## 1 Introduction
### The Main Business Levels of Energy Storage  
The Rocky Mountain Institute has summarized several battery energy storage services based on reports, expert interviews, and internal analysis. This work described 13 energy storage services by the stakeholder groups: independent system operators (ISOs)/regional transmission organizations (RTOs), utilities, and customers. Energy storage can be classified at three different levels: behind the meter, distribution, or transmission level (Figure 3) [76].  
Figure 3: Battery energy storage value chain based on [76; 77].  
ISO/RTO services:
* Energy Arbitrage: In this case, the wholesale electricity needs to be purchased when the locational marginal energy price is low (for example at night) and the electricity is sold back to the wholesale market when locational marginal prices are high [76].
* Frequency Regulation: This is the automatic and immediate response of power to a change in local grid frequency. The regulation is important to ensure that system-wide generation is perfectly matched with system-level load on a moment-to-moment basis to avoid system-level frequency dips or spikes, which can create grid instability [76].
* Spin/Non-Spin Reserves: In the case of an unplanned generation outage, the spinning reserve generation capacity is able to serve the load immediately. The non-spinning reserve (not instantaneously available) is a generation capacity that can respond to contingency events within a short period (<1 min) [76].
* Voltage Support: This regulation ensures continuous and reliable electricity flow across the grid. Voltage on the transmission and distribution system need to be maintained within an acceptable range to ensure that real and reactive power production are matched with the demand [76].
* Black Start: In the case of a grid outage, black start generation assets are needed to restore operation to larger power plants in order to bring the regional grid back online [76]. Utility services:
* Resource Adequacy: In this case, instead of investing in new gas combustion turbines to meet generation requirements during peak electricity consumption hours, utilities and grid operators can pay for other solutions (including energy storage) to incrementally reduce or defer the need for new generation capacity and minimize the risk of overinvestment in that area [76].
* Distribution Deferral and Transmission Deferral: Reducing, delaying, or entirely avoiding utility investments in distribution system upgrades is necessary to meet the projected load growth on specific regions of the grid [76].
* Transmission Congestion Relief: In this case, the ISOs charge utilities to use congested transmission corridors during certain times of the day. The energy storage devices can be deployed downstream of congested transmission corridors to charge/discharge the energy during congested periods and minimize congestion in the transmission system [76]. Customer services:
* Time-of-Use Bill Management: During peak electricity consumption periods when time-of-use rates are the highest, by shifting these purchases to periods of lower rates, behind-the-meter customers can use energy storage systems to reduce their bill [76].
* Increased PV Self-Consumption: This means minimizing the export of electricity generated by behind-the-meter PV systems to maximize the financial benefit of PV systems in areas with utility rate structures that are unfavorable to distributed PV [76].
* Demand Charge Reduction and Backup Power: In the case of grid failure, battery energy storage systems with local generators can provide backup power at multiple scales (industrial operations, daily backup for residential customers) [76].