The RPS mechanism generally places an obligation on electricity supply companies to produce a specified fraction of their electricity from renewable energy sources. Certified renewable energy generators earn certificates for every unit of electricity they produce and can sell these along with their electricity to supply companies. Supply companies then pass the certificates to some form of regulatory body to demonstrate their compliance with their regulatory obligations. Because it is a market mandate, the RPS relies almost entirely on the private market for its implementation. Unlike feed-in tariffs which guarantee purchase of all renewable energy regardless of cost, RPS programs tend to allow more price competition between different types of renewable energy, but can be limited in competition through eligibility and multipliers for RPS programs. Those supporting the adoption of RPS mechanisms claim that market implementation will result in competition, efficiency, and innovation that will deliver renewable energy at the lowest possible cost, allowing renewable energy to compete with cheaper fossil fuel energy sources.