From pure electric charge calculation:
1. Every lightning strike is in the average, just five billion joules. This is about 1,400kWh of energy, if we take as true that there is no loss in storage and transfer.
2. Lightning strikes in a year average around 1.4 billion. And of them, only 25% are ground strikes as the majority (75 percent) are cloud-cloud or intra-cloud which are not able to be utilized. This leaves just 350 million lightning strike that might be harnessed. Also, assuming 100 per cent harnessing of all lightning strikes, no loss in capture, transfer and storage, that is 490,000,000,000kWh/year.
3. In 2009, the world used around 20,279,640,000,000kWh – over 40 times the electrical energy that all the hypothetically harness-able land strikes contain. In essence, every lightning strike we could harness will provide the world with enough power for just nine days!
There’s more. If you’re looking to know the cost to do this:
To record each lightning flash (land strikes are the only ones) it is likely that we will need to build very high structures (think of the Eiffel Tower) approximately one mile apart in a grid which covers the entire globe. This is one tower for every 200,000,000 square meters of earth’s surface.