On-grid solar system - how to control power out from solar system to grid?
If you have a solar system with a grid-tied inverter, the inverter is part of the grid and is pushing onto the grid as hard as necessary to send away the power it has made -- and no harder.  In other words it is just another generator on that grid - like the stinky coal plant and Oroville Dam and Kursk Unit 3 and that offshore wind farm.  Unlike those loads, your grid-tie inverter  won't "back off" or try to load-follow, it will just push until its power is delivered. 

If it can deliver power to your house, that's nice.  From the inverter's perspective, it's part of the grid and your house is just another load served by that grid. 

Your grid-tie solar array runs full-boat if it runs at all.   How does it match up to the grid? By making small adjustments to output voltage. Too low and no current will flow.  Too high and too much current will try to flow. The inverter seeks the voltage where the right amount of current flows.  

If local grid demand is high, voltage will be sagging - and the inverter reduces its voltage to match.  If load is light and voltage is running high, the inverter will raise voltage as necessary to push exactly what it has to give.

The inverter  is also watching for line voltage gone out of bounds.  If voltage spikes much too high, it means the other generators are not backing off (or more likely, the grid is broken) and the grid-tie inverter shuts down.   Also, if voltage goes too low (severe brownout) the inverter will also shut down, partly to prevent "islanding". 

Solar panels notwithstanding, other large generators are designed to react dynamically to varying loads on the grid. If voltage rises high, those generators will "back off" and generate less power.   These work as a counterpoint to solar and wind, which generate at max capacity no mattter what.  Hydro is particularly good at this, it can even "back-pump", intentionally allowing the grid to push the generator and cause it to pump water up back into the reservoir to be used later. A solar inverter can't be pushed, because it's basically an array of diodes.