Design exceedance Q45% versus Q65%

Meaning of Q45%

A design exceedance of Q45% means the plant's design (rated) discharge is the flow that the river equals or exceeds 45% of the time, about $0.45 \times 365 \approx 164$ days a year. The installed capacity is based on this flow:

$$P = \eta_o \rho g\,(Q_{45} - Q_{riparian})\,H_{net}$$

For the other 55% of the time the plant runs at part load, following the river flow. In Nepal, NEA's power purchase practice for ROR projects has moved from about $Q_{65}$ (older practice) to about $Q_{40}$–$Q_{45}$.

Comparison from the seller's (IPP) point of view

Seller's view

• Q45 gives more MW and more total energy, which raises revenue if NEA buys all energy under a take-or-pay PPA. Bank financing and per-MW benefits also favour larger capacity.
• But most of the extra energy comes in the monsoon, when the tariff is lower (dry-season rate is higher than wet-season rate in NEA's posted rates) and when the grid already has surplus, so spillage or curtailment can reduce income.
• Q65 gives a smaller, cheaper plant with higher plant factor and lower risk, but loses wet-season energy.
• The final choice is made by comparing incremental energy and revenue against incremental cost (B/C ratio, IRR) for several exceedance values; with a firm PPA, sellers usually prefer about Q40–Q45.