optimising current set up with a second small inverter...advice please.

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solar-sam asked Dec 11, '23

optimising current set up with a second small inverter...advice please.

I have 2 solar arrays linked into a 5kw hybrid solis inverter with 2 US5000 pylontec batteries

The arrays are both at 140 degrees south and have a 30 degree pitch. My main array is 12 X 420W panels and is located on my main house roof and difficult to access.

My second array is on my brick and tile workshop roof and has 8 panels of 400W each.

However 3 panels on one end of the eight get heavy shade from a large oak which kills production from the 8 until about 09:45-10:30 am...no optimisers yet fitted (option for £120).

Both are tied into my 5 kw inverter which due to the sub optimal position I thought would just supply enough for the 5KW inverter. However it does seem that these more modern panels are performing almost on par with south facing ones going by my solar yields albeit on limited data as only completed in Sept

However performance on sept data instead of a nice solar curve its flat topped at 5kw especially when exporting for at least 7 hrs on a nice day in sept, so I'm pretty sure even more green power could be harnessed or sold in the peak solar months.

Thinking of just buying a cheapie non hybrid inverter say a 2 kw mini and putting 4 of my workshop array on it, not linked to the other or the house in any way just on the AC outlet into my distribution board. Also easy to access and change current config without much trouble.

cost about £300 so good payback.

That way it would feed my household requirements 0.5K/W per hr during daylight, and allow the 4 unshaded panels to produce solar 90mins earlier.

Wasn't originally going to bother but as it now appears I'm going to be paid for exports, there is the incentive to harvest more green energy for the grid.

Have had some success in using timers for freezers etc to utilise solar time and save on battery at night.

Appreciate others opinions

multiple inverters

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Answer 1 · 2024-04-06T12:12:49Z

solar-sam answered · Apr 06 at 12:12 PM

just a follow up from the initial question. now installed a basic non hybrid inverter on 4 panels. These panels were part of a straight 8 which were experiencing shading from each end at start am and 4 pm. so as optimisers not fitted were causing large generation losses.

used a solis mini inverter which cost £ 165 from ebay, and just a AC isolator req, as already had cables and inline fuse.

Pretty impressed with results I'm seeing, seems in the not so sunny south its brief sun then cloud, however seeing total generation soar to over 7 kw, until the sun goes in, but the brief moments do add up in generation. have seen exports to grid increase as well by a noticeable amount.

Export rates allowed are a total joke, as although I'm lucky with a high allowance many are capped at 3.7kw, but if you look at the actual chart data for a good export day and I use the octopus app like many here, it shows in my case not getting even to 3 whereas inverter was chunking out 5kw export for decent periods. guess these are clouds etc, pulling the hourly rates down. however total for each day is accurate.

Does seem crackers to have such low overall export limits while I can import18.5kw (80 amp fuse),or 22.5 if upgraded to 100 amp. Many households now installing fast car chargers (or even 2), which runs for hrs, drawing decent loads.

Grid certainly needs shaking up a bit and relaxation of over tight rules to allow more green energy generation, especially now we are seeing battery costs falling with plyontech 5000 just dropped by £200 from some sellers for example.

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regman54 commented · Apr 06 at 03:55 PM

@solar sam Since you asked:

The problem is when EV fast chargers are introduced into neighbourhoods (never mind the ever increasing population growth) the existing grid and energy production can't cope with this new demand. Even now here in western Canada, where EV sales are slow (and slowing), we are experiencing rotating winter brown outs that never occurred before and we are still months away from peak summertime demand. The grid can only carry so much current and was never designed for such a high increase in demand over such a short period of time so no matter where the energy is produced (roof tops or otherwise) it will cost billions over many years of construction to catch up so simply "shaking up" the grid is not a quick, simple solution.

In the meantime I would suggest for those who want/need fast EV charging they should responsible for producing their own power and stay off the grid until the grid can be properly upgraded.

Cheers.

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Answer 2 · 2024-04-06T14:36:09Z

delf67 answered · Apr 06 at 02:36 PM

"Grid certainly needs shaking up a bit and relaxation of over tight rules to allow more green energy generation,"

I wouldn't hold your breath.... EV sales are plummeting and no one sane is swapping a £2k gas boiler for a £15k heat pump, so daytime domestic consumption at the local level (per transformer) is minimal. Your 5kW export is probably powering the next 10+ houses down your street and the more people that get panels and want to export, the less it's actually needed. In some areas where most houses have panels already, late adopters are not being allowed to export at all.

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Craig Chamberlain commented · Apr 08 at 01:16 AM

I'm swapping a ~£5k oil boiler for an ~£18k heat pump. Does that make me insane too? Or does the £9k government grant plus £7.5k interest free loan and zero VAT preserve my sanity?

To be clear, the heat pump itself isn't £18k but we need to replace 17 radiators to support low flow temperatures. The radiators alone are about £2.5k plus fitting.

It'll be great to get rid of our oil boiler though because it is an external boiler and is around 16 years old, unreliable and in need of replacement. Our oil tank is also in need of replacement so actually the like for like replacement would be £4.5k for the boiler plus £1.5k for the oil tank. So that's £6k to stay on oil or £9k (ignoring the loan) to go with the heat pump. But since the loan is interest free and available, it would be silly not to take it.

Heat pumps get a bad rep sometimes, but that's usually due to a bad design from poor quality installers. If you do your homework, or employ someone to do that for you, then it should work out well.

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delf67 Craig Chamberlain commented · Apr 08 at 09:01 AM

You do understand that the Government doesn't actually have any of it's own money, right? It's OUR money. Any subsidies, grants, loans etc is just them benevolently giving us some of our own money back to us.

You can throw all of the smoke and mirrors at the financials that you want, but swapping something for £5k for something costing over three times as much to do the same job?....

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Craig Chamberlain delf67 commented · Apr 08 at 11:23 AM

And do you realise that "money" has no intrinsic value at all and is all just made up? :)

Yes, I'm well aware of how government finances work and am very happy to take back a very small proportion of the tax I have paid continuously for the last 35 years or so!

If you knew me at all then you'd know that I've not gone into this on a whim and have done extensive homework on heat pumps and all the economics surrounding it. My oil boiler is a combi type unit so has no water storage at all. So part of the heat pump installation is to install a large water cylinder which will then enable me to use excess solar power (currently exporting 4kW) to heat my water. At present my oil boiler starts up every time someone opens a hot water tap and that is massively inefficient just the same as doing lots of short journeys in a car is. So again, part of this installation is actually an upgrade by installing water storage, and by using a heat pump to heat the water, you get much more water heating than you would from a simple immersion heater due to the SCOP factor of the heat pump. eg. 1kW of solar is multiplied by the SCOP factor to become perhaps 2.5kW of heating. Even the new radiators I mentioned are an upgrade because the old ones are also 16 years old and have some signs of corrosion. But more importantly, even if I was installing a new oil boiler, having larger radiators allows for a lower flow temperature which for condensing boilers makes then much more efficient.

So, what you call "smoke and mirrors" is actually more of an exercise in comparing like for like, and simply taking advantage of a government scheme to get back some tax and improving my heating system at the same time. The heat pump itself is just £4908 (https://midsummerwholesale.co.uk/buy/mitsubishi-heat-pumps/mitsubishi-ecodan-8kw-ashp) so is comparable to an external 32kW oil boiler. The water cylinder is around £2600 and radiators around the same again. We're also relocating the heat pump (our choice) so need underground insulated pipes. Then there are labour costs which puts money in local people's pockets.

Lastly, do YOU realise that oil/gas prices aren't ever going to go back to pre-war prices? I used to pay 25p/litre of oil but that spiked to £1.60/litre when the war started and has settled back to around 70p/litre at my last purchase. That costs me around £1500/year to fill the tank a couple of times whereas it was £500/year just 5 years ago. The more I can make use of my solar and batteries to insulate myself (pun not intended) from ongoing fossil fuel increases the better. Yes I know how electricity is generated too, but I have flexibility to consume and export at the most opportunistic time.

The government incentives are purely designed to overcome inertia and to get the market moving forwards. In time the costs of heat pumps and installations will come down. I'm happy to help that happen!

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delf67 Craig Chamberlain commented · Apr 08 at 11:52 AM

Since we dropped the gold standard you are correct, our fiat paper money has no intrinsic value, however we are all forced to give it value because we can't live without it. Try paying your council tax with a basket of home grown vegetables...

I don't doubt for a minute that you have done all of your homework and it makes perfect financial sense for you to go this route. My objection is that the taxpayers of this country are forced to chip in to make it viable for you. If the tech is so good, it should stand on it's own merits.

I've paid tax for over 40 years, can I get a 9 grand handout too please? (Without having to scrap a perfectly good (if inefficient) heating system). I'm sure the manufacture of all the components of your new system had minimal environmental impact....

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Craig Chamberlain delf67 commented · Apr 08 at 01:20 PM

I know you are kidding about the paying bills with home grown veg but we are fortunate to live in a cottage in the countryside and do in fact grow lots of our own food and have a flock of 8 backyard chickens who provide our eggs. We like to be self-sufficient as much as we can, and switching to a heat pump moves us closer to that goal since we no longer need oil deliveries. We are still grid connected yes but generate significant solar for most of the year and in a pinch we could operate without the grid to a large extent. I'm planning to add more solar panels since they are so cheap.

As for tax payers being "forced" to chip in by the government, remember that this is "our" government who we elected, and they are acting on our collective behalf. Obviously not everyone voted for the current government (arguably nobody voted them...) but an election is coming soon and if abolishing such grant schemes is a strong wish of the people then we can elect someone to make that happen. I suspect though that most people will focus on the usual things like health, education, crime and immigration and the amount of money spent on those things, and often wasted in vast sums, is orders of magnitude more than any heat pump grants.

We also took advantage of the EV grants available back in 2016/2017 and that was £4500 + another £1000 to install a charge point. We've also enjoyed paying no road tax during that time and still have the car we bought, which has now done over 80k miles and costs a fraction of the cost of a petrol/diesel car to run.

If our oil boiler was still in tip-top shape then we'd probably not be replacing it, but it is now well past its best and even our boiler maintenance company have said they won't support it beyond this current year. Heat pumps are a very well established technology (aircon, fridges etc) and if you have a post 2006 home as we do (cottage extended and refurbed in 2008/09) then you probably have good enough insulation already and might just need to upgrade a few radiators. Where it's more complex is with older, poorly insulated properties or where it's a terrace and there is nowhere to site the pump. A different solution will be needed in those homes most likely.

If your heating system is still perfectly good then you can afford to wait a while longer by which time the economics of heat pumps and other technologies will be able to work without subsidies. Everyone's a winner! :)

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Answer 3 · 2024-04-07T17:38:26Z

solar-sam answered · Apr 07 at 05:38 PM

very true del67, my plan is to steadily increase my battery storage capacity to become much more self reliant, as I can see some very serious problems looming in the next 15 years or so. Todays production was 35Kw from the main and 7kw from the mini inverter, so as its all green free energy without upgrading panels, I'll most certainly take it. See a lot of vids where they say "don't worry about "clipping", well in my book 7KW is not small beer, and I ran a borehole water pump for a good period today and a high powered compressor which has to run on 16A , all on free green energy, which actually made me feel good as well. Try to plan high use age with sunny days to avoid any imports at all. I'm certainly starting to generate decent credits to my account by my export V import costs.

One problem I do have is on morning start up my 30m/a main house rcd keeps tripping and then resets. Narrowed it down to inverter start up in morning when it boosts voltage to get into grid.

Have seen some recommendations of upgrading to 100ma trip rcd on the solar circuit only to overcome this issue.

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Craig Chamberlain commented · Apr 09 at 11:33 AM

Re the RCD tripping issue, I power my Inverter "AC In" circuit from an 50A MCB rather than an RCD/RCBO. I mainly did this to avoid selectivity issues with downstream RCBOs but also I don't think there is any need to have RCD protection on the feed to the inverter as long as the cable is properly protected (SWA) and not just unarmored cables buried in a wall somewhere.

In my case the inverter MCB is housed in my primary DB and then it runs through surface mounted trunking to the metal changeover switch enclosure and then on through a mix of surface mounted trunking and eventually SWA cable. That SWA then goes underground for 4m and terminates inside another metal DB in my garage where it goes to another changeover switch for future generator connection and finally to my Inverter AC isolator. All the metal parts involved, including the inverter, are earthed with 10mm cable and so any earth fault would trip the MCB well within the required disconnection time. Even a spade through the SWA would almost certainly cause a short circuit trip of the MCB long before any wires could come into contact with a person.

Of course, all of the circuits in DB2 which is connected to the output of the inverter are fully protected by individual RCBOs, and high current circuits that remain in DB1 such as the EV, electric shower and range cooker are all protected by RCBOs too.

I could have gone with either a time delayed RCD or a 100mA one, but couldn't find such a module for my Fusebox branded DB and I prefer not to mix circuit protection device brands if I can avoid it. I could have bought yet another enclosure and mounted the RCD inside that but it just adds complexity, more points of failure and as far as I can see, for little to no benefit.

So that's my view but as usual you should always get professional advice if needed. :)

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