Short answers, pulled from the story.
Variable renewable energy (VRE) refers to sources such as wind and solar power that cannot be adjusted on demand because their output depends on weather conditions. Dispatchable sources, such as dammed hydroelectricity and bioenergy, have stored potential energy that operators can release as needed. Geothermal power is also considered dispatchable and relatively constant.
In 2021, wind supplied 7% of global electricity generation and solar supplied 4%, for a combined share of 11%. In the same year, Denmark, Luxembourg, and Uruguay each generated over 40% of their electricity from wind and solar combined.
Capacity factor is the ratio of electricity a plant actually produces over a period, usually a year, to what it would have produced running at full nameplate capacity the entire time. Wind power typically has an annual capacity factor of 25-50%. Standard photovoltaic solar ranges from 10-20%, while solar thermal power towers with storage can reach 73%.
There is no universally accepted maximum penetration level, as each grid differs in structure, interconnection, and existing capacity. Estimates suggest combined wind and solar can reach 70-90% without regional aggregation, demand management, or storage, and up to 94% with 12 hours of storage. Britain has planned for over 65% by 2030.
Pumped storage hydropower pumps water uphill when generation exceeds demand and releases it through turbines when demand exceeds supply. It is the most prevalent existing storage technology used to support variable renewables, with a typical round-trip efficiency of around 80%.
Norway, Brazil, Manitoba, and Quebec all have high levels of hydroelectric generation that can be ramped up or down to complement wind and solar output. Quebec produces over 90% of its electricity from hydropower, and Hydro-Quebec is the largest hydropower producer in the world. The U.S. Pacific Northwest is also identified as a region where wind energy is well complemented by existing hydropower.