If you really mean gigawatts (unit of power), then you have to multiply it by the time period when this peak power is reached to get the energy output. Lightning bolts have very short durations, tens of microseconds.

Let's be generous and give it 1 millisecond. 1 millisecond times 1 gigawatt is 1 million joules, which is the estimate that the article gives.

It’s also pretty easy to figure where it will strike. Just wire up the ol’ clock tower.

the question is how long.

For example the flash you have/had on standard camera (not smartphone who have LEDs, but the standard compact camera that is just a camera) is about 1kW!!

But this 1kW you have it for about 1ms (milisecond!!). It seems to last longer because the light gets “burned” into your eye.

For 0.2 seconds which is wolfram alpha says is 67.2 kWh

Great Scott!