It's most likely because of our overdrive mechanic. Any excess energy you have is out into your mexes to mine more metal. And because of that you will always spend more energy than you produce. At the same time, as long as your production and utility energy usage doesn't exceed your production, then your storage will be full.
So if you produce more energy than you need, excess still gets used by mexes, if you have your generators aren't connected to mexes, they don't overdrive and the number would show no drain probably.
I hope this explanation was clear... I think it could be explained better.
