Wind energy is part of the growth of renewable energy in Colorado which became the first U.S. state to create a renewable portfolio standard (RPS) by ballot initiative when voters approved Amendment 37 in November 2004. The original version of Colorado's RPS required utilities serving 40,000 or more customers to generate or purchase enough renewable energy to supply 10% of their retail electric sales. In March 2007, HB 1281 increased the RPS and extended a separate renewable-energy requirement to electric cooperatives, among other changes. HB 1001 of 2010 further expanded the RPS. Eligible renewable-energy resources include solar-electric energy, wind energy, geothermal-electric energy, biomass facilities that burn nontoxic plants, landfill gas, animal waste, hydropower, recycled energy, and fuel cells using hydrogen derived from eligible renewables. The PUC has issued rules to implement the RPS. The rules were amended as required by HB 1001 in August 2010. The PUC's rules generally apply to investor-owned utilities (IOUs). Electric cooperatives and municipal utilities serving more than 40,000 customers are still bound to the separate requirement approved by the legislature.