Solar PV Question: >>> prior use of home generated energy

Sterlingtimes

How does a domestic solar PV system know that it is to direct home generated energy to home appliances before using externally generated energy?

Dave_Fowler

Sterlingtimes wrote: »

How does a domestic solar PV system know that it is to direct home generated energy to home appliances before using externally generated energy?

The PV inverter is connected to the house side of the import electric meter. The electric meter reads the nett power used in the house. If you are generating the same amount as you are using, there will be no nett power to read on the import meter. If you are generating less than you are using the meter will read the difference. Should you be generating more than you are using, there will be a nett export which a modern meter will ignore; older meters can run backwards or get quite confused.

Dave F

EricMears

Sterlingtimes wrote: »

How does a domestic solar PV system know that it is to direct home generated energy to home appliances before using externally generated energy?

You're looking at it the wrong way !

Why would your house try and import electricity when its already generating more than you can use and the excess is fighting its way down the same cable so that other people in the street can use it.

There are much more technical reasons and no doubt someone else will attempt to explain in simple language (I see Dave's already tried).

BUT: Why do you drink out of the top of a cup when the bottom is full of tea ?

Sterlingtimes

Dave_Fowler wrote: »

If you are generating the same amount as you are using, there will be no nett power to read on the import meter.
Dave F

Thank you, Dave. I appreciate your explanation. I wondered whether there would be some sort of electrical switch over, but it appears to be much more basic than I thought. That is helpful.

mac2008

Sterlingtimes wrote: »

Thank you, Dave. I appreciate your explanation. I wondered whether there would be some sort of electrical switch over, but it appears to be much more basic than I thought. That is helpful.

Yes it is pretty much that simple. The inverter monitors the incoming mains voltage and sets its output a bit higher. Think of volts like water pressure.


The downside of having no physical switching is that the system will not work without incoming mains voltage, for safety reasons. So no use in a power cut, even if it is sunny!

Ectophile

There's another way of looking at it using Kirchoff's current law:
"At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node." http://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws

Somewhere in your system there must be a 3-way junction where the incoming mains meets the supply to the house and the output of the inverter. In mine, it's the junction box where the installer teed off the supply to install a new mini consumer unit for the inverter.

Using that law, if the house is using more current than the inverter is supplying, then the excess must be coming from the mains. If the inverter is producing more than the house is using, then the excess has to be pushed out to the mains.

thenudeone

Sterlingtimes wrote: »

How does a domestic solar PV system know that it is to direct home generated energy to home appliances before using externally generated energy?

It doesn't, and neither do any of your appliances know "where" the electricity comes from, they just use it.

Very simply - think of your wiring from the meter as a pipe delivering water to your appliances when you turn them on; and your PV as rainwater harvester, and connected to the same pipe somewhere between your meter and your appliance.
When it's raining bucketloads, your rainwater system will be collecting more than your appliances can use, and water would flow "backwards" if you like, in to the water mains. At other times, it will generate very little water, and your appliances will need to import water from the mains.
But neither your water collection system nor your appliances "know" what's happening, and they don't need to. The difference between what you generate and what you use (either plus or minus) is managed automatically by the grid.