Home Energy Glossary

A

• AC Coupling A method of connecting a battery to a solar PV system where the solar panels feed AC power to the home through a string inverter, and the battery converts that AC back to DC for storage. Simpler to retrofit on existing solar installations, but slightly less efficient (~5% loss) than DC coupling. The FranklinWH aPower 2 and Tesla Powerwall 3 (when paired with non-Tesla solar) both use AC coupling.
• Arbitrage The practice of charging a home battery during off-peak hours when electricity is cheap (e.g., $0.12/kWh) and discharging during peak hours when electricity is expensive (e.g., $0.42/kWh). In time-of-use markets like California, daily arbitrage can save $1,500–$2,400 per year. See our ROI scenarios.
• ASIN Amazon Standard Identification Number — a 10-character alphanumeric code that uniquely identifies a product on Amazon. Used in affiliate links to direct buyers to specific products.

B

• Backup Load Center A subpanel containing only the circuits you want to back up during an outage. The battery powers only this subpanel, not the whole main panel. Sizing the backup load center correctly is the single most important factor in keeping battery system costs down.
• Bidirectional Charger An EV charger that can move power in both directions — into the vehicle (charging) and out of the vehicle (V2H or V2G). Required for vehicle-to-home use. Examples include the Ford Charge Station Pro, Wallbox Quasar 2, Emporia bidirectional, GM Energy PowerShift, and Tesla Powershare Gateway. See our bidirectional charger comparison.

C

• Cycle One complete charge and discharge of a battery. Battery warranties typically specify a maximum number of cycles (e.g., 6,000) at which the battery will still retain at least 60–70% of its original capacity. One cycle per day for 10 years = ~3,650 cycles.
• ConnectedSolutions A performance-based battery incentive program run by utilities in Massachusetts, Rhode Island, Connecticut, New Hampshire, Maine, and Vermont. Pays homeowners ~$1,500/year for 5 years in exchange for allowing the utility to draw power from the battery during peak grid events. Stacks with the 30% federal credit.

D

• DC Coupling A method of connecting a battery to a solar PV system where the solar panels feed DC power directly to the battery without conversion. ~5% more efficient than AC coupling but requires a hybrid inverter. Best for new solar-plus-storage installs.
• Depth of Discharge (DoD) The percentage of a battery's capacity that has been discharged. Most LFP batteries can safely discharge to 95% DoD without damage. Older NMC chemistries were typically limited to 80% DoD to preserve cycle life.

E

• End of Life (EOL) The point at which a battery has degraded to a specified percentage of its original capacity — typically 60% or 70%. Most manufacturer warranties guarantee replacement or repair if the battery reaches EOL before the warranty period expires.
• Essential Loads The minimum set of household circuits needed to maintain basic safety and habitability during an outage — typically refrigerator, lights, internet, furnace blower, well pump, and medical devices. Sizing a battery for essential loads only (vs whole-home backup) typically cuts system cost by 50–70%.

F

• Federal Residential Clean Energy Credit Internal Revenue Code §25D — a 30% federal income tax credit on the total installed cost of qualifying residential energy storage (≥3 kWh, charged by solar at least once per year). Non-refundable, carry-forward indefinitely. Runs through 2032 with no phase-down. See our federal credit guide.

G

• Grid-Tied An electrical system that is connected to the utility grid. Grid-tied battery systems can discharge to the grid (in V2G or export programs) and charge from the grid during off-peak hours. Contrast with off-grid systems, which have no utility connection.

H

• Hybrid Inverter A single inverter that handles both solar PV input and battery input, eliminating the need for a separate battery inverter. The Tesla Powerwall 3's integrated inverter is a hybrid inverter, accepting up to 19.2 kW of DC solar input.

I

• Interconnection Agreement A formal agreement with your utility that allows your battery (or solar-plus-storage system) to connect to the grid. Required for any grid-tied system. Some utilities have streamlined agreements for V2H-only systems; V2G systems require more extensive review.
• IRA (Inflation Reduction Act) 2022 U.S. federal legislation that extended and expanded the Residential Clean Energy Credit, raised it to 30%, and confirmed it runs through 2032. The single most important piece of federal policy for home battery economics in the 2020s.

K

• kW (Kilowatt) A unit of power — the rate at which energy is being used or produced at any instant. A typical U.S. home draws 1–3 kW continuously under normal operation, with brief surges to 10–15 kW when large appliances start. Battery continuous output is rated in kW.
• kWh (Kilowatt-Hour) A unit of energy — power multiplied by time. One kW of power used for one hour = 1 kWh of energy. Battery capacity is rated in kWh. The average U.S. household uses about 28 kWh per day.

L

• LFP (Lithium Iron Phosphate) The dominant battery chemistry for residential storage in 2026. Safer than NMC (no thermal runaway), longer cycle life (6,000+ cycles), no cobalt, tolerates deep discharge. All five batteries in our 2026 comparison use LFP.
• Load Shifting See Arbitrage. The practice of moving electricity consumption from one time period to another (typically from peak to off-peak) using a battery or other storage.
• LRA (Locked Rotor Amps) A measure of a motor's starting current — specifically the current draw when the motor is prevented from rotating. Used to compare battery surge capability: a typical 3-ton central AC compressor requires ~100 LRA to start. Higher LRA ratings on a battery mean it can start larger motors.

M

• Main Service Panel The central electrical panel where utility power enters your home and is distributed to individual circuits. Also called a breaker box or load center. If your main service panel is older than 1990 or rated for less than 200 amps, you may need a panel upgrade ($1,800–$3,500) before installing a home battery.
• Microinverter A small inverter attached to each individual solar panel, converting DC to AC at the panel level. Enphase is the dominant microinverter manufacturer. Microinverter systems pair naturally with AC-coupled batteries like the Enphase IQ Battery 5P.

N

• NACS (North American Charging Standard) The EV charging connector designed by Tesla, adopted by virtually all automakers for 2025+ model-year vehicles. Replaces the older J1772 connector for Level 2 AC charging and the CCS1 connector for DC fast charging. If you're buying a 2026 EV, it almost certainly uses NACS.
• NMC (Nickel Manganese Cobalt) A lithium-ion battery chemistry used in most EVs and older home batteries. Higher energy density than LFP but less safe (can experience thermal runaway) and uses cobalt. Largely phased out of residential storage by 2024 in favor of LFP.

O

• Off-Peak The hours when electricity demand is low and utility rates are cheapest — typically 9 PM to 4 PM on weekdays (overnight and midday). The ideal time to charge a home battery or EV. Contrast with peak.

P

• Peak Demand The hours when electricity demand is highest and utility rates are most expensive — typically 4 PM to 9 PM on weekdays in most U.S. markets. The ideal time to discharge a home battery.
• PPA (Power Purchase Agreement) A financing arrangement where a third party owns the solar-plus-storage system on your property and sells you the electricity it produces at a per-kWh rate. Zero upfront cost, but you don't get the federal tax credit and the agreement can complicate home sale. Generally not recommended in 2026.

R

• Round-Trip Efficiency The percentage of energy put into a battery that can be retrieved. If you put in 10 kWh and get back 9 kWh, the round-trip efficiency is 90%. Higher is better. The Enphase IQ Battery 5P leads the segment at 96%; most LFP batteries are 89–95%.

S

• SGIP (Self-Generation Incentive Program) California's flagship home battery rebate program. Pays $0.15–$0.35 per watt-hour of capacity for general-tier applicants and up to $1.00/Wh for equity-tier applicants (low-income or wildfire-prone areas). The single most generous state battery incentive in the U.S. Stacks with the 30% federal credit.
• Smart Panel An electrical panel with per-circuit monitoring and remote control, allowing you to dynamically manage which circuits receive power during an outage or peak-rate period. Examples include the Span Panel, Lumin Smart Panel, and Schneider Square D Energy Center. See our smart panel comparison.
• State of Charge (SoC) The current percentage of a battery's capacity that is filled. A battery at 80% SoC has 80% of its usable capacity remaining. Most home battery systems reserve 5% SoC as a buffer to extend cycle life.
• Surge Output The maximum power a battery can deliver for a brief period (typically 10–30 seconds) to start a motor. Measured in kW or LRA. Critical for backing up loads with high inrush current like air conditioner compressors, well pumps, and furnaces.

T

• TOU (Time-of-Use) A utility rate structure where electricity prices vary by time of day. Typically features cheap off-peak rates, expensive peak rates (4–9 PM), and sometimes a "super-off-peak" midday window. TOU rates are the primary economic driver for home battery arbitrage in California, Massachusetts, Hawaii, and parts of Arizona and New York.
• Transfer Switch An electrical device that automatically disconnects your home from the utility grid during an outage and connects it to your battery system. Required for safe backup operation — prevents backfeeding power to the grid (which can kill utility lineworkers). Modern batteries have integrated transfer switches; older installations use external units like the Reliance Controls Pro/Tran.

U

• Usable Capacity The actual kWh a battery can deliver, as opposed to its nameplate capacity. Most LFP batteries have 95% usable capacity (5% reserved as buffer). When comparing batteries, always compare usable capacity, not nameplate.

V

• V2G (Vehicle-to-Grid) Feeding power from an EV battery back to the utility grid, typically for revenue from grid services programs. The hardware is the same as V2H; the difference is software and utility interconnection. Active V2G programs in 2026 exist in California (PG&E), Maryland (BGE), Massachusetts (Eversource), and North Carolina (Duke Energy).
• V2H (Vehicle-to-Home) Using an EV's battery to power your home during an outage or during peak-rate hours. Requires a bidirectional charger and a transfer switch. See our full V2H explainer.
• V2L (Vehicle-to-Load) The simplest form of bidirectional charging — a standard 120V/240V outlet on the EV itself, max 1.9–3.6 kW, for plugging in tools or appliances directly. No bidirectional charger or transfer switch required.

W

• Watt-Hour (Wh) A unit of energy equal to 1/1000 of a kWh. Battery capacities for small portable power stations are often rated in Wh (e.g., 1024 Wh = ~1 kWh).