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PV Dedicated Circuit?


Gone West
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I've got a PV array on the roof, installed in 2012, and there is a 16A MCB in the CU labelled PV. Is it normal to have this arrangement and does it mean that only what is connected to that MCB can use power from the PVs and that if nothing is connected all the PV output goes to the grid. The PV diagram shows a dedicated PV circuit.

 

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Couple of points from what I can see:

Drawing says 20A supply yet your PV is on a 16A MCB, no big deal just a point.

Not a fan on the PV being connected onto an RCD that feeds other circuits, as in my mind it will effect the disconnection times of the RCD in the event of a fault, you are not alone in the installation being done this way, it appeared to be a common way of doing it, I personally just don’t like it.

Regards to what you are asking in your post, if you are using less energy than you are generating then the excess will go to grid.  In my case the base load of my house without putting anything extra on is about 200W (router, digital clocks etc.) last week at any moment in time I was hardly generating more than 100W as it was so dull, so at no point was my generation greater than my usage.

I have fitted a solar diverter so that if my generation is greater than usage the surplus goes to a heater in a north facing always cold room and not to grid.

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1 hour ago, Roys said:

Regards to what you are asking in your post, if you are using less energy than you are generating then the excess will go to grid.  In my case the base load of my house without putting anything extra on is about 200W (router, digital clocks etc.) last week at any moment in time I was hardly generating more than 100W as it was so dull, so at no point was my generation greater than my usage.

So I have to find what is at the other end of the PV MCB circuit, if anything, and then that will be solely using PV output I guess. If there is nothing connected to the PV MCB circuit how does the PV ouput get used by other stuff on other circuits. How is your solar diverter wired up.

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2 minutes ago, Gone West said:

So I have to find what is at the other end of the PV MCB circuit, if anything, and then that will be solely using PV output I guess. If there is nothing connected to the PV MCB circuit how does the PV ouput get used by other stuff on other circuits. How is your solar diverter wired up.

No it does not matter what circuit is drawing the load.

 

It all gets sorted out by your electricity meter.  That measures what enters (import) or what leaves (export) your house.

 

So if the PV is generating more than the entire load in the house at any instance, then some will be exported.  If the house is using more than the PV is generating at any instant, then some will be imported.

 

The best way to self use as much as you can is use the large appliances (washing machine etc) in the middle of the day.

 

Solar pv diverters automatically monitor import and export and send any surplus power to a dump load, typically your immersion heater to help with self usage.

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2 minutes ago, ProDave said:

No it does not matter what circuit is drawing the load.

So how is the PV MCB circuit wired, is it only supplied by the PVs and therefore when they aren't producing there wouldn't be any power on that circuit?

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1 minute ago, Gone West said:

So how is the PV MCB circuit wired, is it only supplied by the PVs and therefore when they aren't producing there wouldn't be any power on that circuit?

Okay think of it as water.  People find water easier to understand.

 

So your PV is like a borehole, it is producing some water.

All your "circuits" in the house are each using some water.

You have mains water connected as well

 

If your borehole (PV) is producing more water than all the circuits in the house are using, then you will export water to the mains.

If your borehole (PV) is only producing a little bit of water, and the rest of the house is using lots of water you will import water from the mains.

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1 minute ago, ProDave said:

People find water easier to understand.

I'm not understanding, but is that because it's not really a dedicated PV circuit as stated on the drawing. I get what you're saying about the two supplies being lumped together but the drawing says it's a dedicated circuit which to me means a separate circuit isolated from the mains. I hope it's not really a dedicated PV circuit.

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You don’t have to do anything. It’s been fitted since 2012!

hopefully you will be getting fit payments so read the meter every quarter is about all you have to do!

 

unless you want to use the excess and heat the hot water in your tank via the immersion so you will need an inverter and some extra cabling…

 

I think I get nearly 60p for a unit of generated electricity, so all good, no immersion for me so nice and easy.

 

 

Edited by TonyT
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35 minutes ago, TonyT said:

I think I get nearly 60p for a unit of generated electricity, so all good, no immersion for me so nice and easy.

 

 

Oh my oh my. We just hit the 1 year mark since ours was installed , 6600 kWh  generated.  To think if we'd been on that FiT .... £4,000 per year, plus the use of free electricity!

 

I just looked and PVGIS-5 estimated 6882.7 kWh not too shabby especially as we had scaffold by it for 4 (winter) months.

 

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1 hour ago, Gone West said:

So how is the PV MCB circuit wired, is it only supplied by the PVs and therefore when they aren't producing there wouldn't be any power on that circuit?

 

There will always be voltage on that circuit when the grid is connected and the breakers are closed, but no power will flow if the panels aren't producing (and there's nothing else connected).

 

You're making the assumption that because it's called a circuit it is completely separate from the other circuits. It isn't. When all the breakers are on all the circuits are connected together, there is no electrical separation between them. The breakers are there for protecting individual circuits in the event of a fault and convenience for modifications/fault finding etc.

 

 

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1 hour ago, TonyT said:

You don’t have to do anything. It’s been fitted since 2012!

Yes, but I've owned it for less than six months, so  would like to understand how it's been wired and to use it most efficiently. Trying to get SSE to move at anything faster than a snails pace is impossible. They have had all the paperwork for ages but we're yet to see any payment. To be fair it isn't that straightforward as there was a double probate and the property had been empty for a year before we moved in.

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I have never liked the 'water' analogy to describe electrical circuits, so why not describe what actually happens.

Or attempt to.

 

If you take a piece of conducting material, metals are good one.  They are composed of atoms, and the atoms are made, in the simplest model, of the nucleus and orbiting electrons.  At normal temperatures, all is fairly stable and everything stays in place.  There is a little jiggling of atoms going on, but on average, they cancel each other out.

Now if you put a hot flame at one end of the metal, while holding the other end, after a short time, your fingers get hot.  The energy from the flame has somehow travelled along the bit of metal.

Electricity does exactly the same, and in the same way.

What happens in a circuit is that the voltage has enough energy in it to overcome the inherent stability of the material.  It does this by pushing an electron to a higher energy state.  If there is enough voltage, the electro breaks free from its atom, leaving behind a space, which is called a 'hole'. Atoms hate to be unbalanced, and electron hate to be free.  So at the very first opportunity, the hole will get filled by a free electron.  Sometimes this is the same electron that it has just left, other times it is another free electron looking for a hole.

This movement of electrons is what causes current, measured in amps.  The more electrons moved, the greater the current being passed though the metal.

The faster you can move those electron from their atoms to other atoms, the more current you have.  So it is just down to voltage times amps, which we know is power (W = V x A).

The level that the voltage is at, is what sets the speed the electrons 'move' at. The size, and therefore the amount of material, sets the amount of current that can pass for any given speed (or voltage).  Basically a thicker wire has more atoms in it, so more electrons can be moved.

Now imagine what happens when one set of flowing electrons meets another set, coming in the opposite direction.

If the speed and amounts are the same, they will just cancel each other out,  The laws of momentum tell us this.  Mass times velocity, (M x v) + -(M X v) = 0.

Now a PV system, after the inverter, runs an a slightly higher voltage than the incoming mains electricity (it senses this voltage, as well as the phase and resistance constantly).

This means that no electricity can flow up the wires and circuits to heat up the PV modules until they emit light.  Also when the PV is not generating, that pathway is blocked.

When generating, the free electrons, that are looking for empty holes, find another path, the path of least resistance, until they do find those spare holes.  Those holes will be on any connected circuit that has a load on it.  This is really the definition of a circuit, no load and there is no connection between the negative side and the positive side.

So by keeping the PV side at a slightly higher voltage than the incoming supply, electrons are forced to 'look' for those free holes, and if they cannot find any, because the are at a higher energy state, they can physically smash out an electron from the atom it is orbiting.  In doing so, it looses energy, and plops into the vacated hole, leaving the displaced electron to do the same to the next electron.  This bullying by the higher voltage electron eventually forces enough electrons to go back down the grid wires until they meet a load, which may be your neighbours lights.

Now if there are not enough loads on a circuit, and you have two competing sources making free electrons floating about, you eventually get way to many at the elevated energy state.  This causes the metal wire to heat up.  Eventually the wire will get so hot that it will melt, this breaks the circuit and stops current flowing, even if the voltage is still high.  This is why there is a circuit breaker on the PV circuit, it stops wires melting.

Summing up, the higher voltage generated by the PV system allows the electrons to flow to the nearest load, if that nearest load happens to be miles down a wire, they will still flow there.  But what usually happens on a long circuit is that all the pushing and shoving that the electrons do just heats up the wires, the conservation of energy tell us that.

So to use up 'your' electricity, switch on a load, to use your neighbours electricity , or the grid electricity, turn on a load.  To stop your neighbours using your electrify, turn on a load, or turn your incoming supply off, though that will also turn all your electricity off as well.

 

Edited by SteamyTea
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12 hours ago, joe90 said:

Pete, my electrician is Solar qualified, does a lot round our area, can get you his number if you need it ?

Thanks John, it's good to have recommended local trades. We're ok at the moment, I was just interested in why there was supposedly a dedicated PV circuit in the CU and apparently it isn't dedicated.

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13 minutes ago, billt said:

But it is dedicated. That circuit is intended for the connection of the PV inverter only, hence dedicated to the PV system. It's not electrically separate but it is functionally separate.

Thanks, yes I understand that, it was the wording that I found confusing. The 'Dedicated 20A supply from main consumer unit' wording, as it said from CU not to CU.

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The protective device (MCB in the CU) is protecting that dedicated cable and the inverter in the case of fault. Hence the supply is FROM the CU to the PV isolator whichever way the electrons are flowing. Think of it as all roads lead from London, despite being able to drive the other way and get there.

 

When my PV isn't generating I see a tiny (2W) demand on my PV circuit which may just be noise or could be considered the parasitic load of the inverter. Hence when the PV is not producing power, that dedicated circuit is consuming a tiny amount of power from the mains.

 

The 'dedicated' in solar circuits means 'only for the PV connection' not 'electrically separate' as others have explained above. You definitely still get first dibs on the energy produced before the excess gets exported. 

 

Now sit back and be smug about your FIT, some of us aren't so lucky! ?

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1 hour ago, Wil said:

Now sit back and be smug about your FIT, some of us aren't so lucky!

We are lucky having bought a place with PVs and ST and we didn't realise initially that it existed. It'll be good when we get our first FiT payment. 

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41 minutes ago, Gone West said:

We are lucky having bought a place with PVs and ST and we didn't realise initially that it existed. It'll be good when we get our first FiT payment. 

What you going to do with the excess electricity in the summer?

How large is the PV system.

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3 hours ago, SteamyTea said:

What you going to do with the excess electricity in the summer?

How large is the PV system.

It's a 3.84kWp system. When we get sorted we will probably have a 250l or 300l thermal store which will be heated with the ST and PV so we can have mains pressure hot water. Something we miss from our last house. We may at some point get an EV which should soak up some as well.

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1 minute ago, Gone West said:

IIRC it's 1.4kW and it helps to heat the 140l vented DHW tank.

Should easily heat that on a half decent day, even allowing for Cornish Cloud.

Did I tell you that when I moved down it rained every day for 66 days.  That record was broken last year I believe.

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2 minutes ago, SteamyTea said:

Did I tell you that when I moved down it rained every day for 66 days.  That record was broken last year I believe.

Up until the last few weeks the weather has been much drier here that I was expecting. Over the summer there's been less rain here than in SE London. At least I don't have to water the garden as much as I used to. I much prefer the weather down here and no cold east wind.

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