The generators are run by turbines, it's a dam power station (Three Gorges Dam). It has an annual generation of 80TWh/80000000000000KWh. I have to work out the cost per unit of electricity over the operating life of the generating station, so I need to estimate how long it'll last. Is 40 years a suitable estimte?
Ok...you mean the production cost, not how much you get by selling it.
For a hydro dam, the variable cost is extremely low since there is no fuel to buy: you just pay the maintenance and operations staff. The primary cost is the fixed cost of building the dam. So you'd take that and divide it by the total production of the dam over its lifetime.
40 years is probably a good estimate for lifespan before major overhaul (turbine replacement), yes.