Solar batteries 101: key terms you need to know
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If you’ve tried researching solar batteries, you’ve probably come across a few terms you’ve never seen before – or that you haven’t seen since your high school physics class.
To help cut through some of the confusion, we’ve put together a comprehensive solar battery glossary with all of the terms you might come across when trying to find the right solar battery for you.
Absorbent Glass Mat (AGM) battery - A type of sealed lead acid battery that contains a fiberglass mat between the battery plates which holds the electrolyte solution like a sponge, preventing it from spilling if the battery breaks. Read more about AGM batteries here.
AC-coupled battery system - A solar plus storage system that has a solar inverter and a separate storage inverter to charge the battery. AC-batteries are best suited for adding storage to existing solar panel systems. Read more about AC batteries here.
Alternating current (AC) electricity - Electricity in which the electric current changes direction periodically. AC electricity is what is used by most home appliances.
Amperage - Amperage can be thought of as the intensity of the flow of the current, measured in amperes (A).
Amp-hour - The unit of electrical charge that describes how many amps a battery can deliver for one hour.
Anode - The negative electrode from which electrons flow to the cathode.
Backup power - A power source that can be used during a grid power outage.
Battery pack - The final ‘package’ of the battery that includes all battery cells wired together and their protective case. The battery pack may also hold the battery management system.
Capacity - A measure of how much electricity is stored in a battery, typically measured in kilowatt-hours for solar batteries.
Cathode - The positive electrode in a battery in which electrons flow to form the anode.
Cell - The part of the battery that contains the electrolyte, cathode, anode, and separator. Batteries can be made of one cell or multiple cells depending on their design.
Charge controller - A device that regulates the voltage and current going to a battery to prevent overcharging. Read more about charge controllers here.
Continuous power output - The amount of power a solar battery can release over an extended period of time, measured in kilowatts (kW). Most residential solar batteries have a continuous power output of about 5 kW.
Critical load panel - A second electrical panel that a solar battery is connected to that powers certain important appliances during a power outage.
Cycle - When 100% of a battery’s capacity has been discharged and subsequently recharged.
Cycle life - The number of cycles a battery can complete before its performance begins to decrease.
DC-coupled battery system - A solar plus storage system that has one hybrid inverter that works with both the solar panels and battery. DC-batteries are best for installing solar and batteries at the same time. Read more about DC batteries here.
Deep cycle battery - A battery that is designed to discharge 80% or more of its capacity regularly.
Depth of Discharge (DoD) - The amount of electricity that has been discharged from the battery relative to the total capacity of the battery. Most residential solar batteries today have a recommended DoD of 90% or higher, meaning no more than 90% of the battery’s capacity should be discharged to prevent causing damage to the battery.
Direct current (DC) electricity - Electricity in which the electrical current flows in one direction. Solar batteries use DC electricity to charge.
Discharging - When a battery releases power.
Electrode - An electrical conductor that makes contact with a battery’s electrolyte solution so the electrons can pass through the battery. This is where electrons flow to and from. Cathodes and anodes are types of electrodes.
Electrolyte - A chemical solution that allows electricity to flow from the battery’s cathode to the anode.
Energy density - The amount of energy that is stored in a battery in relation to its weight, typically measured in kWh per unit of mass.
Flooded lead-acid battery - A type of lead acid battery that has a free moving fluid electrolyte solution. The electrolyte within the battery evaporates over time, so they require regular maintenance in order to ensure there is enough electrolyte solution within the battery pack. Read more about flooded lead-acid batteries here.
Gel battery - A type of sealed lead acid battery containing an electrolyte gel as opposed to a liquid. The gelled electrolyte prevents spills and leaks and increases the overall safety of the battery. Read more about gel batteries here.
Grid - A network of transmission lines that allows electricity to be delivered from electric producers, like utilities, to the consumer.
Grid-tied solar system - A solar panel system that is connected and works in tandem with the utility grid.
Hybrid inverter - An inverter that combines the features of both solar inverters and battery inverters into one device. Read more about hybrid inverters here.
Inverter - A device that converts DC electricity into AC electricity. Read more about inverters here.
Kilowatt (kW) - A unit of power equal to 1,000 watts. A battery's power output is measured in kW.
Kilowatt-hour (kWh) - A unit of measurement that represents the amount of power released over a certain period of time. A battery’s capacity is measured in kWh.
Lead-acid battery - A type of rechargeable battery that contains lead electrodes and a sulfuric acid electrolyte solution. Read more about lead acid batteries here.
Lithium-ion battery - A rechargeable battery where lithium ions flow through its electrolyte solution. Read more about lithium-ion batteries here.
Lithium iron phosphate (LFP) - A type of lithium-ion battery that has a lithium iron phosphate cathode. LFP batteries offer high performance, stability, and safety.
Load - Any piece of an electrical circuit that consumes power, such as appliances and lights. Also referred to as electrical loads.
Mechanical storage - Uses kinetic energy or gravity to store electricity. Examples include pumped water storage, compressed air energy storage, and flywheels.
Modular battery - Battery packs that can be installed together to increase voltage or capacity.
Net metering - A utility rate program that requires a utility company to credit each kilowatt-hour of electricity produced by a solar panel system at the full retail rate of electricity. Read more about net metering here.
Nickel manganese cobalt (NMC) battery - A type of lithium-ion battery that has a lithium nickel manganese cobalt oxide cathode. NMC batteries are the most common type of lithium ion solar battery.
Nominal capacity - The total amount of energy a battery can store. You cannot necessarily use all of the electricity that is stored in the battery.
Off-grid solar system - A solar panel system that is not connected to the grid at all. Off-grid systems are typically paired with large amounts of solar storage and generators. Read more about off-grid solar systems here.
Overcharging - Charging a battery above its nominal capacity.
Peak demand - The highest amount of electricity consumption on the utility grid over a certain period of time.
Peak demand charge - A charge added to the cost of electricity and other fixed bill charges during a customer's highest demand hour each month.
Peak demand shaving - Managing how you use electricity to minimize the amount of energy you take from the grid during peak demand hours in order to avoid peak demand charges. This usually involves taking electricity from your battery as opposed to the grid during peak demand hours.
Peak power output - The maximum amount of electricity that can flow from a battery for a brief period of time, usually about 10 seconds, so the battery can turn on appliances that require more electricity to power up than they do to run.
Power output - How much electricity can flow out of the battery at any given point in time, measured in kilowatts (kW). The power output typically dictates how many and what appliances your battery can power at once.
Round-trip efficiency - Batteries use a small amount of power to actually store and release electricity. The round-trip efficiency is the percentage of electricity that is available for your home to use after the storage process.
Salt water battery - A type of battery that uses a saline electrolyte solution. These are not currently a mainstream option for residential solar batteries.
Sealed lead acid (SLA) battery - A type of lead acid battery that does not require regular maintenance, as the electrolyte solution does not evaporate. AGM and gel batteries are types of sealed lead acid batteries. Read more about sealed lead acid batteries here.
Solar battery - A rechargeable battery that is connected to and charged by a solar panel system. The stored solar power can then be used to power your home.
Terminal - The electrical contacts that connect the battery to the charging source or the device that needs to be charged.
Thermal runaway - Occurs when a battery cell overheats due to some sort of failure, malfunction, or misuse until the battery is no longer stable and temperature increases cannot be controlled. This can cause the battery to catch fire.
Throughput - The total amount of energy a battery can deliver over its lifetime, typically measured in kWh.
Time-of-Use (TOU) rates - A utility rate structure in which electricity costs more during certain times of the day. Read more about time-of-use rates here.
Usable capacity - The amount of stored energy within a battery that is available for use.
Voltage - The pressure that a circuit's power source pushes electrons through the circuit, measured in Volts.
Watts - A unit of power equal to one joule per second. One watt is equal to one volt times one ampere.