SolarToolsEnergy & Solar Glossary
Plain-English definitions of every term used across SolarToolsPro's calculators, blog posts, and guides. Bookmark this page — it's the reference you'll come back to.
A
AC (Alternating Current)
The standard form of electricity delivered by utilities and used in homes. Electrons reverse direction 50 or 60 times per second (depending on country). Solar panels produce DC, which must be converted to AC by an inverter before use in the home or export to the grid.
AC Coupling
A battery system architecture where the battery has its own integrated inverter. AC from the solar inverter is converted to DC to charge the battery, then back to AC to discharge. Slightly less efficient than DC coupling (90% vs 95%) but easier to retrofit to existing solar.
AFUE (Annual Fuel Utilization Efficiency)
A measure of gas furnace efficiency. AFUE 95 means 95% of the gas energy becomes heat, 5% is lost up the flue. Modern condensing furnaces achieve 95–98% AFUE. Older furnaces may be 80% or lower.
Ampere (Amp)
The unit of electrical current. One ampere = 6.24 × 10^18 electrons per second flowing past a point. Typical home circuits are 15 or 20 amps; an EV charger may draw 32–48 amps.
Autonomy Days
In off-grid solar, the number of days a battery bank can power the home without any solar recharge. Typical design is 2–3 days. More autonomy = more battery capacity = higher cost.
B
Balance of System (BOS)
All the components of a solar system except the panels and inverter: racking, wiring, conduit, disconnects, junction boxes, grounding, and mounting hardware. Typically 10–15% of total system cost.
Battery Cycle
One complete charge and discharge of a battery. Battery lifespan is measured in cycles. LFP batteries typically deliver 6,000+ cycles; NMC 2,000–4,000; lead-acid 1,000–1,500.
BTU (British Thermal Unit)
A unit of heat energy. One BTU = the energy to raise one pound of water by 1°F. Used to rate heating and cooling equipment. 1 kWh = 3,412 BTU. A typical US home needs 25,000–60,000 BTU/hr of heating at design temperature.
Bifacial Solar Panel
A solar panel that captures light on both sides, increasing production by 5–15% when mounted over reflective surfaces (white roofs, snow, light-colored ground). Common in commercial installs; increasingly available for residential.
C
CEC Efficiency
California Energy Commission efficiency rating for inverters. Slightly lower than peak (STC) efficiency but more realistic. Typical CEC ratings: 96–98% for string inverters, 96–97% for microinverters.
Coefficient of Performance (COP)
The efficiency of a heat pump: heat delivered divided by electricity consumed. COP 3.0 means 3 kWh of heat per 1 kWh of electricity. COP varies with outdoor temperature — a heat pump rated COP 3.5 at 47°F may deliver only COP 2.0 at 5°F.
Continuous Power
The maximum power a battery inverter can deliver indefinitely. Tesla Powerwall 3: 11.5 kW continuous. Critical for sizing: must exceed the sum of running watts of all backed-up appliances.
CRI (Color Rendering Index)
A 0–100 score for how accurately a light source renders colors compared to sunlight. Incandescent = 100. Cheap LEDs = 70–80. Quality LEDs = 90+. For living spaces, look for CRI 90+.
Customer Charge
A fixed monthly fee on your electricity bill ($8–$30) that covers the utility's cost of maintaining your connection — meter, billing, wires to your house. Paid even if you use zero kWh. Also called a 'basic service charge' or 'connection fee'.
D
DC (Direct Current)
Electricity where electrons flow in one direction. Solar panels produce DC; batteries store DC. Most homes use AC, so an inverter is required. EV batteries are DC, charged by AC (converted in the car) or DC (DCFC stations).
DC Coupling
A battery system architecture where solar DC connects directly to the battery (no conversion). Higher efficiency than AC coupling (95% vs 90%) but requires a hybrid inverter. Best for new solar+battery installs.
Depth of Discharge (DoD)
The percentage of battery capacity actually used. LFP batteries allow 80–90% DoD; lead-acid only 50%. A 13.5 kWh battery at 80% DoD delivers 10.8 kWh usable. Most modern batteries are rated by usable capacity, not nominal.
Demand Charge
A utility charge based on your peak kW draw during a billing period, not your kWh consumption. Common for commercial customers; rare for residential but growing. If you have a demand charge, staggering large loads (AC, dryer, EV) reduces it.
Distributed Energy Resources (DER)
Small-scale energy generation or storage located at or near the point of use — rooftop solar, home batteries, EVs with vehicle-to-grid capability. DERs aggregate to provide grid services in some markets.
E
EER (Energy Efficiency Ratio)
Cooling efficiency of AC: BTU cooling per watt of electricity. EER 12 = 12 BTU per watt. Higher is better. Modern central AC: 12–16 EER. Mini-splits: 15–22 EER. Older units: 8–10 EER.
Efficiency (Solar Panel)
The percentage of sunlight a panel converts to electricity. Standard panels: 20–21%. Premium panels (SunPower Maxeon, REC Alpha): 22.5–22.8%. Higher efficiency means more power per square foot — matters for small roofs, less so for large roofs.
Electric Vehicle Supply Equipment (EVSE)
The technical term for an EV charger. The EVSE doesn't actually 'charge' the battery — it provides safe AC power that the car's onboard charger converts to DC. Level 1 EVSE: 1.4 kW. Level 2 EVSE: 7–11 kW. DCFC: 50–350 kW.
F
Federal ITC (Investment Tax Credit)
US federal tax credit for residential solar and battery storage. Provided 30% of system cost for installations placed in service through December 31, 2025. Terminated early by the One Big Beautiful Bill Act (July 4, 2025). No federal credit for systems installed after Dec 31, 2025.
Feed-in Tariff (FIT)
A fixed price paid by utilities for solar electricity exported to the grid. Common in Europe and Australia. Different from net metering: FIT pays a set rate regardless of when you export. Often higher than retail for legacy contracts, much lower for new ones.
G
Generation Charge
The portion of your electricity bill covering the cost of producing electricity at a power plant. In deregulated states (TX, PA, MA, NY), you can shop for a competitive generation provider. Transmission and distribution charges still go to your local utility.
Grid-Tied Solar
A solar system connected to the utility grid. Excess production flows to the grid (credited via net metering); deficits draw from the grid. No battery required. The grid acts as a virtual 100%-efficient battery. Most residential solar is grid-tied.
Geyser
South African term for a water heater (typically electric tank). Geysers are one of the largest electricity consumers in SA homes — 30–40% of total kWh. Geyser timers and blankets are common efficiency upgrades.
H
HSPF2 (Heating Seasonal Performance Factor)
Seasonal average efficiency of a heat pump in heating mode. HSPF2 10 = average COP of 2.93. The current US minimum (2023+) is 7.5 HSPF2. Cold-climate heat pumps achieve 10–12 HSPF2. The '2' indicates the new 2023 testing standard.
Hybrid Inverter
A solar inverter that also handles battery charging and discharging. Allows DC-coupled battery systems with single conversion (higher efficiency). Required for new solar+battery installs seeking maximum efficiency.
I
Interconnection
The process of getting utility approval to connect your solar system to the grid. Requires application, engineering review, and sometimes a system upgrade. Typically takes 2–8 weeks. Until interconnection is approved, your system cannot be turned on.
Inverter
A device that converts DC electricity (from solar panels or batteries) to AC electricity (for home use or grid export). Three types: string (central), microinverter (per-panel), and power optimizer (per-panel DC conditioning + central AC inversion).
ITC (Investment Tax Credit)
See Federal ITC. Previously provided 30% federal tax credit for residential solar. Expired December 31, 2025. A separate commercial credit (Section 48E) remains available through 2027 for third-party-owned (lease/PPA) systems.
K
Kilowatt (kW)
A unit of power: 1,000 watts. Measures instantaneous output. A 7.5 kW solar system produces 7.5 kW at peak midday output. A 13.5 kW generator delivers 13.5 kW continuously. Compare to kWh (energy over time).
Kilowatt-Hour (kWh)
A unit of energy: 1 kW of power sustained for 1 hour. What your electric bill measures. A 100W bulb left on for 10 hours uses 1 kWh. At $0.16/kWh, that costs $0.16. US average household: ~10,800 kWh/year.
L
Level 1 Charging (EV)
EV charging from a standard 120V household outlet. Delivers ~1.4 kW (12 amps). Adds 3–5 miles of range per hour. Sufficient for under 40 miles/day driving. Free with every EV; no installation needed.
Level 2 Charging (EV)
EV charging from a 240V circuit (like an electric dryer). Delivers 7–11 kW. Adds 22–40 miles of range per hour. Full charge overnight. Requires 240V circuit installation. Cost: $400–$2,000 installed.
LFP (Lithium Iron Phosphate)
A lithium battery chemistry (LiFePO4) used in modern home batteries and some EVs. Safer than NMC (less prone to thermal runaway), longer cycle life (6,000+ cycles), no cobalt. Slightly lower energy density than NMC. Tesla Powerwall 3, Enphase IQ Battery 5P, and most 2024+ residential batteries use LFP.
Load Shedding
Planned rolling blackouts implemented by utilities when demand exceeds supply. Common in South Africa (Eskom), California (PSPS), and other regions with grid constraints. Drives demand for home batteries and generators.
Lumen
A measure of visible light output. A 60W incandescent produces ~800 lumens. A 9W LED produces the same 800 lumens. When buying LEDs, compare lumens (brightness), not watts (power consumption).
M
Microinverter
A small inverter mounted under each solar panel, converting DC to AC at the panel. Enphase is the leading brand. Benefits: panel-level MPPT, no single point of failure, better shade handling. Cost: ~10% more than string inverters. Warranty: 25 years.
MPPT (Maximum Power Point Tracking)
An algorithm in solar inverters that constantly adjusts the operating voltage to extract maximum power from panels. Without MPPT, panels would operate sub-optimally. String inverters have one MPPT per string; microinverters have one per panel.
N
NEM 3.0 (California)
California's 2023 net metering reform that reduced export credits from ~$0.30/kWh (full retail) to ~$0.08/kWh (avoided cost). Extended solar payback from ~6 years to 10–12 years. Made battery storage economically essential in California. Other states considering similar reforms.
Net Metering (NM)
A utility policy that credits solar customers for excess electricity exported to the grid. 1:1 (full retail) net metering credits every kWh exported at the retail rate. Reduced net metering pays less (avoided cost, typically $0.03–$0.08/kWh). Critical to solar economics.
NMC (Nickel Manganese Cobalt)
A lithium battery chemistry common in EVs and older home batteries. Higher energy density than LFP but shorter cycle life (2,000–4,000 cycles) and higher thermal runaway risk. Being phased out of residential storage in favor of LFP.
O
Off-Grid Solar
A solar system with no utility connection. Requires battery storage sized for multiple days of autonomy, often a backup generator. Costs 2–4× more than grid-tied solar. Necessary for remote properties; optional everywhere else.
Optimization (Solar)
Per-panel DC power conditioning via devices like SolarEdge power optimizers or Tigo. Allows panel-level MPPT and monitoring while keeping a central inverter. Middle ground between string inverters and microinverters.
P
Peak Sun Hours (PSH)
The equivalent number of hours per day when solar irradiance averages 1,000 W/m² (standard test condition). Different from daylight hours — accounts for varying intensity throughout the day. US range: 3 (Pacific Northwest) to 7 (Southwest desert). South Africa: 4.5–6.5.
Peak Demand
The maximum instantaneous power draw during a billing period. Used to calculate demand charges (mostly commercial). For residential, peaks typically occur 5–9pm when AC, cooking, and lighting overlap.
Power Optimizer
A DC-DC converter mounted under each solar panel that conditions power before sending to a central string inverter. Provides panel-level MPPT and monitoring. SolarEdge is the leading brand. Hybrid of string inverter and microinverter approaches.
PSH (Peak Sun Hours)
See Peak Sun Hours. The key solar resource metric for any location. Available from NREL PVWatts (US) or South African Solar Radiation Atlas.
R
Round-Trip Efficiency
The fraction of energy recovered from a battery versus what was put in. LFP batteries: 90–95%. NMC: 88–92%. The 5–10% loss becomes heat. Over 10 years of daily cycling, a 95% efficient battery saves $75+/year versus a 90% one at $0.30/kWh.
R-Value
A measure of insulation's resistance to heat flow. Higher = better insulation. R-30 attic insulation is typical; R-49+ is recommended for cold climates. Wall insulation: R-13 to R-23. Adding attic insulation is one of the highest-ROI efficiency upgrades.
S
SEER2 (Seasonal Energy Efficiency Ratio)
Seasonal average cooling efficiency of AC: BTU cooling per watt of electricity. SEER2 16 = 16 BTU per watt. The '2' indicates the new 2023 testing standard. US minimum (2023+): 14 SEER2. High-efficiency central AC: 18–22 SEER2. Mini-splits: 22–30 SEER2.
Self-Consumption
Using your own solar electricity directly instead of exporting to the grid. Becomes economically important when net metering is reduced or eliminated. A battery enables 70–90% self-consumption; without a battery, self-consumption is typically 30–50% of solar production.
SREC (Solar Renewable Energy Certificate)
A tradable certificate representing 1,000 kWh of solar generation. Some states (NJ, MA, MD, PA, IL, DC) have SREC markets that pay solar owners $50–$400 per MWh generated, on top of net metering savings. Can add $500–$2,000/year to solar economics.
Standby Power
Electricity consumed by devices when they're 'off' but plugged in. Also called phantom load or vampire power. Includes TVs, set-top boxes, game consoles, chargers. Typical US home: 50–150W continuous standby = $70–$210/year. Smart power strips eliminate.
STC (Standard Test Conditions)
Laboratory conditions for rating solar panels: 1,000 W/m² irradiance, 25°C cell temperature, AM1.5 light spectrum. Real-world output is typically 10–20% below STC ratings due to heat, dust, angle, and atmospheric conditions.
String Inverter
A single central inverter that handles an entire string of solar panels. Lowest cost option but single point of failure. Brands: SolarEdge (with optimizers), SMA, Fronius, Sungrow. Typical lifespan: 10–15 years (shorter than panel warranty).
T
Thermal Runaway
A self-reinforcing battery failure mode where heat generation exceeds heat dissipation, leading to fire. NMC batteries are more susceptible; LFP batteries are intrinsically safer. Quality battery management systems (BMS) prevent this in normal operation.
Tier 1 Solar Panel
A classification by Bloomberg New Energy Finance indicating a panel manufacturer's bankability (financial stability). Required by some financing programs. Doesn't directly indicate panel quality, but Tier 1 manufacturers (LG, SunPower, Panasonic, REC, Q Cells) generally produce reliable products.
TOU (Time-of-Use)
An electricity rate plan where prices vary by time of day. Peak hours (typically 4–9pm) cost 2–4× more than off-peak (11pm–6am). Reflects actual generation costs. Shiftable loads (dishwasher, EV, water heater) should run off-peak.
Transformer
A device that steps voltage up or down for transmission. Works only on AC (one of the reasons the grid uses AC instead of DC). Solar systems have a small transformer in the inverter; the utility has large ones at substations.
V
Volt (V)
A unit of electrical pressure. US homes receive 120V (standard outlets) or 240V (dryer, oven, EV charger). Utilities distribute at much higher voltages (4,000V–500,000V) and step down at transformers. Higher voltage = lower current = lower transmission losses.
VPP (Virtual Power Plant)
A network of distributed batteries (home Powerwalls, EVs) aggregated to provide grid services. Utilities pay participants for allowing them to discharge batteries during peak demand. Tesla, Sunrun, and others operate VPPs in California and Vermont.
W
Watt (W)
A unit of power: 1 joule per second. Measures instantaneous energy flow. A 400W solar panel produces 400 watts at peak. A 1,500W space heater consumes 1,500 watts continuously when running. Named after James Watt, the steam engine inventor.
Watt-Hour (Wh)
A unit of energy: 1 watt sustained for 1 hour. 1 kWh = 1,000 Wh. EV batteries are rated in kWh (e.g. 75 kWh). Phone batteries in Wh or mAh (milliamp-hours at the cell voltage).
Z
Zero Export
A solar system configuration that never exports to the grid. Required by some utilities with no net metering, or chosen by homeowners to avoid interconnection paperwork. A battery or smart inverter throttles production to match home consumption.