Please, note the different SI unit of measure: W and Wh
If you are more comfortable with liquids, think of this analogy:
W = flow rate of a pipe, litres per second, gallons per second etc.
Wh = tank capacity, how many litres it contains, how many gallons it contains etc.
k = 10^3 = 1,000
M = 10^6 = 1,000,000
G = 10^9 = 1,000,000,000
"Million kWh" = 10^6 x 10^3 = 10^9 Wh = 1 GWh
So, if you have a 20 GWh tank and you open a tap that can let out 900 MW (0.9 GW), you can empty it completely in 20 / 0.9 hours, that is approximately in 2h 13'