We need to produce enough solar panels to generate net new 20TW every year; 20TW/year as a measure of energy production doesn't make sense as the Watt is a measure of energy transfer. In other words, we need to increase production by 20TW per year every year for 20 years in order to hit the 400TW that we need to be producing all the time (in this model where global energy demand is 400TW)