Energy Storage Metrics: Electric Power and Energy (Power Rating and Discharge Duration)

Power and energy are two key metrics for characterizing a storage system (Eyer & Corey, 2010). Power is the instantaneous measurement of the use of electricity or the rate at which electric energy is consumed. Power is expressed in watts or more commonly, in the context of utility supplied electric energy, kilowatts (1000 watts, commonly written as kW) or megawatts (10⁶ watts, commonly written as MW). Energy is the amount of power consumed, expressed in watt-hours or more commonly kilowatt-hours (1000 watt-hours commonly written as kWh) (Miller, 2012). On a very large scale energy is expressed in megawatt-hours (10⁶ watts-hours commonly written as MWh) but kWh is a more common unit of energy as kWh more closely matches electricity consumption and utility pricing (NSTAR, 2012).

Mathematically the relationship between electric energy and power is :

Power = Energy / Time

Energy = Power X Time

To see the relationship graphically, consider Figure 1, which could be a hypothetical load curve showing the power consumed by a household (in kW) throughout a 24 hour day. Each point on the curve indicates the instantaneous power consumed at a specific time, with a peak of 10 kW at 3pm. The area under the curve is the amount of energy consumed during the 24 hour period, in this case 113 kWh.

Figure 1 Hypothetical load curve showing Power (kW) vs. Time of day (hours).

Consider that Figure 1 could also be a graph showing the power output of an energy storage device during a 24 hour period. During this 24 hour period the energy storage device outputs a maximum power of 10 kW at 3pm and discharges a total of 113 kWh of energy. Under normal operating conditions the storage system’s maximum power output would be the “Nameplate Power Rating” (DOE, 2010). Some energy storage technologies have the short term, emergency capability of discharging at power rates 1.5 to 2 times their nameplate power rating.  The value of this capability, referred to as the equipment’s “Emergency Rating”, can often offset the reduced efficiency and storage equipment damage incurred by this above nominal burst of power (Eyer & Corey, 2010).

The amount of time that an energy storage technology can discharge at its nameplate power rating, before requiring a recharge, is the “Discharge Duration”. Multiplied by the power rating of the device, this is the amount of energy that the storage device can produce. If the energy storage technology, whose power output curve is represented in Figure 1, has a nameplate power rating of 10 kW and a discharge duration of 12 hours, the energy storage device can produce 120 kWh of energy before requiring a recharge. Figure 1 demonstrates a valuable advantage of an energy storage device over traditional power generation equipment. Unlike a traditional generator, most energy storage technology can discharge at significantly variable rates, below its nameplate power rating, without incurring significant inefficiency or equipment damage (Baxter, 2006).

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Works Cited

Baxter, R. (2006). Energy Storage; A Nontechnical Guide. Tulsa, Oklahoma: PennWell Corporation.

DOE. (2010, November 12). Discussion of Data for Smart Grid Metrics and Benefits; Storage System Performance Supplement. Retrieved December 2, 2012, from http://www.smartgrid.gov/sites/default/files/pdfs/energy_storage_system_performance_supplement.pdf

Eyer, J., & Corey, G. (2010). Energy Storage for the Electricity Grid: Benefits and Market Potential Assessment Guide. Albuquerque, New Mexico: Sandia National Laboratories.

Miller, K. (2012, October 26). How To Speak Data Center: Power vs. Energy. Retrieved December 2, 2012, from Network Computing: http://www.networkcomputing.com/data-center/how-to-speak-data-center-power-vs-energy/240010565

NSTAR. (2012). Residential > Rates & Tariffs > Basic Service. Retrieved December 2, 2012, from NSTAR: http://www.nstar.com/residential/rates_tariffs/basic_service.asp