Solar voltaic cells based on crystalline silicon have oparated with a 30% efficiency for experimental cell and 15-20% for commercial units available in 2008, at a cost of around 20-25 cents/kWh, compared to 4-7 cents/kWh for fossil fuel fired power plants and 6-9 cents for those fired by biomass. Part of the high cost results from the fact that the silicon used in the cells must be cut as small wafers from silicon crystals for mounting on the cell surfaces.