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When I talk with groups about solar and wind energy, people ask: “The sun isn’t up all the time and the wind doesn’t blow all the time. How can solar and wind energy power a reliable electric grid?”

About 25 years ago, senior management at a major utility turned down an experimental 8 kilowatt solar project I had proposed because the transmission systems engineers told the CEO that unpredictable solar energy might destabilize the entire utility’s grid. In other words, serious industry professionals claimed to be worried about a solar project representing about 0.00003% of their grid’s capacity.

In hindsight, it is obvious that they were wrong. Several major countries use solar or wind power for more than 10% of the power on their respective grids. Solar and wind are often the lowest-cost options available for new generation capacity and their use is growing fast in most places.

Utility experts are now confident that 50-80% of any grid’s electricity can be reliably and economically supplied by a combination of solar and wind generation. Furthermore, solar and wind aren’t the only renewable generation sources. Hydroelectric and deep geothermal generation are long-proven renewable technologies.

Within the next 10 to 20 years, 100% renewable electric grids on the scale of large countries are expected to become financially and technically feasible. Renewable electricity on the scale of a complete grid has already been achieved in some smaller locations, such as New Zealand and Iceland, which have favorable geology and have emphasized the use of hydroelectric and geothermal generation.

Regular readers won’t be surprised to learn that the difference between “then” and “now” has mostly been driven by successful R&D.

Practical experience with solar and wind systems has been gained by implementing projects similar to the one I had proposed, finding the problems they caused and systematically addressing them. A full list of the relevant technological and regulatory developments would be too long for this space, so I’ll just describe a few highlights.

Over a few panels, solar generation may drop off 80%, abruptly, due to moving clouds. However, on a grid scale, that doesn’t matter. Experience has shown that, over the tens of thousands of panels now on some grids, solar power is very predictable, because sunrise and sunset hours and average cloud cover over hundreds of square miles are very predictable. Similarly, wind lulls that can stop generation at just a few windmills usually aren’t relevant if the grid has hundreds of windmills. Nighttime wind and daytime sun have proven to work well together; winds blow harder at night and the sun is up when wind availability is down.

Faster computers, better forecasts and changes in grid management regulations have each helped to accommodate variable solar and wind power. Utilities used to forecast the balance between supply and demand two hours at a time. Now, systems operators who select the generation systems asked to supply power to any given grid routinely use 15-minute intervals. They are moving towards 5-minute intervals. The shorter the interval, the more predictable solar and wind availability.

To get from 80% to 100% renewables, further work on technologies and regulations is underway and expected to succeed. Critical systems include inexpensive batteries, heat storage and time-of-use metering with incentives for people to use or store energy during specific 15-minute intervals.