Solar and EV Glossary

Ah (ampere-hour)

A unit of electric charge equal to 1 ampere flowing for 1 hour. Ah is used to express battery capacity in terms of charge rather than energy: a 100 Ah battery at 12 V stores 100 × 12 = 1200 Wh = 1.2 kWh of energy. Two batteries with the same Ah rating but different voltages store different amounts of energy; voltage must always accompany Ah to give a meaningful energy figure. To convert: kWh = Ah × V / 1000. The Battery Bank Sizing Calculator outputs the required Ah for a given daily consumption, system voltage and depth of discharge.

Autonomy (also: days of autonomy)

The number of consecutive cloudy or low-sun days a battery bank can sustain the load at the chosen depth of discharge without any solar input. Autonomy directly determines battery bank size: doubling autonomy doubles the required Ah. For grid-tied systems with battery backup, 1 to 2 days is typical (covering a single cloudy day or overnight). For off-grid cabins, 2 to 4 days handles typical weather patterns. Beyond 4 to 5 days, additional battery capacity costs more than the energy it would supply, and partially-cycled batteries degrade faster. The battery bank formula is: Ah = (daily kWh × 1000 × autonomy days) / (system voltage × DoD). Use the Battery Bank Calculator.

Battery capacity (nominal vs usable)

The total energy a battery can store, usually quoted in kWh (nominal) or Ah at a specified voltage. Nominal capacity is the total physical capacity; usable capacity is smaller because discharging below the minimum state of charge (limited by depth of discharge) degrades the battery. A 10 kWh lithium battery with 90% DoD has 9 kWh usable. Lead-acid batteries are typically limited to 50% DoD, so a 10 kWh lead-acid bank delivers only 5 kWh usable. Always size a battery bank using the usable (not nominal) capacity against your daily load. Capacity fades over the battery's lifetime: most lithium batteries are warranted to 70 to 80% of original capacity after 3000 to 6000 cycles.

C-rate

The rate of charge or discharge relative to the battery's nominal capacity, expressed as a multiple of C. 1C means the battery is charged or discharged in 1 hour: a 100 Ah battery at 1C draws 100 A. 0.5C draws 50 A (2-hour discharge); 2C draws 200 A (30-minute discharge). Higher C-rates reduce effective capacity (the Peukert effect) and generate more heat, shortening battery life. Lithium iron phosphate (LiFePO4) cells tolerate 1 to 2C continuous comfortably; most EV fast chargers charge at 0.5 to 1C peak; some fast chargers reach 3C on the right cell chemistry. For a home solar battery, typical charge rates are 0.2 to 0.5C (charging from overnight to full in 2 to 5 hours). The EV Charging Cost Calculator works in kWh added rather than C-rate, which is more practical for home use.

CC/CV charging (constant current / constant voltage)

The two-stage charging profile used by lithium batteries and most modern EV chargers. In the constant current (CC) phase, the charger delivers a fixed current (e.g. 80 A) while voltage rises gradually; this phase fills roughly 80% of battery capacity quickly. Once the battery reaches its maximum voltage (e.g. 4.2 V/cell for NMC), the charger switches to constant voltage (CV) mode: voltage is held fixed while current tapers off as the battery approaches full charge. The CV phase takes longer per kWh added but protects the battery from overcharge. This is why EVs charge 0 to 80% much faster than 80 to 100%: the CV taper slows down the last 20%. Fast DC chargers extend the CC phase as high as the battery thermal management allows.

Depth of discharge (DoD)

The fraction of a battery's nominal capacity that is discharged before recharging, expressed as a percentage. 80% DoD means 80% of the battery's capacity was used, leaving 20% remaining (SoC = 20%). DoD and SoC are complementary: DoD = 100% - SoC. Maximum recommended DoD varies by chemistry: lithium iron phosphate (LiFePO4) can typically be cycled to 80 to 90% DoD with minimal degradation; NMC lithium (common in EVs) is usually limited to 80% for daily use; lead-acid should not exceed 50% DoD for decent cycle life. Deeper discharges stress the battery more and shorten its lifespan. The Battery Bank Calculator uses DoD as a required input to calculate the correct bank size.

EV charging level (L1 / L2 / DC fast)

A classification of EV charging speed based on power delivery. Level 1 (L1): standard household outlet, 1.4 to 1.9 kW (120 V / 16 A in North America), adds roughly 8 to 15 km of range per hour. Level 2 (L2): dedicated circuit with a home charger unit (EVSE), typically 3.7 to 22 kW (230 V single-phase or 400 V three-phase), adds 20 to 150 km per hour depending on power and car. DC fast charging (DCFC, also called L3): bypasses the onboard AC/DC converter, delivering DC directly to the battery at 50 to 350 kW, adds 100 to 400 km per hour. Most home installations use L2. Use the Home Charger Wiring Planner to size the circuit for your L2 charger, and the Charging Cost Calculator to estimate cost per session.

Grid-tied (also: grid-connected)

A solar system that is connected to the utility electricity grid. Excess generation is exported to the grid (often credited via net metering or a feed-in tariff); when generation falls short, the home draws from the grid normally. Grid-tied systems do not require battery storage (the grid acts as a backup) and are simpler and cheaper to install than off-grid or hybrid systems. A key limitation: grid-tied inverters shut down during grid outages for safety reasons (to prevent live power being fed back to a line a utility worker may be repairing). Battery backup or a generator is required for resilience. Most residential solar installations in Europe and North America are grid-tied.

Hybrid solar system

A solar system that includes both grid connection and battery storage. The battery covers evening demand and short grid outages; the grid covers extended cloudy periods and provides export revenue. Hybrid systems use a hybrid inverter (combines solar inverter, battery inverter and grid connection in one unit) or separate components with a transfer switch. They are more expensive than pure grid-tied systems but provide energy independence and backup power. An off-grid system has no grid connection; a grid-tied system has no battery. "Hybrid" strictly covers the combination of both. The Solar Payback Calculator can model the economics of adding battery storage to a grid-tied system.

Inverter

A device that converts direct current (DC) to alternating current (AC). In a solar system, the inverter converts the DC output of panels (and batteries) to the 230 V / 50 Hz (or 120/240 V / 60 Hz) AC that household appliances use. Types: string inverter (one unit for all panels in series, most common for residential), microinverter (one per panel, maximises yield on shaded roofs), hybrid inverter (integrates battery charging and grid connection). Inverter sizing should account for the total panel array peak power plus a surge margin for motor loads (typically 2 to 3x rated power for brief startup). Use the Inverter Sizing Calculator to determine the required rating from your load list.

kW (kilowatt, unit of power)

A unit of power equal to 1000 watts. Power measures the rate of energy transfer or consumption at an instant in time. A 3 kW electric kettle converts 3000 joules of electrical energy to heat every second while it is on. A 5 kWp solar array produces up to 5 kW of electrical power at peak irradiance. An EV home charger rated at 7.4 kW delivers that power continuously while charging. kW tells you the rate; kWh tells you the total energy over time. The relationship is: kWh = kW × hours.

kWh (kilowatt-hour, unit of energy)

A unit of energy equal to 1 kilowatt of power consumed for 1 hour. Your electricity bill is measured in kWh. A 100 W bulb running for 10 hours uses 1 kWh. A solar panel producing 400 Wp under peak conditions for 5 hours generates 2 kWh. An EV with a 77 kWh battery can store 77 kWh of energy, and adding 46 kWh (charging 20 to 80%) costs approximately 46 × electricity tariff in euros or pounds. kWh is an energy unit; kW is a power unit. Confusing the two is the most common error in solar and EV discussions: "My car has 77 kWh" is correct (energy); "My charger is 7.4 kWh" is wrong (it should be 7.4 kW). Use the kWh to Euro Calculator for cost calculations.

kWh/100 km (EV energy consumption)

The standard European metric for electric vehicle energy efficiency, analogous to litres/100 km for petrol cars. It states how many kilowatt-hours of energy the vehicle consumes per 100 km of travel. Lower is better. Typical values: compact EVs 13 to 17 kWh/100 km; mid-size sedans 16 to 20 kWh/100 km; large SUVs 20 to 28 kWh/100 km. Consumption rises significantly at motorway speeds and in cold weather. To calculate range: range (km) = battery capacity (kWh) / (kWh/100 km) × 100. The North American equivalent is miles per kWh (mi/kWh) or Wh/mile. See also Wh/km for the per-kilometre variant. Use the EV Charging Cost Calculator to estimate cost per 100 km.

kWp (kilowatt-peak, also kW_p)

The peak power output of a solar panel or array under Standard Test Conditions (1000 W/m² irradiance, 25 °C cell temperature, AM1.5 spectrum). A 400 Wp panel has a kWp of 0.4; a 10-panel array of 400 Wp each has a system size of 4 kWp. kWp measures the rated capacity of the array; actual output is always lower in real conditions because irradiance is rarely 1000 W/m², cells run hotter than 25 °C in summer, and inverter and wiring losses apply. Annual energy yield: kWh/year = kWp × annual peak sun hours × system efficiency. For a 4 kWp system in the UK (900 annual PSH equivalent), with 80% system efficiency: 4 × 900 × 0.80 = 2880 kWh/year. Use the Solar Panel Output Calculator.

MPPT (Maximum Power Point Tracking)

A control algorithm used in solar charge controllers and inverters to continuously find and operate the panel at its maximum power point: the voltage at which the panel delivers its highest wattage for current irradiance conditions. A solar panel's I-V (current-voltage) curve has a knee where power peaks; this point shifts with irradiance and temperature. MPPT electronics sweep the operating voltage to track this peak, extracting 20 to 30% more energy than a fixed-voltage PWM controller in typical conditions. MPPT controllers can also accept higher panel voltages and step down to the battery voltage, allowing longer cable runs with thinner wire. MPPT is the recommended choice for any system larger than a few hundred watts. Compare with PWM.

Off-grid

A solar (or other renewable energy) system with no connection to the utility electricity grid. The system must be entirely self-sufficient: the panel array, battery bank and charge controller must cover 100% of the load including the worst-case low-sun season. Off-grid systems are typically sized for worst-month irradiance (winter peak sun hours) rather than annual averages, to avoid running out of power in the darkest months. They are more expensive per kWh than grid-tied systems because large battery banks add significant cost. Common applications: remote cabins, caravans, boats, rural properties where grid connection is prohibitively expensive. Compare with grid-tied and hybrid.

Panel efficiency

The fraction of incident solar energy that a panel converts to electrical energy, measured as a percentage. Panel efficiency = Pmax / (irradiance × panel area). At 1000 W/m² irradiance: a 400 Wp monocrystalline panel measuring 1.7 m² has efficiency = 400 / (1000 × 1.7) = 23.5%. Commercial mono-PERC panels typically achieve 19 to 22%; TOPCon and heterojunction (HJT) cells reach 22 to 24%; polycrystalline panels are 16 to 18%. Higher efficiency means fewer panels or less roof area for the same output. Panel efficiency is not the same as system efficiency: system efficiency (typically 75 to 85%) accounts for inverter losses, wiring losses, temperature derating and soiling. The Solar Panel Output Calculator uses system efficiency as a separate input.

Peak sun hours (PSH)

The equivalent number of hours per day during which solar irradiance averages 1000 W/m² (the reference level for panel ratings). PSH is not the same as daylight hours or sunshine hours: a location with 10 hours of daylight but variable cloud cover might receive only 4 peak sun hours. To calculate daily energy yield: daily kWh = system kWp × PSH × system efficiency. For a 4 kWp system with 80% efficiency and 3.5 PSH: 4 × 3.5 × 0.80 = 11.2 kWh/day. PSH varies by location, season and panel tilt. The UK averages 2.2 to 2.8 PSH annually; Spain averages 4.5 to 5.5 PSH; Australia 4.5 to 6.0 PSH. Use the Peak Sun Hours reference table for your region, and the Solar Panel Output Calculator to apply it.

PWM charge controller (pulse-width modulation)

A simpler, cheaper type of solar charge controller that connects the panel directly to the battery when charging is needed, pulsing the connection on and off to regulate current as the battery nears full charge. Because the panel voltage is pulled down to match the battery voltage, any excess panel voltage is wasted as heat rather than converted to useful energy. PWM controllers are only efficient when panel voltage closely matches battery voltage (a 12 V panel on a 12 V battery). They lose 20 to 30% of potential yield compared to MPPT in most real installations, and do not support higher-voltage panels. PWM is acceptable for very small systems (under 200 W), simple portable setups, or where cost is the overriding constraint. For anything larger, MPPT pays for itself quickly.

Range (EV)

The maximum distance an electric vehicle can travel on a fully charged battery under a defined test cycle. Official figures use WLTP (Worldwide Harmonised Light Vehicle Test Procedure) in Europe, or EPA combined in the USA. WLTP ratings are typically 10 to 15% higher than real-world range; EPA figures are closer to real-world. Actual range varies with speed (motorway driving at 120 km/h uses 30 to 50% more energy than city driving), ambient temperature (batteries lose 20 to 40% capacity at -10 °C), HVAC use, and load. Range = battery capacity (kWh) / consumption (kWh/100 km) × 100. A 77 kWh battery with 18 kWh/100 km consumption gives 77 / 18 × 100 = 428 km WLTP range; real-world at 130 km/h might be 280 to 320 km.

Regenerative braking

A feature of electric and hybrid vehicles in which the electric motor acts as a generator during deceleration, converting kinetic energy back into electricity and storing it in the battery, rather than wasting it as heat in friction brakes. The amount of energy recovered depends on driving style, speed and the car's regeneration calibration. In city driving with frequent stops, regeneration can recover 15 to 25% of the energy used in acceleration. At steady motorway speeds, there is little deceleration and regeneration contributes minimally. Most modern EVs offer adjustable regeneration strength: light regen (coasting feel, like petrol cars) through to strong "one-pedal driving" where lifting the accelerator provides enough deceleration for most city stops without touching the brake pedal.

Solar irradiance (unit: W/m²)

The power of solar radiation incident on a surface per unit area, measured in watts per square metre (W/m²). The solar constant (outside the atmosphere, perpendicular to the sun) is approximately 1361 W/m². At sea level on a clear day with the sun at zenith, irradiance on a horizontal surface is about 1000 W/m², which is the reference value for solar panel ratings (STC). Typical irradiance varies from below 200 W/m² on overcast days to 900 to 1100 W/m² on clear summer days. The integral of irradiance over time gives solar irradiation (Wh/m² or kWh/m²), which is the total energy delivered. Peak sun hours are numerically equal to daily irradiation in kWh/m².

State of charge (SoC)

The remaining charge in a battery expressed as a percentage of its nominal capacity. 100% SoC = fully charged; 0% SoC = fully discharged (or at the battery management system's cutoff). SoC and depth of discharge are complementary: SoC = 100% - DoD. Estimating SoC accurately is a challenge for battery management systems (BMS): open-circuit voltage gives a rough approximation (for lithium, voltage changes little between 20% and 80% SoC, making it unreliable in that range), so modern BMS units use coulomb counting (integrating current over time) corrected by periodic full-charge resets. EVs display SoC as a percentage or estimated range on the dashboard.

STC (Standard Test Conditions)

The laboratory reference conditions under which solar panels are rated: irradiance of 1000 W/m², cell temperature of 25 °C, and AM1.5 solar spectrum (equivalent to sunlight through 1.5 atmospheres of air, approximately a 48° sun angle). STC are chosen for reproducibility and international comparison, not because they represent typical outdoor conditions. In practice, cells routinely run at 45 to 70 °C in summer, reducing output by about 0.3 to 0.5% per degree above 25 °C. A panel rated 400 Wp at STC may produce only 360 to 380 Wp on a hot summer day. Irradiance is also rarely 1000 W/m² at the panel due to cloud cover, haze and panel tilt. System efficiency factors in the Solar Panel Output Calculator account for these real-world deviations from STC.

Wh/km (EV energy consumption per kilometre)

An alternative expression of EV energy efficiency: watt-hours consumed per kilometre travelled. Wh/km = kWh/100 km × 10. A car consuming 18 kWh/100 km uses 180 Wh/km. Wh/km is useful for per-trip calculations: multiply trip distance by Wh/km to get energy used. It appears in EV charging cost calculators because it links directly to battery capacity (kWh) and range: range (km) = battery capacity (Wh) / consumption (Wh/km). Typical values: 130 to 160 Wh/km for compact EVs; 160 to 220 Wh/km for mid-size; 220 to 300 Wh/km for large SUVs and vans. See also kWh/100 km. The EV Charging Cost Calculator accepts consumption in Wh/km for range estimates.

Frequently Asked Questions