1. Consider heating your house on a cold but sunny day. You could heat it with electricity from the grid, by having large windows letting the sun in, by having solar panels power a heat pump, or by burning some dried out weeds that you have lying around. All of these would work but they would get counted very differently. Using the grid gets counted as energy in the 15 TW figure, so might solar panels (who knows if they are not connected to the grid). But compare solar panels connected to a heat pump, to having the sun stream in through glass windows. This is pretty much the same thing, however one may get counted as using energy, the other definitely will not.Now compare solar panels to the weeds. Solar panels take energy from the sun, convert it to electricity which then heats the house. So does growing weedy plants, chopping them down and burning them, but with the energy stored chemically rather than electrically. The amount of sunlight hitting the weeds over their lifetime would be huge, of which only a small fraction is made use off when you cut them down and burn them. So how much energy do you “use” when you heat your house this way?