Energy Storage

When collecting intermittent energy from sources such as wind or solar, production will peak and trough along with the weather and environment, and these fluctuations disrupt overall energy availability.
Energy storage is therefore essential to any intermittent energy system, and is currently the biggest obstacle when trying to transition away from fossil fuels.

Sustainable energy storage is very valuable, but not completely essential to a modern community.
The ability to have energy, on demand, allows for a whole variety of modern technology and techniques, not least of all, reliable computing (for scientific and practical purposes such as automation), modern life support systems and techniques, and refrigeration for medicines and other perishable items.
These storage systems exist and are reasonably sustainable at this point in time.

Technology is still improving in this area, but considering how much storage would be required to power a city or even a small town at modern energy consumption levels – our first priority is reducing our energy usage until energy technology improves further. That being said, there are many sustainable, practical storage methods available than are currently quite common.