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Remember that electricity flows freely through wires, so there is no way to direct the specific energy from solar directly to the Powerwalls. The only thing under control is how much power the Powerwalls are generating or absorbing. The lines are just a way of making the power flows easier to understand. When the Powerwalls lag a bit in adjusting their charging/discharge rates or there is a rounding error causing a mismatch in values, the end result can be that extra flows are shown for a short amount of time.
Note that it would be just as correct to describe the situation as .4 kW going from the grid to the Powerwall with the house being supplied by .4 kW from the grid and .4 kW from solar, just not as intuitive or representative of the intent of the schedule.
Yes, but isn't it the higher voltage that the inverter puts out that determines to which load it will flow to? The grid presents as a load when its voltage is less than the inverter. A Grid Tie inverter only has one connection to a service panel or sub panel and current will flow to all the panels behind the meter based on their loads before any excess goes to the grid. The panels are bussed together by their breakers on the main panel with other load breakers.Devices like these are not all bused together on a single wire with electricity flowing in every direction. The flow of energy is directed and gated by the electronics. So it very much does decide how much energy from the solar cells is directed to which destination, the power wall, the home or the grid.
Yes, but isn't it the higher voltage that the inverter puts out that determines to which load it will flow to? The grid presents as a load when its voltage is less than the inverter. A Grid Tie inverter only has one connection to a service panel or sub panel and current will flow to all the panels behind the meter based on their loads before any excess goes to the grid. The panels are bussed together by their breakers on the main panel with other load breakers.
Perhaps we are saying the same thing.
The Tesla PowerWall is AC coupled to the Grid Tie solar inverter. I assume that is what you mean when you mention those devices have to have a common power buss in order to work. As I mentioned earlier the main service panel is common to all the breakers and subpanels, except when the grid is down and, as you mention, the transfer switch disconnects everything except the critical loads (or the whole house), the Grid Tie inverter and the PowerWall.........
I don't know exactly how Tesla or anyone else wires their systems, but if they want the solar power to charge the batteries it can be done by wiring it all together on a single power bus and letting the components talk to one another to draw what is intended for that moment. Any demand that isn't satisfied by the solar would come from the grid. But to use the battery when the grid is down definitely requires a contactor to isolate the grid and requires the solar inverter to operate stand alone. Otherwise you are right, the power can all be bused on a single set of wires.
Devices like these are not all bused together on a single wire with electricity flowing in every direction. The flow of energy is directed and gated by the electronics. So it very much does decide how much energy from the solar cells is directed to which destination, the power wall, the home or the grid.