Yes, take a look under techs/magitech/resources/food. This is a resource where you can store a certain amount of it and it also creates a steady flow. The reason energy source has a negative value for energy is because that is the cost (it's a bit of a hack). In other words, if it has a negative cost, then it is actually giving you energy. Energy is a static resource. There is a certain level of supply and there is a certain level of usage. Excesses are not stored. Food is consumable. If your consumable resources go negative, a number of your units will take damage every 30 or 60 seconds until they die off (thereby lowering your consumption rate) or you get more food to feed them!