Bitcoin miners in aggregate pay for electricity to process blocks of transactions, and the number of blocks is predictable (one every 10 minutes or so). The calculable metric is consumption (of electricity or water) per block. Each block can contain one or thousands of transactions, depending on demand and size (in terms of memory consumption). Currently, there are around 3-4,000 transactions per block, but earlier this year, the number was more like 1,000.