Solar PV voltage drops DC AC

[revor]

Had quotes for 6.4 Kwp  solar pv ground based array with Solar Edge 5Kw inverter. From the array to the CU is about 60 M underground duct already in place. There is the option of having the inverter indoors close to the CU which will be DC from the panels (which is what installer has quoted for ) or the inverter (IP65) could be outside and send AC to the house. I was once told but cannot recall where I got it from, nor the reason, that it was a no no to send DC underground for that distance. Does anyone have a view. Also is the voltage drop calculation w.r.t cable CSA the same for DC as it is for AC. Other than12v projects from my electronic days years ago I know nothing about higher volts DC.

Thanks

 

[binky]

OK, DC cables in ducting can be done provided it is totally dry in the duct  - water affects DC cabling when constantly immersed, it will last about 5 years as I have seen on solar farms. SolarEdge inverters are IP65 so can go under the array in the shade. Unless you have shading issues though, it's a waste of money, and enhanced performance for an unshaded array is neglible against the additonal costs. If you want a 'quality' inverter fit SMA, although your installer may be looking at max DC intput on a single phase inverter? Chinese inverters such as Solis and Solax seem well made and are significantly cheaper, they may not last quite as long, but at less than half the price of SMA / Solaredge, it's hard to justify the additonal costs. Given the efficiency of modern inverters 6.4 kWp seems a tad oversized for a 5kW inverter. I have found many panels will output more than stated, and undersizing inveters it's not necessarily the best idea - it can choke the output. The counter argument is enhanced winter outputs, ie when the sun is weaker., but as modern inverters are around 97% efficient at converting DC to AC, this argument is a little out of date. 

Given such a large array, it may well be also worth considering battery storage, unless you have high demands in the house? A 4 kW array can generate 25 / 30 kW  a day between April to September given a sunny day.  

 

[revor]

Thanks binky great info there. Went back to proposed installer about this issue and they came back saying had not realised the run was 60 m and yes the inverter should be outside. The information was on the drawing I sent, was  on the quotation as he had copied my drawing, I had also sent photos showing the ground, array going on in relation to the house, so obvious was a cable run. I had also marked the Naylor duct run on the drawing also. Obviously not read what I sent which was at their request. The first quote had a 3.6 inverter until I questioned it. Very worrying really as I am a novice even though read a lot and finding errors like this questions whether there is other stuff I won't pick up. Yes I am considering a battery think the Tessla is best for storage amount although pricy and only available as far as I can make out through Tessla authorised installers. I am also dubious about having solar edge as don't have shading issues but there seems to be an advantage from diagnostics point of view but whether that is worth the extra is questionable.

Its a large array because when I got planning permission was limited to 16 panels but since then they have almost doubled in output. The ones I am considering are 390 w so will get 60% more output than would have done few years ago

 

[ProDave]

I am interested how you now justify spending so much on solar PV with no Feed In Tariff.

I managed to (just) justify my 4KW system by buying very cheap and installing myself, and I might just get payback in 6 years from self usage.  If I had to pay someone to install it the payback time would have been way too long.  And even 4KW can be difficult to self use all you generate, so 6KW you will be exporting a lot of that, at the moment without payment.

 

[revor]

Yes it is quite a financial investment depends on how much you use I guess. It has cost you to install as your time could have been spent doing your work and earning so has cost you your hourly rate in a way.

The greater the consumption the better the payback. Considering the battery so we use the stored daily energy  in the evenings, the battery needs to be filled and emptied every day to make that pay, and next year an EV (quite a few small vehicles being launched which  will bring the price down somewhat with volume and as manufacturers compete with one another, and it will be time to change our car anyway.)The saving on petrol will be fairly substantial and will contribute to the overall payback. Another way of looking at it, is what is the annual return on your investment I calculate about 4% which is better than any bank or building society but not as good as a neighbour who got in early and his return is about 10% based on the FITS. There are tariffs now coming available which will buy your production, take a look at what Octopus has to offer. If you have excess and not on FIT (guess if not MCS registered you would not qualify anyway) might be of some benefit to you. I have not factored this in my calculations as the idea is I use as much as I can so hopefully will be little to send to the grid. The government have decreed that those contributing to the grid should be paid for it.  https://www.solarpowerportal.co.uk/news/beis_unveils_smart_export_guarantee_to_replace_export_tariff

The theory is what you produce in the day you use, or store in battery/ and or a thermal store via immersion heater. Come evening you use what is in battery, anything left over charges your car, the idea is you use all you have stored in the battery then if car still not charged fully, you top up overnight from the grid at a cheap rate. I am only looking at these options at moment but it is not just the payback  it is a contribution to reduce our CO2 emissions. Anyway why should those who struggle to pay their energy bills contribute to subsidising those who can afford to invest in renewables. We are in an area where we often get power outages usually in the evenings so a backup via a battery (and auto disconnect from the grid) would avoid us loosing power. During the summer we should not need the grid but if we did  it would be minimal  the winter months are a different kettle of fish though. From where we are sitting there is a lot going for it getting the spec right will be crucial. What was the kit you installed?

 

[ProDave]

Mine was a simple 4KW array bought cheap from ebay, total installed cost £1500.  My prediction is I will self use £250 worth of electricity per year giving a 6 year payback.  

I am presently self using 95% of what I generate.  Export this year so far is just 48KWh, and most of that is at mid day when the panels produce more than the immersion heater can dump if nothing else is using power.  It helps being semi retired in that one of us is usually in for part of the day so can use the washing machine, tumble dryer and dishwasher (one at a time) in the middle of the day.

Be careful comparing "return on investment" with a bank account,  Money in the bank remains there to be drawn upon. Money spent on PV is gone, so your ROI is 0 until you have had enough saving from it to recoup the install cost.

Charging an EV?  Fine if you are retired and the car is at home to be charged.

I keep looking at batteries but think they have a way to fall yet before they are viable.  With my small array and being able to self use most of it, there is not much to be gained by having battery storage, though arguably storing it in a battery to use in the evening is better than heating hot water with it (as there are more efficient ways to heat hot water) With a larger array, it will be harder to self use all of it real time so batteries might make more sense.

Re the payment for export scheme. As predicted, you can only sign up for that if you have paid a premium to have the system installed by an MCS contractor.  Given the small amount of export I have the export payments would be tiny (about £2.40 so far this year) so clearly if I had had to pay hundreds of pounds for an MCS install, that extra cost would never be repaid by the paltry export payment.  It infuriates my why they continue to feed the gravy train and insist on MCS contractors.  Just about any electrician is competent to connect an inverter safely, why be forced to use a monopoly scheme that charges way more just so you can claim what should be available to all?

Incidentally when setting up my panels, I configured them as an East / West split rather than all south. that was to try and get a useable generation for a longer part of the day, rather than a big peak at mid day, in the hope it might make self use easier, but at the expense of lower total generation.

 

[revor]

 Good points pro dave that is a cheap install. My inverter would be close to that alone. I agree about you comment re cost of batteries, and looks like close up on the horizon batteries will be super capacitors which promise to give better performance  at lower cost.  I am getting to old waiting for the next development so will have to bite the bullet soon.

 

[ProDave]

Panels were just over £1000 from ebay.   Tranergy 3.68Kw inverter new from ebay £255. The rest was cable, mounting hardware, generation meter, isolators etc I used Unistrut being much cheaper than the "proper" aluminium mounting channels.

 

[binky]

I you are buying an EV it will have a battery of at least 10kW on a Hybrid, 20 -30 Kw on electric only, so I wouldn't be that keen on a solar battery, and certainly not the overhyped Tesla, which is so massive it basically can only go outside ( I was a registerd installer),  BYD offer a good battery with far more bang for your buck. BYD being the biggest car battery manufacturer in the world.  Batteries do pay for themselves, but they aren't as good as the panels themselves. Water heating gadgets are good - if you have  a hot water tank or jacuzzi. For a ground array, diagnostics is a doddle - you don't need scaffolding. Nothing should go wrong with the panels anyway, for at least 30 years. Any faults are usually installer related, namely badly done cable joins, or very rough handling. NB Dave's panels were second hand from a solar farm, and I wouldn't give the steam off my wee for a Traversty inverter  :^O . 

What are your leccy bills like? £1k / £2k / £3k per annum? Do yuo have a lot of leccy gadgets that eat power? Are you retired and home a lot during daylight hours, a vampire perhaps ?

 

[revor]

I did eye the BYD but made in China. I avoid stuff from there  for a variety of reasons. Tesla does have a high capacity and can deliver 5 kw continuously and  7 kw for 10 secs at any one time useful for our Kw programmable induction hob. Space is not an issue as when building our house (farmhouse conversion plus extension included a plant room for all the services.) But things are changing very quickly. Don't know what leccy bills will be as still building our house (mainly self) and living in part of it (bed and bath). Live on a small holding and have outbuildings we use but is on separate supply to the house. In total used nearly 7500 KwH between them but split will change I guess. So bill in total not that big but is going to rise with inflation and cost of smart meter roll out is recovered (claimed in places at £400 a household) and as we expand the business. We are here all the time so its not as if we don't have the opportunity to use what we produce. We have a friend who did a DIY install of 17Kw and does struggle to use it all. He has south facing array and E/W array he got an installer to do the 2 big inverters, so you can have too much generation. He gets some payment for what he sends to the grid but is a pittance.

 

[binky]

Nothing wrong with BYD, it's a well made unit, they make the batteries that go in EVs, and the nice thing is that is a stacking system, so you can add further batteries units with little hassle. I'm not a fan of Chinese products, so I wouldn't recommend these unless I though it was worth it, they are also usually sold with SMA controller,, which is the important bit.  However as space isn't an issue, I would go with LG units. Your hob is very unlikey to be taking full whack at any one time, and discharging batteries quickly doesn't do them a lot of good.

As  you are working from home, then a larger PV array makes sense, your base level loadings will be relatively high - PC, printer, lights, modems etc etc all add up to a reasonable load of, at a guess about 0.5/1 kW.  So a larger array will cover that base laod even in winter and possibly on a rainy day. Given that you are in North Wales, famous for mountains by the sea ( my granny used to live in Colwyn Bay), you no doubt get a lot of cloud cover, again a larger array helps. It also has to be said once you start fitting solar, the cost of making the array bigger isn't that high, the expensive bit is the base cost of doing any installation. I've just had a quick look at inverters SMA do a 5kW sungle phase unit that will take upto 7.5kW of panels £960, and Solaredge have released  a 6Kw single phase unit that will take upto 9kW of panels £960 + the optimisers. Solis do a 6kW unit that takes 6.6kW of panels at around £500. Prives inclusive of VAT. If you do go down the Solaredge route,  I woud suggest buying panels with the optimisers built in, this halves the number of cable joins behind the panels, and chances for a duff joint to cuase trouble with water ingress.

Incidentally, with the demise of the FiT rates, registering a system with MCS is now pointless, the only people you need to talk to is your local DNO  - I would do this before parting with money as they may not approve a system over the nominal 3.68 kW (that's the inverter) they have to allow. I would also check your grid voltage, if this is floating high towards the max 253V allowed on the national grid, it may prevent the system form working properly. Again the DNO should be able to reduce this, but only they can do that.

 

[revor]

Thanks ever so much binky very good of you giving me these options and the wisdom of your experience and knowledge. It is very helpful. I have not committed myself to anything yet, but because the proposed installer has made so many mistakes so far, I am going to kick them into touch. I have done some digging around the DNO bit and downloaded  the G59 regulation/ criteria so will give them a call next week.  The hob  (not installed yet)  is 7 Kw but can be programmed via the controls to 4kw it's is the start up current I am concerned about. From what I have measured recently incoming voltage is 242 volts steady was not the case 10 yrs ago it could fluctuate between 190 and 260 as you watched the multi-meter. Don't have a data logger so can't check fluctuations easily. Been lot of new cabling done recently, now got new all the way from the transformer in the road 120m away to the CU in the house has not stopped the power outages though they are not as frequent as they were. Important that I can disconnect from the grid in event of an outage so can use the battery. The Tesla has a back up device that will do this disconnect are there other means of doing this?

 

[binky]

most battery controllers have an 'emergency power function' for essential loads or can be programmed to work under such conditions - tend to be resrictive on what they will power. You will need a 'change-over switch' which effectively isolates you from the mains supply, (and possibly a commando socket and a small genny to ensure limited but continuous power if your outages are long). You need to isolate from mains supply somehow as other wise you could be backfeeding the neighbours, and it's a safety function for the DNO workers. P14 of the attached manual shows a rough schematic. NB look for 'AC' battery packages, these work independently of the solar array. I would suggest this set-up because of the distance to your array - you need to get a connection to an amp-clamp or energy-flow monitoring meter to control the battery charging. SMA does a certain amount of this via wi-fi, and can work over wifi-bridges. Not sure what happens if you lose power and the internet goes down?  MIght be worth a quick call to SMA technical help, don't be put off by German phone number, thay usually have an English fella to answer the calls from UK.

With regards to DNO, if they object to the size of array, you can use export limitation to comply with thier requirements(again SMA are good for this and DNOs like SMA). I often ask how big I can go, and hope I don't get the predantic prat who always says 3.68 kW max, and never any more than that! Not looked at Solar edge from this pont of view,, but I would assume th same functionality.  Chinese gear can also be set for this that I have used.  NB G59/83 have been superseeded by G99 / 100.

As for your installer, it sounds like he would sell sand to the Arabs and doesn't really know what he's on about!  It's popular to sell Solaredge singing it's enhanced features, but it's really just a ploy to up-sell! 

View attachment Solis_RHI_series_inverter_20180621.pdf

 

[revor]

Thanks for he above. Got in touch with DNO won't discuss before have a filled in application. As you correctly say  binky the EREC have changed, G83 has become G98 and G59 G99. The forms look pretty onerous to me as do not have the knowledge to answer lot of the technical stuff. Confusingly the ENA released an amendment to G59 on 5th May a few days after the new EREC's came into force became redundant before becoming effective!!.