Of all the solar irradiation that falls on a PV plant, we lose a lot of it before we have kWh of energy that we can sell.
The chart below highlights just fifteen of the key ways those losses can occur (and it isn't intended to be an exhaustive list). It also divides them into where, along the chain from sunshine in to electricity out, these different losses happen.
The biggest loss is within the module of course: even the best commercial, single-material PV cells only convert into retreivable electrons around 20% of the total solar energy they absorb. Heat the cells up and this efficiency falls further (the 'temperature effect'). This can account for a sizeable loss on - ironically - hot, sunny days!
However some losses occur because light never reaches the module in the first place, others occur within the array of modules, then further energy is lost in converting from direct to alternating current and upping its voltage for export into the grid.
Even assuming the plant is up-and-running ('available'), there can be circumstances where electricity cannot be exported because of grid congestion or lack of market demand (though in some regulatory regimes, plant owners are insulated against financial loss in these circumstances).
While many of these losses are individually small (often a percent or less), they do all add up!