Not sure that’s true. And mining on a CPU is even less efficient. Your hash rate will be way lower unless you’ve got a really high-end system with a really low latency bus and RAM. And if your hashrate is too low, it would take months for you to find a single block unless you’re pooling with a bunch of others and splitting the profit. It’s quite variable, but very, very few people can make profit on any popular coins. Too many people to compete with to find a block.

Oh and don’t forget cooling cost. The fans in the computer, the fans in your house, and your air conditioner in your house need to disipate the heat and there’s a lot more generated per clock cycle from a CPU than a GPU using comperably old technologies. If you live somewhere that you’re producing more electricity with solar than you consume, then it’s probably not a cold climate.