Batteries 101 Series: How to Talk About Batteries and Power-To-Energy Ratios

April 13, 2016 by Joyce McLaren

This series explains more about batteries and the power-to-energy ratios. This is the second part of a two-part series. Read part one of the series.

As solar and other renewable energy technologies become more mainstream, the public becomes more familiar with the language of photovoltaics (PV). Even if most people don't have a thorough understanding of how it works, homeowners with a PV system may know the difference between the rated capacity of the system (expressed in kilowatts) and the amount of electricity that the system actually produces (expressed in kilowatt-hours). Homeowners may be confident explaining their 5-kilowatt system produces about 7,000 kilowatt-hours per year.

With the price of batteries falling, the emergence of Tesla’s Powerwall, and other home energy storage options from different manufacturers, consumers are faced with learning the lingo of yet another energy technology. While their understanding of PV is applicable, batteries offer new concepts to master.

The specifications of a battery system generally provide a kilowatt (kW) rating as well as a kilowatt-hour (kWh) rating. PV-savvy consumers who are new to the energy-storage world are at risk of misinterpreting the significance of these ratings by directly translating their understanding of PV systems to battery systems. When describing a battery system (whether or not it's attached to PV panels), it's necessary to indicate the power-to-energy ratio; that is, to fully understand the capabilities of a particular battery system, one must know both the kilowatt rating and the kilowatt-hour rating.

For batteries, the power rating (measured in kilowatts) indicates how much power can flow into or out of the battery in any given instant. It's similar to the capacity rating of a PV system (also measured in kilowatts), which indicates how much power can theoretically come out of the PV system in any given instant. However, one common mistake is to use the term capacity when referring to the kilowatt rating of the battery system. The more accurate term is the power rating of the battery.

The energy rating, or battery capacity, of the battery system is measured in kilowatt-hours and provides an estimate of the amount of energy that can be stored. The energy rating is the measure of how much electricity the system can deliver or absorb over the course of an hour. This is similar to the energy output of a PV system over time (which is also measured in kWh).

The important difference is that, unlike the PV system, battery systems are designed to maximize either the power rating or the energy rating, depending on their intended use.

As discussed in an earlier blog post, battery-system owners may tap into multiple value streams to realize shorter payback periods. Commercial-building operators may use batteries to reduce utility demand charges as well as energy charges through peak shaving. In the PJM Interconnection, some behind-the-meter battery system operators bid to provide frequency regulation and are compensated for providing this grid service. Different end uses call for different energy-to-power ratios.

If the battery system will be used primarily to provide frequency regulation, the battery system needs to charge and discharge many times over short durations of time. A system used in this type of scenario will be designed with a higher power rating. If a battery system will be used primarily to provide peak-shifting or must provide backup power in case of a grid outage, the battery needs to be able to discharge over a longer period (e.g., 2 – 5 hours) and is designed with a higher energy rating.

As reported by Green Tech Media in its Q3 2015 Energy Storage Monitor, deployment of grid-scale energy storage shifted from a focus on systems with high energy ratings (MWh) to avoid curtailment of renewable generation, to systems with higher power ratings (MW) to provide quick-responding frequency regulation in the PJM market.

Knowing the power-to-energy ratio of a battery system provides a better understanding of its intended use and capabilities. Understanding that battery systems are designed with different power-to-energy ratios helps a person realize why it's difficult to compare the costs of two battery systems even when they use identical chemistries (e.g., lithium-ion). While PV systems may be compared by their capital cost in $/kW (or by their life-cycle cost of energy in $/kWh), the different power-to-energy ratios of battery systems makes an apples-to-apples comparison more problematic. Given this, the industry is still determining the most accurate way to report on battery systems and their costs—and the public is still learning how to interpret those reports.