Energy sources are sustainable if the following conditions are met:
- The long-term availability of the energy source that guarantees to meet any present or future consumption needs.
- The energy source must be replenishable without human intervention.
- The amount of energy consumed to exploit the available resources should not exceed the amount of energy these resources produce.
Countries in the world that are renewable energy leaders are: Costa Rica, Iceland, Norway, Portugal, Germany and China.
In Iceland almost 100% of all energy consumed is renewable energy. The energy need is largely met through hydroelectric and geothermal sources which are sustainable energy sources. Nine out of every ten houses in the country are heated directly with geothermal energy. The National Power Company of Iceland operates 18 power stations producing sustainable and renewable energy.