Electronic – Why can’t I use solar PV system while disconnected from grid

I don't have experience with the specific models you are referencing but I do have experience with phase locking AC power sources. The bicycle analogy above is exactly how mechanical generators work and with solid state power inverters to the point of the inverter "pushing" on the grid however you don't want the grid to ever "push" on your inverter. The inverter watches the 60hz sine wave from the grid connection to trigger the driver to keep the output in phase with grid power. This is going to be a high impedance detection circuit so any source should do the job such as your pure wave inverter off a car battery. The listed tolerance on the sine wave is well outside the normal tolerance for mains. How precise is the frequency of the AC electricity network?

While connected to the grid it is critical the inverter is synced to the grid frequency and phase as tight as possible to prevent the inverter from seeing the grid as a load. If the grid were to have just gone down and your system started automatically the phase could be 90deg out of sync when the grid power is restored which would be worse than shorting the output of your inverter. This is why they inverter relies on an external clock source and shuts down if one is not present. Disconnected from the grid you have a self contained power system and the phase and frequency relationship to the grid is meaningless but the frequency does need to be close to 60hz for any devices you maybe running on that power.

(1) The inverter output will be exactly the line voltage, having raised the (local) line voltage according to the source impedance of the supply line and the current developed As the inverter is connected to the line, its output is the (locally measured) line voltage by definition. The "meter spins backwards" when generation exceeds use, because it senses the current direction. Often there would be separate meters for consumption and generation.

^{simulate this circuit – Schematic created using CircuitLab}

This is a residential distribution network, your house is No.2, the third from the pole transformer and the only one with solar power. Currently it's in shadow, so the line voltage reduces slightly further from the pole, and the last house (No.3) sees 109V.

When the sun comes out, the line voltage at the pole remains 115V, but your local line voltage also becomes 115V.

No. 1 is now fed from both sides and it's line will rise from 113V to 114V, and No.3 is still 2V below your line voltage, now seeing 113V.

Weather Line No.1 No.2 No.3
Cloudy 115V 113V 111V 109V
Sunny 115V 114V 115V 113V

(2) Where are you and what connection regime are you in? If you're in the UK and conforming to the G83/2 standard for a single phase residential connection, the answer is up to 16A. (Higher output is allowed on a separate standard, G59/2 for 3 phase connections). Any excess power is simply not drawn from the panel (assuming this is a grid tie system with no local storage)

None of this applies outside the UK.

(3) Pretty much nothing visible, maybe a very slightly higher voltage and marginally cleaner waveshape (assuming your inverter is legal and conforms to the applicable standards, including harmonic distortion.

The G83/2 specification illustrates the requirements for protection against overvoltage, out of tolerance frequency and waveform shape - and the all-important consideration of disconnecting immediately if the grid fails, to prevent electrocuting the engineers trying to repair it!

Here's an article on the rules in California to get you started on the US standards - which may well vary from State to State.

Grid regulation is a topic in itself - it isn't automatic, someone monitors it and adds or removes generation capacity to keep its parameters (voltage and frequency) within limits.

One characteristic of traditional (spinning metal) generators is that as you load them, they slow down a little, which reduces their output voltage and their contribution to the grid - transferring their load to others, which slow down in turn - arriving at a concensus on the actual mains frequency. You can watch this process in the UK in real time here. At the moment, it's reading 50.007Hz, so there's no need for additional capacity, but if it falls to 49.9Hz, phone calls will be made and some other power source will be turned on...

The importance of this for grid tie inverters is the way they can affect demand and power flows in the short term in unpredictable ways, taht the grid isn't currently designed for.