When the sun goes down or the wind falls silent, renewables like solar and wind still need to keep the lights on. Enter long-duration energy storage, an emerging class of technologies that can store power for many hours or even days.

As the world electrifies and shifts to clean energy, this capability grows more important than ever. The U.S. Department of Energy (DOE) defines long-duration energy storage (LDES) as systems capable of delivering electricity for 10 or more hours.

Let’s break down what LDES is, why it matters, and how it could reshape the energy transition.

What Is Long-Duration Energy Storage?

In simple terms, long-duration energy storage refers to systems that can store electricity (or energy in another form) and release it across extended timeframes—typically 8–10 hours or more, and in some cases days or even seasons.

The exact definition varies by region and study. For example, the DOE uses the 10-hour benchmark. Compared to short-duration storage (like lithium-ion batteries that usually discharge in under 4–6 hours), LDES tackles longer time horizons and larger scale.

That makes it essential for smoothing out not just daily peaks but longer gaps in supply and demand.

Why Long-Duration Storage Matters

Here is why this technology matters:

• Renewables such as solar and wind are variable. They generate when the sun shines or wind blows—not necessarily when demand is high. Large‐scale storage helps align supply with demand. 
• Short-duration storage fills immediate gaps. But as renewables increase in share on the grid, longer gaps and “rare events” (for example several days of low wind) become more problematic. This is where LDES comes in. 
• Having LDES in place supports grid reliability, energy security, and decarbonization. It enables systems to keep operating when variable generation is low and ensures clean power remains available. In short, when we talk about building a resilient, clean grid, LDES is one of the foundational pieces.

Types of Long-Duration Storage Technologies

LDES covers a range of technological approaches, each with strengths, trade-offs, and applications. Here’s an overview:

• Flow batteries (e.g., vanadium, iron-air) — Electrochemical systems that store energy in liquid electrolytes. They offer scalability and longer durations than typical lithium-ion batteries. 
• Compressed air energy storage (CAES) and liquid air energy storage (LAES) — Mechanical/thermodynamic systems that compress air or liquefy it during surplus, then expand or reheat it to generate power. Good for longer durations and larger scale. 
• Pumped hydro — The largest existing form of long-duration storage globally. It uses water pumped to an upper reservoir then released to generate electricity when needed. 
• Thermal storage and hydrogen — These may store energy across very long time horizons (days to seasons) using heat reservoirs or converting electricity into hydrogen or other fuels. Each technology comes with cost, siting, regulatory and market-model considerations. But together they form a toolbox for managing energy on longer time scales.

Leading Companies and Projects in LDES

Here are some of the innovators and projects pushing LDES toward commercial reality:

• Form Energy — Working on iron-air batteries designed for multi-day discharge.
• Highview Power — Developing liquid-air energy storage systems.
• ESS Inc. — Building flow-battery systems for long-duration storage.
• Malta Inc. — Developing grid-scale thermal energy storage systems.

Challenges and the Road Ahead

LDES is promising, but it is not without hurdles. Here are the key challenges:

• High upfront cost and long development time. Infrastructure for long-duration storage often requires more capital and more complex siting than batteries.
• Market and regulatory models still catching up. Many electricity markets are designed around short-duration resources; lengthy durations require new business models and revenue streams.
• Technology risk and scale risk. Some technologies are still early stage and need to prove commercial viability.
• Policy and procurement frameworks. Government and utilities will need to adopt procurement strategies that value long-duration flexibility, not just short reserves. For example, states like California, New York and Massachusetts have begun defining LDES metrics and targets. Looking ahead the winners will be those technologies that can combine duration, cost-effectiveness, siting flexibility, and scalable deployment in real-world systems.