Alternative solar supply available at point of use Domestic kitchen

Question

Hello newbie : 
 Imagine Domestic kitchen Usual array of sockets and under counter appliances 
 Wish Introduce Alt solar supply sockets alongside existing grid supply sockets , Thus : User can select alternative ( solar ) supply as opportune arises at point of use ( battery storage/ inverter system) 
 Eg plug microwave to alternative socket as power opportunity ( battery charge is available ) 
 (Alt supply sockets are entirely dedicated from inverter as typical OFF grid system ) 
 Question Is there anything in regs preventing this ; Thought: The sockets must be somehow labelled / identified as alt supply Any ideas to meet regs? 
 
 Remark : similar to external generator supply but with NO changeover switches . Outlets are dedicated 
 
 thanks Ms Otis

mapj1 · Accepted Answer

Regs wise there is nothing stopping a second Ring or Radial circuit being connected to another supply, so long as things are clearly labelled and it is obvious how and where to isolate it if need be. However it would be most unusual, and may be a bit confusing for a non- technical user if some sockets were live or dead depending on when the sun last came out. Normally UK practice is that the inverters are "grid tied" that is to say they connect to the house mains, and when generating push current back in the opposite sense to a normal load, so the voltage rises rather than falls moving from substation to house. That way the socket has a supply all the time, but uses the inverter when available. 
 Thinks to watch - 
 1) earthing systems will need to be co-ordinated (i.e. solidly connected) so that there is no chance of a user straddling the two systems getting a shock from the cases of one appliance plugged into each.Also, as parts of the system may be live even when the external mains is off, then the external supply earth cannot be relied upon as the sole means of earthing. -the external earth may be disconnected while it is being repaired. 
 2) The performance of the inverter under overload or fault will be very different to the mains supply, and may not be enough to reliably trip a normal breaker. This may mean a non0standard arrangement is needed. 
 Can I ask why like this as opposed to the more conventional ? Mike.

AJJewsbury · Answer

OK, here's my attempt at a diagram (it's not intended as a full design or even a recommendation, just what I think you're suggesting - so as to keep the discussion on track).

AJJewsbury · Answer

Sun and moon: If moon there is no PV ? Mostly you have nothing in peak hours.? Just grid ? Unless you have storage battery ? 
 
 Agreed. 
 
 Ok so you have to charge it ? Is that PV by day ? Or is it also grid charge in case :Is that Off peak electricity for that ? Can we believe that ? 
 
 With a grid-connected system you have both options. Conventional fixed-hours off peak is indeed an option, perhaps more attractive are the likes of Octopus's Agile tariff, based on a smart meter, it follows the per-half-hour wholesale prices - which sometimes can be very cheap - occasionally negative unit prices (you're paid to consume electricity). All part of the brave-new-world smart grid idea to help balance demand with supply whatever times of day. 
 
 And how exactly do you size that battery capacity ? 
 
 Ah, that's one of those six million dollar questions. It's a classic case of diminishing returns - the first kWh will be well used and pay for itself quite quickly - the 10th kWh will be less well used, and the 100th kWh only used a few times a year, and you'd still have some risks of no power during a long power cut. About 1 days worth of consumption is roughly where most commercial products are. Per kWh batteries are disappointingly expensive - how deep are your pockets? 
 
 Grid tie So now you must have import export meter ? AND G9 xx anti islanding in case of power cut ? 
 
 You probably don't need an export meter unless you want someone to pay per unit for what you export (my grid tied PV system ran on an old spinning disc meter under the FIT for 10 years) - the only requirement is that the meter isn't allowed to run backwards (and for payments the FIT just assumed you exported 50% of generation). If you want power during a power cut, then yes you'd need an islanding relay and appropriate controls (but compared with everything else they're relatively simple). 
 
 Your installation costs and kit are rising ? 
 
 You pays your money and takes your choice of course. Grid tied is probably simpler to install (least disruption to the existing installation and no duplication of circuits, and side-steps problems with the protection of d.c. cables in odd situations), but yes, does require equipment with a little more sophistication. 
 
 Excess solar energy If you have it would surely just waft around your home system hopefully being absorbed by some standby thing you have left on during the day ? 
 
 If electricity "wafts" about, you've got some very serious problems with you installation. With a grid-connected system any excess is exported (unless there's a power cut of course). With an off-grid system it's either diverted to something useful (an immersion heat is a common one) but that requires extra equipment, or the PV charge controller shuts down once the battery is full (so a complete loss of generation). You could leave some small appliance on the off-grid sockets permanently to reduce the changes of that happening perhaps - but you then reduce the energy available when you want it (and the small appliance stops working once the battery is flat). Many people go on holiday for a fortnight during the summer - which can be a surprisingly large percentage of the available solar power, without the significant loads being available. 
 - Andy.

Grumpy · Answer

Personally, and I have no idea/couldn't care less if it it is proscibed by "the regulations", I am absolutely against the use of blue mpde water pipe, brown soil pipe and white/grey waste pipe as cable ducting. And I've come across them all. It is asking for trouble imho. As specific, easily identifiable, cable ducting is readily available why would you use anything else?

AJJewsbury · Answer

Andy could you please itemise your system 
 
 I've only got a simple grid-tied PV system (2.16kWp (9off 240W panels), with a 2000W inverter. No battery. It just supplies the house in general, any deficit automatically made up from the grid, any excess exported. (To some ways of thinking you could argue I use the grid as a battery.) 
 Overall it generates around 1800kWh per year - although that might average to around 5kWh/day, it's very variable with both season and daily weather. May/June this year were pretty sunny and I was getting around over 300kWh/month (i.e. around 10kWh/day on average) Dec/Jan was rubbish - barely 1kWh/day on average - many days with next to nothing at all - often several days in a run when it's foggy or the panels covered with snow. 
 If it help, here are a couple of screen shots from my (emon based) monitoring system (it's not the most accurate in the world, but does give an indication). Blue lines show local demand, yellow PV generation (so where there's yellow above blue, I'll be exporting to the grid, blue above yellow means I'm importing). 
 June-ish: 
 
 Dec-ish:

gkenyon · Answer

Anne Otis said: Question Is there anything in regs preventing this ; Thought: The sockets must be somehow labelled / identified as alt supply Any ideas to meet regs? 
 There may be issues with UK legislation (Regulation 21 of ESQCR) if there is no switching to completely disconnect the live conductors of the grid. 
 A floating supply or IT earthing arrangement is not recommended in general for domestic use in the UK. Also, Chapter 82 of BS 7671 requires the "backup" supply to be a switched alternative to the grid, not a "floating supply", so really it's not covered in the national wiring code in the UK. One key thing to remember is that, even if the manufacturer only provides one socket-outlet, the first thing users at home will do is connect multi-way extension leads to power equipment, some of which is connected into telecomms installations etc. - so what you thought was "floating" isn't any more. 
 See also the following in the IET Code of Practice for Electrical Energy Storage Systems Second Edition: 
 
 Section 9.4 (and in particular 9.4.5 regarding limitation of use of IT system) 
 Appendix F, item F1 (page 157) 
 
 I'm fairly sure the MCS standard for battery storage installations doesn't permit it either. 
 
 So, in summary whilst product standards, and to an extent installation standards, might appear to permit the use of a "floating backup supply", in practice in the UK for installations that are to be operated by ordinary persons as defined in BS 7671, we would strongly recommend the "floating backup socket" functionality is disabled when the inverter is installed, or, if power backup is required by the client, select an inverter that supports a TN-S earthing arrangement in island mode, along with switching that meets the requirements of BS 7671 and the legislation (Regulation 21 of ESQCR). Relays alone are not suitable for disconnecting from the grid in the UK - the switching device should provide isolation according to IEC 61140.

Anne Otis · Answer

thanks again Andy and all I was getting excited there it was do able > BUT These examples seem to be all about changeover switches Isolating 
 circuits from grid / reconnecting to inverter : 
 If your still with me Please check again the concept : Perhaps I am breaking new territory If you would please indulge me one more time : to be sure 
 GO and stand in your kitchen : Look at the socket outlets Now imagine next to EACH is installed another : These added sockets to be fed directly and independently connected via solar/ battery inverter (separate distribution board) All separate: wiring : everything: 
 These "alternate" sockets go live whenever ( you) consumer turns on the inverter . The grid sockets are also live same time. Now imagine you consumer go and choose alt supply source by simply unplugging any appliance eg microwave and plugging into adjacent "alt" socket: Thats the point of it 
 No changeover switches to existing loads Inverter system was thought as fully independent : 
 OH damn !!!! by your good observations we see a problem with earthing to appliances IF we introduce a separate TT earth. One chassis at different potential to others. 
 Last ditch is could we avoid sperate TT earth and simply link the inverter earth system to incoming grid earth. ? ( I think you are saying that would mean BOTH Ground and Neutral were linked. Inverter to grid incoming. 
 Question remains IS this possible and so avoid changeover switches converting alt source to island Grateful thanks again to all still on this thread 
 Ms O

Alternative solar supply available at point of use Domestic kitchen

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