Currently, the limit of technology is fueled by crystals that emit heat energy. They appear as glowing quartz crystals. Those crystals are unique to this world and are mined in Dandnar, which are traded/stolen to the point where each region has access to them. The crystals can be depleted of energy like batteries, so they must be mined constantly. They grow at an accelerated rate to other minerals which allows them to be replaced without much effort.

As far as actual technology that uses these batteries, simple appliances like heated baths, stoves, and ovens are primarily the luxuries. The crystal batteries can also charge-up held items like weapons and tools. So axes chop harder, swords slash faster, ect. (Like in Harvest Moon games). The batteries have a limited supply of energy, so charged attacks are used sparingly or if there are other batteries to replace the expired ones.