The Optimal Storage Duration and Depth for Battery Storage Systems

Table of Contents

• What does the storage duration/storage depth of a battery storage system mean?
• What storage depth is right for my battery storage project?
• Which storage configuration is best suited for current and future requirements?
• Why do 2-hour storage systems dominate the market?
• Why should battery storage systems be able to store energy for longer and longer periods?
• Is a 4-hour battery storage system economically viable?
• Why can 4-hour storage systems achieve higher returns?
• What are the advantages of 4-hour storage systems for co-location projects?
• Is a 4-hour storage system always the better choice?
• Conclusion on the optimal storage duration or depth of battery storage systems

Summary

For many new battery storage projects, a storage duration of 4 hours is emerging as a strategic advantage. 4-hour storage systems can shift more energy, take advantage of wider price windows, and combine multiple revenue streams—such as arbitrage, intraday trading, grid services, and colocation—more flexibly. 2-hour storage systems remain viable in certain business models but are more heavily dependent on current short-term markets.

Investors today should therefore evaluatestorage duration not only based on current revenue streams but also on long-term marketing flexibility, site conditions, and future market requirements. This blog post helps youevaluatethe appropriate storage duration for the designof your battery storage project.Properly sizing a battery storage system today also means anticipating tomorrow’s market. For project developers and investors, this raises a fundamental strategic question:

What does the storage duration or depth of a battery storage system mean?

The storage duration or depth of discharge describes how long a battery storage system can provide its rated power. For example, a 4-hour storage system can store twice as much energy at the same power level and deliver it over twice as long a period as a 2-hour storage system.

• 10 MW power + 20 MWh capacity = 2-hour storage system 

• 10 MW power + 40 MWh capacity = 4-hour storage 

Both systems have the same power output but differ in the amount of available energy, or storage capacity.

Conclusion: The longer the storage duration, the more energy can be stored, shifted, and sold. The storage duration therefore directly influences the potential revenue models for a battery storage project.

What storage depth is right for my battery storage project?

The choice of the right storage depth should always be based on the planned marketing strategy, the investment horizon, and the expected market conditions. 

Anyone planning a battery storage system today should not only consider current revenue sources but also take into account how electricity markets, storage costs, and marketing opportunities might develop in the coming years.

Conclusion: Making investment decisions based solely on today’s revenue situation is short-sighted. Looking in the rearview mirror is not enough to predict the evolution of a dynamic and increasingly volatile energy market. Instead, it is essential to analyze various scenarios and take potential market, technological, and regulatory developments into account early on. These factors in particular could significantly influence the economic viability of different storage capacities in the coming years.This results in a battery storage system that not only fits today’s market conditions but is also prepared for tomorrow’s requirements.

Which storage configuration is best suited for current and future requirements?

The German battery storage market is evolving; storage duration in hours has now become the new “megawatt.”  For a long time, the formula for battery storage in Germany was simple: react as quickly as possible to capture revenue in service markets such as primary control power.
But the rules of the game are changing. The optimal depth of discharge for a battery storage system depends on several factors:

• the revenue model,  

• the investment horizon,  

• the marketing strategy  

• and expected market trends.  

While 2-hour storage systems continue to have viable applications today, current market analyses show that 4-hour storage systems are becoming increasingly important. An analysis by Modo Energy shows that, for 2026, 4-hour systems are projected to yield the highest returns in the German market: 4-hour storage systems achieve an IRR of 12.2% and outperform comparable 2-hour systems—which yield 10.8%—by 1.5 percentage points. 

The analysis shows that the market is shifting away from quick returns toward a long-term trading strategy. 2-hour systems are the standard today. They are capital-intensively optimized and are profitable in current markets. However, more storage capacity in the system leads to shrinking margins in traditional niches. In addition, competition within short time windows is increasing dramatically. And negative electricity prices, as well as high price spreads, are persisting over increasingly longer periods.

In short: A 2-hour storage duration is therefore becoming less and less sufficient. A 4-hour storage duration is becoming the economic “sweet spot.” The sweet spot is the point at which investment costs and return potential are perfectly balanced. For project developers and investors, choosing the storage duration is therefore becoming more of a strategic decision rather than merely a technical sizing issue.

Why do 2-hour storage systems dominate?

Many of the battery storage systems installed today are designed for market conditions in which rapid response and high power output are more important than a long storage duration. A 2-hour system is particularly effective when it comes to typical revenue sources such as providing balancing energy, intraday trading, mitigating short-term price spikes, and grid-supporting services such as atypical grid usage. 

For these applications, shorter storage durations are often still sufficient at this time. Currently, about 83% of the battery storage systems in operation in Germany consist of systems with a storage duration of less than two hours. (Source: https://modoenergy.com/) 

However, those who focus solely on today’s short-term revenue opportunities underestimate the potential risks posed by future market and regulatory changes. In the medium term, regulatory adjustments and new market mechanisms in particular could significantly alter revenue prospects.

Why should battery storage systems be able to store energy for longer and longer periods?

With the accelerated expansion of solar and wind energy and the new regulatory framework currently being developed as part of the fundamental grid tariff reform, not only is the power grid changing, but so is the structure of the electricity markets. We are seeing increasingly frequent periods of very low electricity prices followed by phases of significantly higher prices. This high price volatility on the EPEX spot market means that the time-shifting and storage of energy are becoming enormously important. 

This isbecause battery storage systems with longer storage durations can

• store more energy temporarily, 

• take advantage of wider price windows, 

• absorb more surplus renewable energy, 

• and market energy over longer periods of time. 

As a result of this trend, project developers are increasingly exploring storage durations of four hours or more.  

In short: A 4-hour system ensures maximum flexibility for today and the future. As markets evolve, prices fall, and changing conditions open up new opportunities, a 4-hour system in particular realizes its full yield potential over its entire lifecycle.

Is a 4-hour battery storage system economically viable?

Based on the current assumptions in the Modo Energy study (“What’s the optimal battery duration in Germany?” – Research | Modo Energy), a 4-hour system currently yields the most attractive returns in the German market. For projects commissioned in 2026, Modo Energy calculated an internal rate of return (IRR) of 12.2%. This represents a 1.5 percentage point advantage over a comparable 2-hour system with an IRR of 10.8%. 

However, it is important to note that these results are based on the model assumptions of this study and do not represent a universal statement applicable to every project. Actual profitability depends on the project location, marketing strategy, financing, and market developments.

Why can 4-hour storage systems achieve higher returns?

The appeal of longer storage durations stems from the interplay between costs and revenues. On the revenue side, 4-hour storage systems can take advantage of more arbitrage opportunities, balance out longer-lasting price differences, open up additional marketing windows, and even combine different revenue sources (multi-use storage). 

At the same time, costs do not increase proportionally with storage duration in all areas. Many infrastructure components remain largely the same regardless of capacity—such as grid connections, substations, transformers, control and monitoring technology, and parts of the construction infrastructure. As a result, the specific investment costs per megawatt-hour stored decrease as storage depth increases. The analysis by Modo Energy specifically highlights this effect.

This is why 4-hour systems are increasingly becoming the strategic standard:

• Arbitrage depth: Price spreads can be exploited much more effectively over longer periods of time. 

• Multi-use strategies: More capacity means greater flexibility to serve various revenue streams—such as trading and services—in parallel. 

• Future-proofing: A more robust revenue profile that is less vulnerable to short-term market saturation and regulatory changes. 

What are the advantages of 4-hour storage for co-location projects?

The co-location operating model is currently emerging as a key driver of profitability for existing PV and wind farms, as this model combines generation and storage into an economically optimized system.  

In addition, some newer plants receive no market premium or compensation for electricity fed into the grid during periods of negative prices. Instead of selling electricity at low or negative prices during such time windows, a battery storage system can temporarily store the energy and sell it at a later time. In co-location projects, greater storage capacity provides additional flexibility.

A 4-hour storage system can:

1. store significantly larger amounts of energy,  

1. bridge longer periods of negative prices, and 

1. shift the stored electricity more strategically into economically attractive market windows.  

The goal is not only to produce renewable energy but also to place it on the electricity market with temporal flexibility. 

Electricity prices, grid connections, and market opportunities vary depending on the location. The generation profile determines how regularly and for how long energy is available for storage. And the marketing strategy determines whether the storage system should primarily respond in the short term or shift larger amounts of energy over several hours.

Is a 4-hour storage system always the better choice?

No, because the optimal storage duration is always project-specific. A 2-hour storage system can still make sense if the business model is heavily geared toward today’s balancing power markets, the focus is on short payback periods, or investment costs need to be kept as low as possible. 

However, many of these models are based on market conditions that are currently changing. Numerous simulations and market outlooks suggest that, as renewable energy continues to expand, longer storage durations will become increasingly important.  

The question, therefore, is not so much whether 2-hour storage works today, but rather which storage duration will still best meet future market requirements in five or ten years. 

A 4-hour storage system can therefore offer advantages when arbitrage revenues play an important role, multiple revenue streams are to be combined, co-location projects are planned, or there is a long-term investment horizon. 

In summary: The key issue is not which storage duration is fundamentally “better,” but rather which best fits the respective business model, investment horizon, and expected developments in the energy markets.

Conclusion on the appropriate storage duration or depth for battery storage systems

Anyone following current developments no longer has to guess. A 4-hour system, operating at two cycles per day, can be deployed in the market for up to 16 full-load hours daily—twice as many as a 2-hour system. This means more price windows, greater flexibility in shifting energy, and a broader revenue base across trading, services, and colocation. In short: less dependence on a single revenue model. 

At the same time, there is also a natural limit to longer storage durations. The greater the storage depth, the more the number of possible full cycles per day becomes the limiting factor. Therefore, even longer storage durations do not automatically create more economic benefit but must be suited to the specific use case. 

From today’s perspective, there are strong indications that the market is shifting away from the historically dominant 2-hour systems toward 4-hour and, in some cases, 6-hour storage systems. These offer an attractive balance of marketing flexibility, revenue potential, and operational feasibility. Ultimately, however, a variety of factors—including location, grid connection, generation profile, investment horizon, and marketing strategy—will determine whether a 2-hour, 4-hour, or, in the future, even a 6-hour system will provide the greatest economic benefit for your next project.