Cells are wired in series to increase the voltage of the array. The currents of the cells will add. Cells are wired in parallel to increase the current of the array. A most important equation in electronics. Voltage x Current.
Power is measured in Watts.
The power varies with the angle the light hits the cell. It varies approximately sinusoidally. This question can only be answered with suggestions and experience. There are no hard facts or formulas to state, since every array will have different constraints that it needs to fit. You also need to know how much power you will need, both the voltage and the current.