The green line shows Swanson's law, a 20% decrease in price for every doubling of cumulative shipped photovoltaics. The blue line shows actual world wide cumulative installations vs. average cell price. Original source data * "1977–2013:" Bloomberg, New Energy Finance, (archived) * "2014:" based on average sales price of $0.36/watt on 26 June 2014 from EnergyTrend.com * "2015:" based on average sales price of $0.30/W on 29 April 2015 from EnergyTrend.com * compare to current spot-market prices here.