I have a common electrical panel fed by the grid. I have a 5.2 kW solar array backed in to the lowest breaker slot on a 30 amp breaker. The system was professionally installed and I never had a chance to ask the electrician, how on earth do the loads "choose" to use the solar power coming from the inverter before using the power from the main? Does the meter on the outside of my house have something in it to where as soon as anything higher than 120 volts is sensed over a load it automatically stops being powered from the grid until the load goes higher than what the inverter can provide ?
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Electrons are fungible — you can't tell the difference between one electron and another. Therefore, no actual switching is required — your power meter simply measures the net power consumption of your house, which is the difference between what your loads require and what the solar array is providing.
In some jurisdictions, if that net value becomes negative — in other words, you're generating more power than you need — the meter does measure the power that you're feeding into the grid separately, because the rate at which you sell power to the grid is not the same rate at which you purchase it.
If the solar inverter is producing very-slightly more voltage than the grid then power is fed back into the grid. That's point 1 and, behind point 1 is a fair bit of technicality to make this happen in a dignified way.
Point 2 is that if, the solar inverter is pushing power back into the grid then, any load connected on the household MUST be exclusively taking power from the solar inverter.
If the load from the household cannot be totally provided by the solar inverter, then there is still a very good chance that the design is such that it can provide some of those watts needed but, it rapidly gets to a point where the grid supplies all the power.
Dependant on the tech involved this "marginal region" might be a little bit wider than other installations but, no switches are necessary.
With competing generators, it's haggled out by voltage pressure in a sort of electron economy. Suppose your house has 4kw of load, and the resistance of the wires to your home drops your breaker-box voltage to 239v. The solar panel comes online and pushes 10kw. It's going to increase voltage until that amount of power flows, competing with the grid for your local load, and competing to push its power onto the grid against wire resistance. Imagine a "constant wattage power supply" that bumps up voltage until voltage*current = target wattage. For a solar grid tie inverter, it's just like that.
For instance the solar might push the voltage (at your breaker box) up to 241.5 volts, if necessary to push through wire resistance. Other houses in your block will draw preferentially from your solar due to higher voltage/less resistance. That will increase voltage on your street by some millivolts, which will make grid power seek other streets with lower voltage, etc. The whole system balances out, simply using voltage as a passive means to route current to where it's most needed. Back at the natural-gas generating station, if they see system voltage getting high (from too much generation compared to load), they will back off their output to keep voltage in spec.