Mean efficiency is what we use when determining C1 (fees for electricity) for holders.
Below is an example explaining how this indicator is obtained.
Let's say you have two NFTs.
One of them has a power of 2 TH/s and an energy efficiency of 50 W/TH. The second has a power of 128 TH/s and an energy efficiency of 25 W/TH.
To get the average energy efficiency of your NFTs, you need to add 50 and 25, and then divide them by 2.
You get 37.5 W/TH, which actually doesn't reflect reality because the weight of the second NFT is much larger.
That's why Mean Efficiency is necessary. In this case, the calculations will look like this: (2*50 + 128*25)/(2+128) = 25.38 W/TH
Knowing and seeing your Mean Efficiency indicator, it will be easier for you to understand how many watts your NFTs will require on average.