Variable renewable energy (VRE)
Renewable generation whose output varies with weather and time of day, mainly wind and solar PV, and cannot be fully dispatched.
Variable renewable energy (VRE) refers to sources whose instantaneous output is set by the resource rather than the operator: chiefly solar PV and wind, plus run-of-river hydro, tidal and wave energy. The IEA characterises VRE by four properties: variability, uncertainty (forecast error), location-specificity and non-synchronous connection through power electronics.
Wind and solar together produced roughly 17% of world electricity in 2025 (Ember), up from about 4% in 2015. Several systems run much higher annual shares: Denmark above 55% wind and solar, South Australia around 70%, Spain and Germany above 40%.
The IEA describes six phases of VRE integration. At low shares (below about 5 to 10%) impacts are minor. Above roughly 15 to 25%, systems need more flexibility: interconnection, storage, demand response, flexible plants and better forecasting. At high shares, issues such as curtailment, negative prices, declining capacity credit and reduced system inertia become central.
VRE also experiences value deflation: because output is correlated across plants, market prices fall in the hours when they generate most. This "cannibalisation" effect is visible in the solar capture rate, which in several European markets fell below 60% of the baseload price in 2024.
Related terms
All terms →- Curtailment
Deliberate reduction of a generator’s output below what it could produce, usually wind or solar, because of grid, market or system constraints.
- Duck curve
The shape of net load over a day in solar-rich grids: a deep midday dip as PV output peaks, followed by a steep evening ramp.
- Capacity credit
The fraction of a resource’s nameplate capacity that can be counted on to meet peak demand, used in adequacy planning and capacity markets.
- Inertia (grid)
The kinetic energy stored in synchronously rotating generators and motors that resists sudden changes in grid frequency.
- Dispatchable generation
Generation whose output can be scheduled, raised or lowered on request by the operator, independently of weather.
- Grid-forming inverter
An inverter that sets its own voltage and frequency reference, behaving like a voltage source that can stabilise weak grids and provide synthetic inertia.