Closely related to energy consumption is the concept of total primary energy supply (TPES), which – on a global level – is the sum of energy production minus storage changes. Since changes of energy storage over the year are minor, TPES values can be used as an estimator for energy consumption. However, TPES ignores conversion efficiency, overstating forms of energy with poor conversion efficiency (e. g. coal, gas and nuclear) and understating forms already accounted for in converted forms (e. g. photovoltaics or hydroelectricity). The IEA estimates that, in 2013, total primary energy supply (TPES) was 1. 575 × 1017Wh (= 157. 5 PWh, 157,500 TWh, 5. 67 × 1020joules, or 13,541 Mtoe) or about 18 TW-year. From 2000–2012 coal was the source of energy with the total largest growth. The use of oil and natural gas also had considerable growth, followed by hydropower and renewable energy. Renewable energy grew at a rate faster than any other time in history during this period. The demand for nuclear energy decreased, in part due to nuclear disasters (Three Mile Island in 1979, Chernobyl in 1986, and Fukushima in 2011). More recently, consumption of coal has declined relative to renewable energy. Coal dropped from about 29% of the global total primary energy consumption in 2015 to 27% in 2017, and non-hydro renewables were up to about 4% from 2%.