Electricity costs here will double in the spring and I believe the ramp down back to 'normal' rates could take 2 years.

 

[binky]

Rough numbers, it's 4amps per Kw, so 24 kw is 96 amps. Now I would be looking at sizing the inverters to suite your existing cables, buy there's a little problem with that, for solar we like 1% transmission losses, where as your existing cables will be specced for 3% or 5%. So we need to look at the actual cable sizes and hope someone got scared and over rated the cable sizes. It's also normal practice to undersize inverters, so an inverter rated as 6kw can be connected to say, 8kw of panels. I'll spare you the details of how that works for the moment as I'm quite a long way into a bottle of rum....

 

[deanwilts]

Thanks Binky - yep I accept the cabling will need to be double-checked. All I know is there are 2 x 40 amp circuit breakers on the consumer unit they feed into in the house (there's a 2 x satelite consumer units in the outbuildings). I'm assuming for now they're OK. If they're not it'll be an overbudget item.

It's also normal practice to undersize inverters, so an inverter rated as 6kw can be connected to say, 8kw of panels.

That was my assumption - given we're unfortunately not in California . I'm assuming in reality I won't get anywhere near 24kW and that, if I do on some crazy summer day, the inverters will just draw what they need based on internal resistance etc and not get blown up.

That said my instinct is not to go for too small inverters - I'd rather overspec them. I'm a fan of over-engineering. There seems to be plenty of 6, 8 and 12kw inverters out there.

In terms of undersizing inverters, would, say, an 8kW inverter with a 12kW max rated array be sensible? Are there any broad guidelines?

Thank you for your input - I really appreciate it.

 

[King Arthur]

Interesting discussion and some good points. I entirely agree that PV payback periods have dropped over the past 15 years when I dismissed them, plus the increase in grid electricity costs also helps reduce this, which is why FITs have been able to also be reduced. In fact, I'm very tempted to now look again at a PV system based on today's prices and tech. I wonder what sort of improvements the next 15 years will bring? But that's the price early adopters pay - a bit like EV owners today.

I also agree with the 'passive haus' point. It could surely be the way forward for new builds but the problem is the huge legacy of old housing stock. A bit like the mains gas problem really - the cost of replacing 50, 60, 70% of all our homes is simply too big an issue for people to comprehend. And then there's the 'heritage' people who won't even give me permission to fit DG or put solar panels on my roof because the house is of 'historic importance' and therefore listed to legally restrict what an Englishman can do with his castle (so to speak)!

Just one point regarding "Generally cat 5 is good for about 100m". In fact, the 100m limitation is based on the cable being able to support the maximum bandwidth (bit rate) and is the manufacturer's way of saying if you limit yourself to 100m then the cable will support the 100Mbps specified speed and be certified as such (the 100m 'spec' is actually 5m stranded patch cable + 90m solid cable + 5m stranded patch). Shorter lengths will support much higher speeds which is why, in practice, Cat5 will easily support Gigabit (1000Mbps) LANs in most homes and small offices without the need for a costly upgrade to Cat5e or even Cat6 cabling.

As for longer distances, it's quite possible to run useful speeds at well over 100m. In fact this is a recognised mode of operation for IP security cameras because they require nowhere near 100Mbps speeds and many network switches are specifically configurable to provide a lower speed on ports connected to IP cameras in order to enable well over 100m cable runs.

I know this because there are a pair of swans that nest each year near me and I wanted to set up a CCTV camera to monitor their progress. I used an IP camera powered via PoE so (no separate PSU issues) and the total Cat5e cable run was about 220m. I connected it to a TP-Link 8-port LAN switch (TL-SG108PE), which supports PoE, and configured it to enable "extend mode" on the relevant port. The manual says "Enabling Extend Mode on the port extends the maximum transmission distance from 100 m to 250 m and limits the maximum speed to 10 Mbps." I can confirm it worked perfectly throughout last summer. Even 10Mbps must surely be fast enough for a PV control system!

 

[King Arthur]

I'd love a wind turbine. It's the height of them that makes it an issue - more about relationship with the neighbours than planning law. The PV array site is completely out of view of the neighbours but in full south facing sun all day due to the topography (it slopes slightly to the North).

Best advice I've read about wind turbines is to buy an accurate wind monitor and run it for a year before spending any serious money.

High wind speeds yield more power because wind power is proportional to the cube of wind speed, which is another way of saying that the rated output drops off really, really quickly at lower wind speeds. Unless you're in a very windy area they'll likely be the least efficient way of extracting energy from the environment.

Of course, like any system, a wind turbine could be sufficiently over-spec'd so that it provides sufficient power for your needs at low wind speeds and then just dump the excess on a properly windy day - all it needs is sufficiently deep pockets But it's that sort of trade-off that makes all this syetm design stuff so challenging and interesting.

 

[Trendywendy]

I’m just a consumer but have read your post with interest. If I can pass on a nugget of advice for your planning stage it’s this - make sure you choose an inverter that can provide an EPS function and have installation include the necessary wiring and socket. We had 14 panels fitted in October and 10.4 kw of battery storage. Yes it was relatively expensive but we were aiming for as much autonomy as possible while still needing the grid for the winter months. We were dismayed yesterday during a sunny daytime power cut, that the system just shut down. It was explained to us that it was to protect any engineers working on the grid - fair enough. But as there is a work around this available as an ‘optional extra’ we were a bit miffed that it was something you had to opt into! Rant over, bear it in mind for your design. Who wouldn’t want to be able to use their battery storage in a power cut. Shakes head in despair. Good luck with your plans.

 

[binky]

I’m just a consumer but have read your post with interest. If I can pass on a nugget of advice for your planning stage it’s this - make sure you choose an inverter that can provide an EPS function and have installation include the necessary wiring and socket. We had 14 panels fitted in October and 10.4 kw of battery storage. Yes it was relatively expensive but we were aiming for as much autonomy as possible while still needing the grid for the winter months. We were dismayed yesterday during a sunny daytime power cut, that the system just shut down. It was explained to us that it was to protect any engineers working on the grid - fair enough. But as there is a work around this available as an ‘optional extra’ we were a bit miffed that it was something you had to opt into! Rant over, bear it in mind for your design. Who wouldn’t want to be able to use their battery storage in a power cut. Shakes head in despair. Good luck with your plans.

All the inverter/battery systems I've seen have an EPS function, the problem is usually retro fitting to an existing electric installation. For a full redevelopment or new build, this is not a problem as it's easy to wire a dedicated circuit.

 

[binky]

Interesting discussion and some good points. I entirely agree that PV payback periods have dropped over the past 15 years when I dismissed them, plus the increase in grid electricity costs also helps reduce this, which is why FITs have been able to also be reduced. In fact, I'm very tempted to now look again at a PV system based on today's prices and tech. I wonder what sort of improvements the next 15 years will bring? But that's the price early adopters pay - a bit like EV owners today.

Just one point regarding "Generally cat 5 is good for about 100m". In fact, the 100m limitation is based on the cable being able to support the maximum bandwidth (bit rate) and is the manufacturer's way of saying if you limit yourself to 100m then the cable will support the 100Mbps specified speed and be certified as such (the 100m 'spec' is actually 5m stranded patch cable + 90m solid cable + 5m stranded patch). Shorter lengths will support much higher speeds which is why, in practice, Cat5 will easily support Gigabit (1000Mbps) LANs in most homes and small offices without the need for a costly upgrade to Cat5e or even Cat6 cabling.

PV tech isn't changing much anytime soon from what I've seen. Inverters run at 92% minimum these days, a vast improvement over efficiency drop off to around 70% only 10years ago. Panels keep improving slowly, up from 13% to around 20% these days, but I've always reckoned surface area is as important as efficiency. I'm not a great fan of undersizing inverters, that advice was based on the inefficient old style units. Personally I prefer size for size inverters, there's many things affect PV outputs, panels are plus tolerance, cloud edge effect can cause short lived surges, and you get the highest outputs in colder clear weather, like May time before the heat of summer. Wet summers are surprisingly good too, the rain washes dust off the panels.
Your comments on net cable sizes are interesting, I work off official recommendations, but have often wondered if that's nonsense .
Trouble is when working with customers, it can be very embarrassing / expensive if you get that wrong...

 

[King Arthur]

Your comments on net cable sizes are interesting, I work off official recommendations, but have often wondered if that's nonsense .
Trouble is when working with customers, it can be very embarrassing / expensive if you get that wrong...

Official recommendations are rarely nonsense but they are often 'safe' and 'quick' and they avoid the need to have a more fundamental understanding of the system and technology in question. A big part of the success of structured cabling systems that use 'Category' cabling was the 'kit of parts' and 'rules' approach, which made it relatively easy for almost anyone to implement a data networking infrastructure quickly and reliably. It's the difference between being a BSc/MSc/PhD electronics engineer who can design a system from basic principles and someone who can put together a bunch of off-the-shelf parts using an instruction manual. Like the guys in your local garage who can fix your car by replacing parts but couldn't design and build one from scratch. Knowing how something works and being able to use it is nowhere near the same as being able to design it. Eg, we all know how to use (say) a wifi router but how many of us could actually design and build one ourselves?

As for not being an 'official recommendation' to extend Cat5 cabling beyond 100m, it all depends. Yes, 100m will guarantee a certain performance but there are also other options for different performance points. Same as electrical cabling really. What's the current rating of 2.5mm2 T&E? Well there isn't just ONE is there - it all depends.

But you're absolutely right about the risks of getting it wrong for customers, which is why it's good to have a 'rule book' to blame if things do go wrong. Just like the wiring regs.

 

[binky]

It's the difference between being a BSc/MSc/PhD electronics engineer

But you're absolutely right about the risks of getting it wrong for customers, which is why it's good to have a 'rule book' to blame if things do go wrong. Just like the wiring regs.

Misuse of the title 'engineer' pees me off, it gets applied to almost anyone who uses tools or gets their hands dirty.

I;ve used Ethernet over Power devices a few times to overcome longer data runs, and avoid expenisve armoured data cable. That seems to work reasonably well,

 

[King Arthur]

Agreed - wouldn't get away with it in Germany where 'engineer' is a properly recognised and respected professional qualification and gives the holder a recognised title, ie 'Ing', in the same way as 'Dr'.

We do have the Chartered Engineer (CEng) qualification in the UK of course, but you won't find many of them in your local garage or electrical wholesalers

 

[binky]

Agreed - wouldn't get away with it in Germany where 'engineer' is a properly recognised and respected professional qualification and gives the holder a recognised title, ie 'Ing', in the same way as 'Dr'.

We do have the Chartered Engineer (CEng) qualification in the UK of course, but you won't find many of them in your local garage or electrical wholesalers

I don't know it it's true, but someone told me the French automatically register qualified engineers in their equivalent of the yellow pages and it's not uncommon for them to work a main job and private work. Completely different status to the UK where accountants are regarded higher


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