Speak storage fluently with these key terms

Home energy storage systems are becoming increasingly popular in the US. Solar installers say that one-third of customers nationwide express interest in energy storage, while on EnergySage's Marketplace, that number is even higher, with over 70 percent of solar shoppers also interested in energy storage.

With so many new terms to digest, we thought a glossary of common terms and concepts related to energy storage and batteries would be helpful. Read on for some of the key terms you're likely to come across while researching batteries.

Charging

You are adding electricity to your battery. This is the exact same as charging your phone. You can either charge your battery with the electrical grid or from an onsite renewable resource, like solar panels, on your house.

Discharging

You are removing electricity from your battery to power your house. Similar to how your phone battery level decreases over a day as you use it. Just as the more apps you use on your phone, the quicker the phone battery will run out, the more appliances you power in your home with your battery, the quicker your energy storage system will lose charge.

Battery chemistry

Different batteries have different methods of storing electricity. While some, like flywheels, use a mechanical system to store kinetic energy as potential energy, most residential energy storage solutions use chemistry to store electricity. The most common chemistries for batteries are lead-acid and lithium-ion - you can read our comparison of lead-acid and lithium-ion battery technologies here.

When reviewing information on the size of an energy storage system, it's important to make a distinction between power and energy. At a high level, power is the size of the pipe–how much electricity is the maximum that you can push through at one time–whereas energy is the flow through the pipe–how much electricity has moved through the pipe total over a certain period. There are several battery-specific terms that help describe the energy and power of the systems.

Power

Power is rated in kilowatts and measures the maximum amount of electricity an appliance requires in any instant. Power is an important metric for batteries because it determines how many appliances and home applications a battery can power concurrently. To understand the battery size you need, it's useful to compare the power rating of a typical residential energy storage system to the power requirements of the appliances you want to run with your battery.

Appliances in your home range from using several Watts of power to several hundred or even thousands of Watts of power. At the low end, a standard compact fluorescent (CFL) light bulb uses about 15 Watts, and a phone charger uses less than 10 Watts—both a refrigerator and an LED TV pull about 200 Watts of power. Meanwhile, at the high end, appliances like clothes washers and dryers, air conditioners, and even hair dryers or coffee makers use over a thousand Watts of power.

Energy capacity

The energy of a storage system is rated in kilowatt-hours and represents the amount of time you use your appliances. In other words, energy is power consumption multiplied by time: kilowatts multiplied by hours to give you kilowatt-hours. To understand the energy sizing of batteries, you need to know how long you want to run your appliances with your battery. Running many appliances for a long period would require a larger battery from both a power and energy perspective while running fewer appliances for a shorter period would require a smaller one.

Continuing the example of power requirements of home appliances from above, keeping ten CFL light bulbs on for six hours uses nearly 1 kilowatt-hour of electricity (10 CFLs * 15 Watts per bulb * six hours). A television or refrigerator may use 1 kilowatt-hour of electricity over 24 hours, depending on how often the TV is turned off and on and to what temperature the refrigerator is set. On the other hand, running a central air conditioner could use 10 kilowatt-hours per day.

Max power (kW)

The absolute maximum amount of power a battery can discharge over a short burst. This is typically described as the power output over several seconds to several minutes. Max power is important because the power requirement for certain appliances–notably a sump pump–spikes when you turn them on. As a result, for a battery to power certain appliances, you will need one that can push out a large quantity of power over a short period to allow you to turn on those systems.

Continuous power (kW)

The constant amount of power a battery can discharge over a prolonged period. The continuous power of a battery will always be lower than the max power. If you plan to run many power-intensive appliances concurrently with your battery, then you will need a battery that has a higher continuous rated power to be able to power many appliances all at once.

Total energy (kWh)

How much electricity is stored in the battery in total when fully charged. Expressed in kilowatt-hours, this is an energy metric that demonstrates the amount of electricity that would be available if you could fully discharge your battery to zero.

Usable energy (kWh)

The amount of electricity stored in the battery that is available for consumption. Batteries have their own native load requirements. In other words, batteries require a certain amount of electricity to continue running. As a result, not all the electricity stored in a battery is available for you to consume. If you try to use beyond the usable energy of a battery, it may have detrimental impacts on the health and longevity of your battery.

Depth of discharge (%)

The depth of discharge for a battery is the extent to which a battery can be discharged safely while ensuring enough energy is left to power the battery's electric requirements. Overall, depth of discharge is another metric for the amount of energy stored in a battery that is available to you. A standard depth of discharge is between 80 percent and 95 percent.

Round-trip efficiency (%)

The amount of electricity required to charge a battery and the amount available to discharge is not one-to-one. In other words, there are electricity losses during converting electricity into stored energy and then back again into usable electricity. Losses are typically low, in the 5 percent range, and round-trip efficiency is a metric that explains the ratio of electricity required to charge versus electricity available to discharge. Importantly, while depth of discharge measures the amount of electricity stored in a usable battery, round-trip efficiency measures how much electricity is required to store that energy in the first place, which is not one-to-one due to electrical losses during the charging process.

Many batteries provide additional benefits beyond just the ability to charge and discharge. How those benefits work and whether they are important can help you decide on different battery options.

Islanding

If you are interested in continuing to power your house from your battery in the event of an outage on the electrical grid, then you need a battery capable of "islanding." Batteries with this capability will recognize when the power grid is down and cut your house off from the electrical grid so that it can continue to operate smoothly, effectively turning your home into an energy "island."

Black start capability

Appliances require electricity to turn on. So how does your battery turn on if the electricity is out and your battery is out of juice? In that case, your battery must be able to return online. The idea of "black start" comes from redundancy built into the electrical grid, where there are power plants capable of turning on when the entire electrical grid is down so the system can run again. Black start is important when you anticipate needing to power your home with stored solar energy during prolonged outage events, such as in places frequently hit by major storm events.

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