Ramp rate is a termed used in power generation to express how quickly a power plant’s power output is changing, either ramping up (increasing) or ramping down (decreasing). So it is expressed in units of power over time (e.g. MW per minute). Different power plants have different achievable ramp rates: nuclear power plants have slow ramp rates compared to the much faster ramp rates available with hydro or gas power plants, for example. Renewable sources such as solar and wind can ramp up and down very rapidly, though driven not by choice but because of sudden changes in the available resource (i.e. the weather).
A power system has to match power supply with power demand at any point in time, and demand can vary up and down – sometimes rapidly. Indeed, you may hear the term ramp rate used in the context of power demand rather than power supply.
Power plants with the ability to ramp up and down quickly are important elements of a flexible power supply mix, particularly in meeting peak demand (which often happens for relatively short times). Those that can only react more slowly tend to run more steadily, providing so-called baseload power.
Even if some sources are able to ramp up and down quickly, it doesn’t mean it’s the preferred way to run them. Power plants have an output (usually close to their rated power) at which they are operating most efficiently, so ramping up and down means operating them less efficiently (i.e. more primary fuel usage and cost for the same electricity output).
The system impact of power plants which ramp up and down according to the weather depends on how these changes in supply coincide with changes in demand. For example if solar/wind output ramps up at the same time as demand increases, this reduces the ramp rates required of other sources to meet this rising demand. However, if not, the ramp rates required of other sources increases: for example if wind/solar output is falling at the same time as demand is increasing, other sources must ramp up fast enough to not only meet increased demand but to replace the falling wind/solar output too.