A Basic Unanswered Question but fundimental to a householder

[DickiePhitt]

None of the PV companies I've dealt with so far has been able to explain what, to me, is a basic question. In a property with a grid connection and a PV installation what ensures that the energy from the array is used preferentially to that coming through the utility meter?

Reverting to the (I know , oversimplified) water analogy. I envisage such a system as a big tank of water ( the grid) and a small tank of water (the PV output) connected together at the outlet to feed a tap (house load). Turn the tap on and water (power) is taken from both tanks, not just the small one.

So what is different with a PV installation? What makes the house load pull preferentially from the PV up to the capacity of the system?

 

[Deleted member 26818]

I could be completely wrong here, as I don't deal with PV in any way shape or form.
However it would seem to me, that when the PV system produces energy, that energy has to go somewhere.
So unless there's a storage facility, or some means of dissipating the energy produced, it will be used.
Think of it like the small tank of water, but with only one entrance and one exit, and no means of stopping the water entering.
Either the water will exit, or the tank will burst.

 

[Bradzz]

If I’m right you are asking what makes a house use the total load of the PV output.

 

[Bradzz]

I could be completely wrong here, as I don't deal with PV in any way shape or form.
However it would seem to me, that when the PV system produces energy, that energy has to go somewhere.
So unless there's a storage facility, or some means of dissipating the energy produced, it will be used.
Think of it like the small tank of water, but with only one entrance and one exit, and no means of stopping the water entering.
Either the water will exit, or the tank will burst.

As Spimlondon has said, also the pv install does not go through the meter, and when the pv install is in use the grid meter slows down, so you use less power from the grid, and when you are not using equipment you add back to the grid, thus lowering your bills.

 

[DickiePhitt]

So taking what Spinlondon has said, and going back to my 2 tank analogy - the second being the big one (grid). Where the grid differs is that, unlike the PV output, it does have more than one exit (other consumers). As a result the property takes the PV power preferentially up to the output of the array. Its as though the small tank has a variable output 'pump' on it

Bradzz, it wasn't the question of using the total, it was the issue of why the house would use the available PV power in preference to the grid - in my 2 tank analogy it would be more 50:50. As Spinlondon suggests the PV is going to push it through whereas the grid isn't (simplistically speaking, if I understand correctly).

Thanks guys

 

[malcolmsanford]

The PV system is working at a higher voltage than the grid mains system, your network voltage is 230V say and the PV system is producing say 260v, so the invertor will feed the higher voltage into the system.

 

[Worcester]

@DickePhitt

What malcomsandford has said is correct (though exaggerated!)

The inverter, in order to export the electricity creates its output at a slightly higher voltage than the grid. (It is constantly monitoring it to make sure it is.) This can cause problems where the grid is already running overvoltage (see below), and sometimes we have to ask them to reduce it.

To use your water analogy, that means it has a greater 'head' / pressure, so unless that output is used locally (wether it's connected before or after the meter is irrelevant, except that yuou don't want to be paying for your own electricity!) it will be exported, and it could well be that when you are not using it, your neighbour is!

This last point is why, when we are connecting up a number of houses in close proximity we have to ask the DNO (Network Operator that fixes the power supply , not who you buy it from) for prior approval for connections, because after a while, the neighbourhood overall will be an exporter, even if each installation is less than 3.68kWp (by the way they have talked about the implications of going > 3.68kWp and <4kWp haven't they!)

 

[malcolmsanford]

Now now worcester mate I was using the said voltages as an example I was not even going to go into technical reasons................:shades_smile:

 

[DickiePhitt]

All makes sense now. Hate not quite understanding things, especially when there's so much money involved. Thanks for the enlightenment.

PS Yes, Worcester, I'm ok on the significance of the 3.68kWp value.

Thanks again, guys.