The per-annum value is the total amount of energy generated over the course of the year. The Wannapum dam is rated at 1,400 MW; if it generated that 24x365, it

The per-annum value is the total amount of energy generated over the course of the year. The Wannapum dam is rated at 1,400 MW; if it generated that 24x365, it would generate 12.3 million MWh. Since it actually generates 4 million MWH, it is running at about 30% capacity. That said, a $90M dam in the 1950s would be a lot more expensive today. Partly due to inflation, but also for extra effort to preserve river ecology, etc. that nobody cared too much about back then.