Calculating the intensity of an electromagnetic wave
If I’m given an electronmagnetic wave with an energy of 1 eV, how do I calculate its intensity ?


You don't have enough information to calculate the intensity.

In addition to the energy received, you also need to know over how wide an area the wave front was spread, and how long it took to receive the given amount of energy.

Practically, when we want to measure the intensity of an EM wave, we use a device like an antenna or a photodiode. This allows us to measure the power received (rather than energy). And by knowing the effective area of the antenna or photodiode we can quickly calculate the intensity. If we want to know the energy received, we have to do a post-processing calculation (integrate the received power over time). 

So a realistic problem would be if you know the received power in watts, and the area of your receiver in \$\rm m^2\$, you would calculate the intensity from those values.

It's possible, for example when measuring the output of a pulsed laser, to know the received energy rather than the received power. In that case if you really want to know the peak intensity of the pulse, you'll need to do a more involved measurement, for example with a higher-bandwidth receiver, or using auto-correlation, to determine the pulse shape before you can find the intensity.