What is the difference between SoH, Usable Energy and Availability?

All three metrics - SoH, Usable Energy, and Availability - can be expressed either as a percentage or as an absolute amount of energy, typically in megawatt-hours (MWh). They provide distinct perspectives on the condition and performance of a battery storage system. All of them are are calculated with the Nominal Energy as reference, which typically equals the Installed DC Energy Capacity in MWh.
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1. State of Health (SoH)

SoH indicates the overall health of a battery system and is defined as:

SoH = Nominal Energy - Aging

SoH strictly reflects the aging of the battery and does not consider imbalances or other temporary factors.

• It gradually decreases over time as the battery naturally degrades.

• It is typically calculated on a weekly or monthly basis.

• Key applications include:

  • Planning system expansions (Site Augmentation)

  • Monitoring the business case over time

  • Managing warranty and performance guarantees

Key applications include:

2. Usable Energy

Usable Energy represents the actual energy available for use and is defined as:

Usable Energy = Nominal Energy - Aging - Recoverable Energy

• Aging represents energy loss due to degradation, and cannot be recovered.

• Recoverable Energy accounts for:

  • Imbalances

  • Weak or limiting cells

  • Non-operational components

  • Other influencing factors

Recoverable Energy accounts for:

• Imbalances

Imbalances

This Usable Energy metric can fluctuate daily due to:

• Balancing activities

• Component replacements or maintenance

The primary focus is on the "Recoverable Part": identifying currently unused energy and determining actions to restore its availability.

Analogy:
Imagine your Usable Energy as the full size of a beer glass. If you fill this glass slowly and carefully, you can fill it completely, just like discharging at very low power allows you to extract all usable energy.

However, if you try to fill the glass as fast as possible, foam (akin to system inefficiencies) prevents it from being completely full. This is what the Penalty Risk Assessment metric shows - the portion of energy you can utilize when operating at maximum (nameplate) power.

From experience, to fully fill a beer glass to the brim, you slow down the filling speed (i.e., reduce charging or discharging power) toward the end. The same principle applies when maximizing usable energy in a battery system.

3. Availability

Availability measures the proportion of operational components and is defined as:

Availability = Nominal Energy - Non-operational Components

• It specifically considers non-operational (often referred to as "offline") components.

• It is calculated either as 0/1 * Nominal Energy per component.

• It is a subset of the factors influencing Usable Energy.

• This metric is particularly relevant in the context of warranty and performance guarantees assessments

The figure below illustrates the three different KPIs in comparison:

Figure 1: The three KPIs compared

Summary

These three metrics capture different effects and provide distinct insights into the battery storage system:

In a new and fully functional storage system, all three values should be nearly identical. Over time, they will diverge due to various factors. By definition, Usable Energy will always be the lowest value since it considers all possible limitations.