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Old 10-28-20, 03:48 AM   #1
Barrowman
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Default Using my panels without the grid

Hi everyone just found this site and hope I can get something sorted out.
I have 16 panels which are wired as 2 sets of 8. I assume their output is at 192 volts.
The system is a grid tied one here in the U.K.
What I am wondering is if I can use the panels unmodified on their own off grid.
It's a 4Kw system.
If I fed one set to a PC power supply and the other to a separate PC power supply I am thinking I could wire one 12 volt output from one with a 5 volt output from the other so I could charge some batteries,
I know it would only give around 10 amps at 17 volts but that would be better than nothing and I think it might be possible to develop a more powerful system based on this.
Do you see any problems with trying this?
I am assuming that the output voltage from the panels would not rise much so as to cause any problems for the PSU input.
Forget this I didn't realise this had a transformer input and wasn't what I thought.


Last edited by Barrowman; 10-29-20 at 03:47 AM..
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Old 10-29-20, 03:45 AM   #2
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Although the unit I originally had was not what I thought I have been looking at how the PC power supply works. I have opened up a fairly recent one and think now it would work. I guess I didn't understand fully how they do work. Also I am wondering if I could run several of the outputs in parallel with some way of load balancing.
Alongside this I have a few old laptop chargers as well as some wall warts and maybe they could be used to provide lower power chargers.
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Old 10-29-20, 09:34 PM   #3
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Might want to take a look at server power supplies, commonly available into the low kW range. Modifying one to have MPPT is going to be tricky even if you're advanced with electronics.
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Old 10-30-20, 03:49 AM   #4
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Quote:
Originally Posted by NiHaoMike View Post
Might want to take a look at server power supplies, commonly available into the low kW range. Modifying one to have MPPT is going to be tricky even if you're advanced with electronics.
I like the idea of using server power supplies.
I was thinking that the 17 volts would feed a charge controller so no need to mess with them. Can't it work like that?
We tend not to think we might be without power for long here in Britain but we lived for 11 years on a Scottish island fed by a single undersea cable from the mainland. Something went wrong one winter and the whole island was without power for 3 days.
I have been trying to find out what the lowest input voltage needs to be for a charge controller as I will have to make sure I wire up the power supplies to at least reach this level but up to now all I can find is the maximum voltage and power. Does anyone know any values please?

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Old 10-30-20, 07:19 PM   #5
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I can see a few issues:
If you get a feed in tariff (as it's the UK), they won't like you messing with the DC from panel to inverter. You could inject DC here, and "make" money!
Normally a 2kW string of 8 panels would be at a higher voltage - I'd expect more like 70V per panel, so around 500V.
4kW at 12V is a lot of current - normally people would try and avoid such heavy currents, due to the losses in the copper wiring.

A much simpler option, avoiding the possible legal issues & dangerous side with the high voltage DC, would be to charge up batteries from AC, and then use them for certain loads. You could set a timer to charge the batts at mid-day, so they always get charged, and usually it's free. 12V lighting is a very suitable load, maybe power for a laptop. It won't charge up from PV when there's a power cut, but it's a lot simpler.
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Old 10-31-20, 07:22 AM   #6
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Quote:
Originally Posted by Robl View Post
I can see a few issues:
If you get a feed in tariff (as it's the UK), they won't like you messing with the DC from panel to inverter. You could inject DC here, and "make" money!
Normally a 2kW string of 8 panels would be at a higher voltage - I'd expect more like 70V per panel, so around 500V.
4kW at 12V is a lot of current - normally people would try and avoid such heavy currents, due to the losses in the copper wiring.

A much simpler option, avoiding the possible legal issues & dangerous side with the high voltage DC, would be to charge up batteries from AC, and then use them for certain loads. You could set a timer to charge the batts at mid-day, so they always get charged, and usually it's free. 12V lighting is a very suitable load, maybe power for a laptop. It won't charge up from PV when there's a power cut, but it's a lot simpler.
I can see that I could inject DC but it would surely cost me more to generate it than I would get paid wouldn't it?
Or am I missing something?

I am surprised that you think the voltage will be so high.
I just checked an ebay listing which shows the following
Quote:
Solar panels
250 watts

Dimensions (H x W x D) in mm: 1485mm x 992mm x 40mm

Weight in kg: 15.8

Cell type: multicrystalline

Power max .: 250 watts

Open circuit voltage max .: 28.16 V

Max voltage: 33.80 volts

Max current: 8.88 A.

Short-circuit current max .: 9.16 A

Module efficiency: 16.97%
Which is in line with what I expected.

It's interesting that you suggested a timed battery charging regime. I had considered doing that and am in any case going to implement a small system to keep my spare car battery and my wife's mobility scooter battery in reasonable condition seeing as my wife and I are old and in poor health so I am rarely using my car.
Still that's no good for such a time when the grid is off for any length of time as the inverter will not run.
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Old 10-31-20, 11:48 AM   #7
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Hi Barrowman - we're in the UK too, got our 4kWpk solar 6 years ago. I still have the paperwork - we have 2 strings of 10 "Kinve" panels, each panel max power is at 36V*5.5A, and 45V peak, so each string could have 450V dc from it. The "Samil" solar inverter we have can cope with up to 550V.
I think that in the 2kW-10kW range high voltages around this level are favoured, as it keeps the DC cable current lower, so cheaper, but isn't so high voltage to need specialised components. I think that ebay one that you found is intended to be connected via a PWM controller to a 24V battery, and that grid tied ones we have are usually higher voltage. You could google the particular inverter model that you have, and it will say the max voltage.

We get a "Feed In Tariff" of 17p, and 2p per kWh, so 19p/kWh overall. That is slightly more than the cost of electricity. It is odd, I agree. I would be more comfortable with just using the grid as a battery than the FIT (which is pretty much gone now in the UK).

When did you get solar - do you get the FIT or not ?
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Old 10-31-20, 01:17 PM   #8
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We moved back to England about 5 1/2 years ago. The system had already been in place since early 2013 so we got a straight transfer from the last owners.
The cables from the panels look as if they could easily handle 10 amps. I've forgotten what the inverter is and not feeling up to climbing up into my loft right now.
In my younger years I did a lot of electronics stuff including repairing the old colour CRT TV's ( 25 KV DC on the anode ) for family, friends and workmates so well aware of the need for caution.

Well if the output of each string is around 500 volts then I could wire the inputs of 3 psu's in series and then I would have a wider range of combination of outputs to wire in series to give suitable voltages to feed the charge controllers.

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Old 10-31-20, 07:03 PM   #9
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Wiring dc smps power supply inputs in series is unlikely to work. Maybe if they were identical and designed for it, but its a recipe for a bang I fear. Any mismatch, and one of them would get all the volts, and will instantly fail, then the next too. Those psu have turn on delays, overcurrent trips - again, mismatch, bang.
In 2013 the fit was alive and well - perhaps its a ‘rent a roof’ scheme - the panels are owned by a company, you get free electricity from them, they get the feed in fariff? If the fit is involved, you mustn't connect to the dc cables. Aside from the danger too.
There are solar controllers designed for off grid I think. I think the newer Tesla Powerwall can ‘Island’, among others.
How often do you get powercuts? We have one every few years, 5mins maybe. Fridges, freezers, laptops just ride through - maybe a few lights would be good to keep on, but that’s it. Unless you had a serious ‘islanding’ system, it won’t cope with normal house loads.
A timed battery charger is so much simpler and cheaper to the above though.
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Old 11-01-20, 04:52 AM   #10
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Okay thanks for the information Robi. I certainly do get FIT. I get a cheque every 3 months and average about 60 per month.
I guess I will go for the timed battery charging, As you say it's very simple.
We have only 1 power cut which lasted several hours, but quite a few lasting from a few seconds up to about 5 minutes. My main reason for what I had intended was that although our home is on fairly high ground the immediate area is a flood plain.
Now with so many changes in the weather we are getting many more days with heavy rain and we have seen what happens in areas with flooding and total lost of power for long periods.

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