why does density matter?
Well, for a start, it tells you HOW BIG a 1 kg cube of TNT is. Curiously, this is important to the end result...
And by the way, Watts (W) are Joules (J) per second.
There is no such thing as watts-per-second
There ARE Watt-hours or kWh (kilo Watt hours). {There are even watt-seconds!} W-H or kWh are used to measure or compare steady power output/consumption.
A 1 Kw electric bar heater consumes 1,000 J/sec
It delivers 1 kWh of energy per hour