Define reserve power

Reserve power is the generating capacity available in a power system over and above the expected peak demand, kept to supply load when units fail, are under maintenance, or when demand is higher than forecast.

$$\text{Reserve} = \text{Available capacity} - \text{Peak demand}$$

It includes spinning reserve (running units), non-spinning reserve (quick-start units) and cold reserve (units that need hours to start). It is usually expressed as a reserve margin of about 15–25% of peak demand. Example: a system with 1,150 MW capacity and 1,000 MW peak has 150 MW (15%) reserve.