solar panels connected in series and parallel
Since in electrical the above equation P= V*I is possible mathematically, is it also possible practically ??


Basically yes. That equation exists because it matches real world observations. However, you must also account for other equations that match real world observations. Here's one that is especially relevant in this context:

$$ P = I^2 R $$

This is Joule's first law. If you have 1205A, R better be very small. R here is the resistance of the wire carrying your 1205A. Let's say it's just \$1m\Omega\$. Then:

$$ P = (1205A)^2 \cdot 0.001\Omega \approx 1452W $$

That's an awful lot of power to be wasted just in the wire. It's probably getting pretty hot.

It's usually easier and more economical to make wires with strong insulation that won't break down under high voltage than it is to make wires with low resistance, since this requires large amounts of expensive metals. Thus, when people have a lot of electrical energy to transfer, as a power plant does, they tend to use high voltage rather than high current.

Regarding the calculation of how much power you can get out of some number of solar panels, you need not make the math so complex. If one panel gets you 250W, then two gives you twice that much. I'm not sure how you are "combining in series and parallel", but to get 518750 watts, you need, one way or another:

$$ \frac{518750W}{250W} = 2075 \text{ panels} $$

If you connect the panels in series, then their voltages add, but the current remains the same. If you connect them in parallel, then the currents add, but the voltage remains the same.


  And after converting direct current to alternating current will there be a power change ?


There is nothing that makes AC or DC inherently have more or less power than the other. However, any way you might realize such a conversion will involve some losses, so the output electrical power will be somewhat less than the input power, with the difference likely converted to heat in your conversion device.