Glossary

The DC power glossary

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AC (alternating current): Current that reverses direction many times a second (50 or 60 Hz). Used by grids and most household appliances.
Absorption stage: The charging phase where a charger holds a constant voltage while current tapers as the battery fills.
Amp (A): Unit of electric current — the rate of charge flowing through a conductor.
Amp-hour (Ah): Battery capacity: one amp for one hour. Multiply by voltage to get watt-hours.
Ampacity: The maximum continuous current a conductor can carry without exceeding its temperature rating.
AWG (American Wire Gauge): Wire size system where smaller numbers mean thicker wire; 4/0 is very thick, 18 is thin.
Balancing: BMS function that keeps individual cells at matching voltage so the pack can use its full capacity.
BMS (battery management system): Electronics that monitor and protect a lithium battery: voltage, current, temperature and cell balance.
Busbar: A solid conductor that distributes high current to several cables neatly and safely.
C-rate: Charge or discharge current relative to capacity; 1C empties a battery in one hour, 0.5C in two.
CCS (Combined Charging System): EV connector family that adds two DC pins below an AC plug: CCS1 in North America, CCS2 in Europe and much of the world.
Charge controller: Device that regulates solar panel output to charge a battery safely. See MPPT and PWM.
Charging curve: A vehicle's charging power plotted against state of charge; power typically tapers as the battery fills.
CHAdeMO: A DC fast-charging connector standard from Japan, used by some older EVs elsewhere.
Circular mil (cmil): Unit of wire cross-sectional area used in voltage-drop calculations.
Clipping: Lost solar output when panel DC power exceeds what the inverter can convert.
DC (direct current): Current that flows in one direction. Produced by solar cells, stored in batteries, used by electronics and EV packs.
DC-DC charger: Converts one DC voltage to another, e.g. charging a house battery from a vehicle alternator.
DC fast charger (DCFC): Off-board charger that supplies DC straight to an EV battery, typically 50–400+ kW.
DC/AC ratio: Solar array DC rating divided by inverter AC rating; slightly above 1 is common.
Degradation: Gradual loss of capacity (batteries) or output (solar panels) over time.
Depth of discharge (DoD): How much of a battery's capacity is used in a cycle. 80% DoD means discharging to 20% state of charge.
Efficiency (EV): Distance per unit of energy, e.g. miles per kWh or Wh per km.
Float charge: A low maintenance voltage after full charge; lead-acid needs it, LiFePO4 often does not.
Fuse: A sacrificial link that opens on over-current to protect wiring from overheating.
Ground fault: An unintended current path to ground; can be dangerous in PV arrays and must be detected.
Hybrid inverter: An inverter that manages solar input, battery charging and AC output in one unit.
Interrupt rating (AIC): The maximum fault current a fuse or breaker can safely stop.
Inverter: Converts DC to AC. Rated by continuous and surge watts.
J1772: North American AC charging connector (Type 1), used for Level 1 and Level 2.
J3400 (NACS): SAE standard for the North American Charging Standard connector, carrying AC and DC through one compact plug.
kW vs kWh: kW is power (rate); kWh is energy (amount). A 10 kW charger running one hour delivers 10 kWh.
Level 1 / Level 2: AC charging at 120 V (≈1.9 kW) or 208–240 V (≈7–19 kW) in North America.
LFP / LiFePO4: Lithium iron phosphate chemistry: long cycle life, thermally stable, 3.2 V per cell.
Microinverter: A small inverter on each solar panel, converting to AC at the roof.
MPPT: Maximum power point tracking: a charge controller that adjusts voltage and current to extract the most power from panels.
Net metering: Billing arrangement crediting exported solar energy against energy you buy; rules vary widely.
NMC: Nickel manganese cobalt lithium chemistry: high energy density, used in many EVs.
Ohm (Ω): Unit of resistance. Voltage = current × resistance.
Onboard charger (OBC): Converter inside an EV that turns AC from a Level 1/2 station into DC for the battery.
Peak sun hours: Daily solar energy expressed as hours of 1,000 W/m² sunlight; used to size arrays.
Preconditioning: Heating or cooling an EV battery before fast charging so it can accept more power.
PWM controller: Simpler, cheaper solar controller; less efficient than MPPT when panel and battery voltages differ.
Round-trip efficiency: Energy you get out of a battery divided by energy you put in.
Series / parallel: Series adds voltage; parallel adds capacity (amp-hours).
Shunt: Precision resistor used by battery monitors to measure current and track state of charge.
State of charge (SoC): How full a battery is, as a percentage.
State of health (SoH): A battery's current capacity compared with when new.
String (PV): Panels wired in series to raise voltage; residential strings can exceed 400 V DC.
Surge rating: Short-term power an inverter can deliver to start motors and compressors.
Taper: The reduction in charging current as a battery approaches full.
V2H / V2G / V2L: Vehicle-to-home, vehicle-to-grid and vehicle-to-load: using an EV battery to power a house, the grid or devices.
Volt (V): Unit of electrical potential difference — the "pressure" pushing current.
Voltage drop: Voltage lost along a cable due to its resistance; keep it under about 3% for most DC circuits.
Watt (W) / watt-hour (Wh): Power and energy. Watts = volts × amps; watt-hours = watts × hours.