The chart shows that the difficulty measures how much electricity is required to mine bitcoin. The higher the difficulty, the longer an ASIC must run to find a block; thus, it will spend more electricity. ASICs differ in their energy efficiencies, and in the chart, we have used the Antminer S19, a relatively energy-efficient model.
The Antminer S19 currently requires 181 MWh to mine one bitcoin. This is down 11% from 204 MWh in May when the difficulty sat at an all-time high.