Solar Energy Glossary
50+ solar terms explained in plain English. Whether you're sizing your first system, negotiating with installers, or just curious about how solar works — this glossary covers the terminology that matters. Back to Learn →
A
- AC/DC Ratio
- The ratio of your solar array's DC capacity to your inverter's AC output rating. A ratio of 1.2:1 (e.g., 6 kW DC panels feeding a 5 kW AC inverter) is common and often optimal, as panels rarely produce full rated power.
- Azimuth
- The compass direction your solar panels face, measured in degrees clockwise from true north. In the Northern Hemisphere, azimuths between 150° and 210° (roughly south) are optimal for annual production.
B
- Bifacial
- A solar panel that captures sunlight on both its front and rear sides. Bifacial panels can generate 5-15% more energy than monofacial ones when mounted on reflective surfaces like white roofs or sand.
C
- Carbon Offset
- A reduction in CO₂ emissions that compensates for emissions elsewhere. Your solar system generates carbon offsets by producing clean electricity that displaces fossil-fuel-generated power.
- Clipping
- When a solar array produces more DC power than the inverter's AC rating can convert, forcing the inverter to cap output. Moderate clipping (1-3% of annual production) is often economically acceptable.
D
- Degradation Rate
- The annual percentage of power output a solar panel loses over time. Modern N-type panels degrade at approximately 0.4% per year, retaining ~88% of initial output after 30 years.
- DoD (Depth of Discharge)
- The percentage of a battery's capacity that has been discharged. A battery with 90% DoD can safely use 90% of its rated capacity before needing recharge. Higher DoD means more usable energy per cycle.
E
- Efficiency Rating
- The percentage of sunlight a solar panel converts into electricity. Residential panels range from 20% to 25% efficiency. Every 1% gain allows roughly 15-20 watts more per standard panel.
G
- Grid-Tied
- A solar system connected to the utility grid. Grid-tied systems can draw power when solar production is low and export excess generation back to the grid through net metering.
H
- HJT (Heterojunction Technology)
- A high-efficiency solar cell technology combining crystalline silicon with thin amorphous silicon layers. HJT cells achieve 24-25.5% efficiency with extremely low temperature coefficients, ideal for hot climates.
- Hybrid System
- A solar system that combines grid connection with battery storage. Hybrid inverters manage power flow between panels, batteries, home consumption, and the grid — enabling backup power and time-of-use optimization.
I
- Interconnection
- The process of connecting a solar system to the utility grid, including the technical and administrative steps required. Utilities require an interconnection agreement before your system can export power.
- Irradiance
- The power of solar radiation received per unit area, measured in watts per square meter (W/m²). Standard test conditions assume 1,000 W/m² irradiance. Real-world irradiance varies with latitude, season, and weather.
- ITC (Investment Tax Credit)
- A US federal tax credit for solar installations, currently 30% of system cost through 2032. The ITC reduces your federal income tax liability dollar-for-dollar and is the single largest incentive for US residential solar.
K
- kWh (Kilowatt-Hour)
- A unit of energy equal to one kilowatt of power sustained for one hour. Your utility bill is measured in kWh. An average US home consumes approximately 900 kWh per month.
- kW (Kilowatt)
- A unit of power equal to 1,000 watts. Solar system size is rated in kW — a typical residential system is 6-10 kW. To convert: an 8 kW system produces roughly 8 kWh in one peak sun hour.
L
- LCOE (Levelized Cost of Energy)
- The lifetime cost of electricity generation divided by total energy produced, expressed in cents per kWh. Solar LCOE has fallen below $0.04/kWh for utility-scale projects, making it the cheapest new electricity source.
- LFP (Lithium Iron Phosphate)
- A lithium-ion battery chemistry using iron phosphate as the cathode. LFP batteries offer longer cycle life (6,000-10,000 cycles) and superior thermal safety compared to NMC, now dominating residential storage.
- Li-ion (Lithium-Ion)
- A family of rechargeable battery chemistries using lithium ions. In solar storage, the two main variants are LFP (safer, longer-lived) and NMC (higher energy density). All modern home batteries are lithium-ion.
M
- MWh (Megawatt-Hour)
- A unit of energy equal to 1,000 kilowatt-hours. Utility-scale solar farms are measured in MWh of annual production. Your home system typically produces 8-15 MWh per year.
- Microinverter
- A small inverter mounted behind each individual solar panel that converts DC to AC at the panel level. Microinverters eliminate single-point failure and optimize each panel independently, making them ideal for shaded or complex roofs.
- Mono (Monocrystalline)
- Solar cells made from a single crystal of silicon, offering higher efficiency (20-25%) than polycrystalline cells. Nearly all modern residential panels use monocrystalline cells with PERC or TOPCon technology.
- MPPT (Maximum Power Point Tracking)
- An electronic system that continuously adjusts the electrical operating point of solar panels to extract maximum power under varying conditions. Modern inverters use MPPT to optimize energy harvest throughout the day.
N
- Net Billing
- A compensation structure where exported solar energy is credited at a rate different from (typically lower than) the retail electricity rate. Unlike net metering, net billing imports and exports are priced separately.
- Net Metering
- A billing mechanism that credits solar system owners for excess electricity exported to the grid at full retail rate. Your meter literally spins backward. Net metering policies vary by state and are being phased down in some regions.
- NMC (Nickel Manganese Cobalt)
- A lithium-ion battery chemistry using nickel, manganese, and cobalt. NMC offers higher energy density than LFP but shorter cycle life and higher cost, making it increasingly rare for new residential installations.
- NOCT (Nominal Operating Cell Temperature)
- The temperature a solar cell reaches under real-world conditions (800 W/m² irradiance, 20°C ambient, 1 m/s wind). NOCT ratings (typically 42-46°C) give more realistic performance predictions than STC lab ratings.
O
- Off-Grid
- A solar system completely disconnected from the utility grid. Off-grid systems require large battery banks and often backup generators, making them significantly more expensive than grid-tied systems.
P
- Peak Sun Hours
- The equivalent number of hours per day when solar irradiance averages 1,000 W/m². A location with 5 peak sun hours receives roughly 5 kWh of solar energy per kW of installed capacity per day.
- PERC (Passivated Emitter and Rear Cell)
- A solar cell technology adding a dielectric passivation layer on the rear of the cell to improve light capture. PERC panels achieve 20-22% efficiency and remain the dominant budget option for residential installations.
- Poly (Polycrystalline)
- Solar cells made from multiple silicon crystals melted together. Polycrystalline panels are blue in color and less efficient (15-18%) than monocrystalline. Largely phased out for residential applications in favor of mono PERC.
- Power Optimizer
- A DC-DC converter attached to each solar panel that maximizes power output before sending DC to a centralized string inverter. Optimizers combine the cost advantage of string inverters with panel-level optimization.
- PPA (Power Purchase Agreement)
- A contract where a third party installs and owns a solar system on your property and you buy the electricity it produces at an agreed rate, typically lower than utility prices. You pay for power, not equipment.
- PTC (PVUSA Test Conditions)
- A more realistic solar panel rating standard than STC, using 1,000 W/m² irradiance, 20°C ambient temperature, and 1 m/s wind speed. PTC ratings are typically 10-15% lower than STC and better reflect real-world output.
- PTO (Permission to Operate)
- Final approval from your utility company allowing your solar system to be connected to the grid and begin exporting power. PTO is the last step before activation and can take weeks after installation.
R
- REC (Renewable Energy Certificate)
- A tradable certificate representing 1 MWh of renewable electricity generation. Solar system owners can sell RECs separately from the physical electricity, providing additional revenue in some markets.
- Round-Trip Efficiency
- The percentage of energy retained after charging and discharging a battery. An LFP battery with 95% round-trip efficiency loses only 5% of energy per cycle. Higher efficiency means lower operating losses.
S
- Smart Meter
- A digital electric meter that records consumption in near-real-time and communicates with the utility. Smart meters enable time-of-use billing, net metering, and utility monitoring of solar exports.
- Solar Array
- The complete collection of solar panels in an installation, including mounting hardware and wiring. A typical residential array contains 15-25 panels covering 300-500 square feet of roof space.
- SREC (Solar Renewable Energy Certificate)
- A state-specific version of RECs generated exclusively by solar systems. SREC markets exist in states with Renewable Portfolio Standards, with prices varying from $10 to $400+ per certificate depending on supply and demand.
- STC (Standard Test Conditions)
- Industry-standard lab conditions for rating solar panels: 1,000 W/m² irradiance, 25°C cell temperature, and air mass 1.5 spectrum. STC ratings represent ideal conditions; real-world output is typically 10-15% lower.
- String Inverter
- A centralized inverter that converts DC power from multiple solar panels wired in series (a "string") into AC power. The most cost-effective inverter type for simple, unshaded roof layouts.
T
- Temperature Coefficient
- The percentage of power a solar panel loses for each degree above 25°C. A coefficient of -0.30%/°C means a 10.5% power loss at 60°C cell temperature. Lower coefficients (closer to zero) are better for hot climates.
- Tilt Angle
- The angle between your solar panels and the horizontal ground. Optimal tilt approximately equals your latitude for year-round production, though trade-offs exist between summer and winter optimization.
- TOPCon (Tunnel Oxide Passivated Contact)
- A high-efficiency N-type solar cell technology using an ultra-thin tunnel oxide layer to reduce recombination losses. TOPCon dominates new manufacturing capacity in 2026, achieving 22-24.5% module efficiency with excellent temperature coefficients.
- TOU (Time-of-Use)
- A utility rate structure where electricity prices vary by time of day, with peak hours (typically 4-9 PM) costing more than off-peak. Battery storage can shift solar generation to peak periods for higher savings under TOU rates.
V
- VPP (Virtual Power Plant)
- A network of distributed home batteries coordinated by software to provide grid services. VPP participants earn payments for allowing their batteries to discharge during peak demand events while maintaining backup reserves.