Solar Energy Glossary

Clear definitions of solar energy terms every homeowner should know before going solar.

A B C D E G H I K L M N O P R S T U V

A

Azimuth: The compass direction a solar panel faces, measured in degrees from north. In the Northern Hemisphere, south-facing (180 degrees azimuth) panels produce the most energy. Southwest and southeast orientations (150-210 degrees) produce 95-98% of optimal output.

B

Battery Storage: Rechargeable battery systems (typically lithium-ion) that store excess solar electricity for use during nighttime, cloudy periods, or grid outages. Popular options include Tesla Powerwall (13.5 kWh), Enphase IQ Battery (5/10/15 kWh), and Generac PWRcell (9-18 kWh).
Bifacial Solar Panel: A solar panel that captures light on both the front and rear surfaces, generating up to 10-20% more energy from reflected ground light (albedo). Most effective on ground-mount systems with light-colored surfaces underneath. Gaining popularity in commercial installations.
Ballasted Racking: A solar mounting system for flat roofs that uses weighted blocks (ballast) to hold panels in place without penetrating the roof membrane. Eliminates roof leak risk from mounting holes. Common on commercial flat roofs. Requires structural engineering to verify roof load capacity.

C

Cost Per Watt: The total installed cost of a solar system divided by its rated capacity in watts. The national average is $2.50-$3.50 per watt before incentives. After the 30% federal tax credit, effective cost is $1.75-$2.45 per watt. Lower cost per watt means better value.
Combiner Box: An electrical enclosure that combines the output of multiple solar panel strings into a single feed to the inverter. Contains fuses or circuit breakers for overcurrent protection. Typically located near the array on the roof or ground mount.
Charge Controller: A device in off-grid and battery-based solar systems that regulates the voltage and current from solar panels to the battery bank. Prevents overcharging and deep discharge. Two types: PWM (cheaper, less efficient) and MPPT (more expensive, 15-30% more efficient).
Clipping: Energy loss that occurs when solar panel output exceeds inverter capacity, causing the inverter to limit (clip) the power. Minor clipping during peak hours is acceptable and expected with DC-to-AC ratios above 1.0. Excessive clipping reduces system ROI.
Capacity Factor: The ratio of actual electricity produced over a period to the maximum possible if the system operated at full rated power 24/7. US residential solar capacity factors range from 14% (Washington) to 24% (Arizona). Higher capacity factors mean more energy per installed watt.

D

Degradation Rate: The rate at which solar panel output decreases over time. Most panels degrade 0.3-0.5% per year, meaning a panel produces about 87-92% of its original output after 25 years. Premium panels like SunPower Maxeon degrade at only 0.25%/year.
DC-to-AC Ratio: The ratio of total solar panel DC capacity to inverter AC capacity. A 1.2:1 ratio (e.g., 7.2 kW of panels with a 6 kW inverter) is common and intentional -- panels rarely produce rated power simultaneously, and slight oversizing maximizes energy harvest.
Duck Curve: A graph of electricity demand minus solar production throughout the day, shaped like a duck. As solar adoption grows, midday electricity demand from the grid drops dramatically, then ramps steeply in the evening when solar production drops and air conditioning runs. The duck curve drives TOU rate adoption and battery storage economics.

E

EPC: Engineering, Procurement, and Construction -- the full scope of work to design, source materials, and build a solar installation. EPC contractors handle the entire project from initial design through final inspection and Permission to Operate.

G

Grid-Tied System: A solar installation connected to the utility grid. Excess production is sent to the grid (earning credits via net metering), and grid power is available when solar production is insufficient. About 95% of residential solar installations are grid-tied.
Ground-Mount System: A solar installation mounted on frames anchored to the ground rather than on a rooftop. Ground-mount systems can be oriented at the optimal tilt and azimuth for maximum production. They cost 10-20% more than rooftop due to racking and foundation costs but are ideal when roof space is limited.

H

HJT (Heterojunction): Heterojunction Technology combines crystalline silicon with amorphous silicon thin-film layers. HJT cells have excellent temperature coefficients (less power loss in heat), low degradation, and 22-24% efficiency. Used by REC Alpha and Panasonic EverVolt.

I

Inverter: A device that converts the DC (direct current) electricity produced by solar panels into AC (alternating current) electricity used by your home appliances. The two main types are string inverters (one unit for the whole system) and microinverters (one per panel).
Interconnection Agreement: A formal agreement between a solar owner and the utility company that establishes the rules for connecting a solar system to the grid. Required before your system can be energized. Processing time ranges from 2 weeks to 6 months depending on the utility.
IBC (Interdigitated Back Contact): A solar cell design where all electrical contacts are on the rear of the cell, eliminating front grid lines that shade the surface. IBC cells achieve the highest residential efficiencies (22-25%). Used by SunPower Maxeon. More expensive to manufacture.
Interconnection Queue: The backlog of solar projects waiting for utility review and approval to connect to the grid. Interconnection queues in some utilities are 3-12 months long, delaying when your system can be turned on. This is the most common cause of delays in going solar.

K

Kilowatt (kW): A unit of electrical power equal to 1,000 watts. Residential solar systems are typically sized in kilowatts (e.g., a 7 kW system). One kilowatt of solar capacity produces about 1,200-1,800 kWh of electricity per year depending on location.
Kilowatt-Hour (kWh): A unit of electrical energy equal to one kilowatt of power sustained for one hour. Your electric bill is measured in kWh. The average US home uses about 900 kWh per month (10,800 kWh/year).

L

Load Profile: A graph or data showing your electricity consumption patterns throughout the day. Understanding your load profile helps size a solar system correctly. Homes that use most electricity during the day benefit more from solar without batteries.
Levelized Cost of Energy (LCOE): The total cost of a solar system over its lifetime divided by total energy produced, expressed in cents per kWh. Residential solar LCOE in the US is typically 5-10 cents/kWh, compared to 12-25 cents/kWh for utility electricity. Lower LCOE means better economics.

M

Microinverter: A small inverter attached to each individual solar panel that converts DC to AC at the panel level. Provides panel-level optimization (shade on one panel does not affect others), monitoring, and eliminates single-point-of-failure risk. Costs more but lasts 25 years.
Monocrystalline: A type of solar cell made from a single continuous crystal of silicon. Monocrystalline panels are the most efficient (20-23%) and most common type used in residential installations. Identifiable by their uniform dark appearance.
MPPT: Maximum Power Point Tracking -- an algorithm used in charge controllers and inverters that continuously adjusts voltage and current to extract the maximum possible power from solar panels under varying conditions (temperature, shading, cloud cover). MPPT delivers 15-30% more energy than PWM controllers.

N

Net Metering: A billing arrangement where excess solar electricity sent to the grid earns credits on your electric bill. When your panels produce more than you use, the meter effectively runs backward. Policies vary significantly by state and utility.
NABCEP: North American Board of Certified Energy Practitioners -- the gold standard certification for solar installers. NABCEP-certified installers have demonstrated knowledge of system design, installation, and code compliance. Always verify your installer holds NABCEP certification.

O

Off-Grid System: A solar installation completely independent of the utility grid. Requires substantial battery storage to power the home at night and during cloudy periods, plus a backup generator for extended low-sun periods. Costs 2-3x more than grid-tied systems.

P

Photovoltaic (PV): The technology that converts sunlight directly into electricity using semiconductor materials. Solar panels are made of photovoltaic cells, typically silicon-based, that generate DC electricity when photons strike the cell surface.
Peak Sun Hours: The number of hours per day when solar irradiance averages 1,000 watts per square meter. A location with 5 peak sun hours receives the equivalent of 5 hours of full sunshine. This metric is key for estimating solar production. Arizona averages 6.5 peak sun hours; Washington averages 3.5.
Permission to Operate (PTO): Final approval from the utility company allowing your solar system to generate electricity and connect to the grid. Issued after inspection and interconnection agreement are complete. Until PTO is granted, your system must remain off.
Power Optimizer: A DC-DC converter attached to each solar panel that optimizes the output of individual panels before sending power to a central string inverter. Made primarily by SolarEdge. Provides panel-level monitoring and shade mitigation at lower cost than microinverters.
Power Purchase Agreement (PPA): A solar financing arrangement where a company installs panels on your roof at no cost and you buy the electricity they produce at a fixed per-kWh rate, typically below your utility rate. Similar to a lease but charges are based on actual production rather than a fixed monthly fee.
Polycrystalline: A type of solar cell made from multiple silicon crystals melted together. Slightly less efficient than monocrystalline (16-18%) but historically cheaper. Identifiable by their blue speckled appearance. Losing market share to more efficient monocrystalline panels.
PERC: Passivated Emitter and Rear Cell -- a solar cell technology that adds a passivation layer to the rear surface, reflecting unabsorbed light back through the cell for a second pass. PERC cells achieve 20-22% efficiency and have been the industry standard since 2019.
Performance Ratio: The ratio of actual solar energy output to theoretical maximum output, expressed as a percentage. Typical residential systems achieve 75-85% performance ratio. Losses come from temperature, shading, soiling, wiring, inverter efficiency, and panel mismatch.

R

Racking/Mounting System: The hardware that secures solar panels to your roof or a ground structure. Roof-mounted racking uses rails and clamps attached through the roof decking. Ground-mount systems use concrete footings and metal frames. Mounting type affects both cost and aesthetics.
Rapid Shutdown: A NEC (National Electrical Code) requirement that solar panel systems must be able to quickly reduce voltage to safe levels for firefighter safety. Module-level electronics (microinverters or power optimizers) satisfy this requirement. String inverters require additional rapid shutdown devices.

S

String Inverter: A central inverter that converts DC to AC for an entire string of solar panels. More affordable than microinverters but the whole string is limited by the lowest-performing panel. Typical lifespan is 10-15 years.
Solar Investment Tax Credit (ITC): A federal tax credit that allows you to deduct 30% of the cost of installing a solar energy system from your federal taxes. The 30% rate is available through 2032, then steps down to 26% in 2033 and 22% in 2034.
SREC (Solar Renewable Energy Certificate): A tradeable certificate representing the environmental benefits of 1 megawatt-hour (MWh) of solar electricity. Some states require utilities to buy SRECs, creating an additional income stream for solar owners. SREC values range from $10 to $300+ depending on the state market.
Solar Panel Efficiency: The percentage of sunlight energy that a solar panel converts into electricity. Most residential panels are 19-23% efficient. Higher efficiency means more power per square foot, which matters when roof space is limited. SunPower Maxeon leads at 22.8%.
Solar Payback Period: The number of years it takes for cumulative solar savings to equal the initial system cost. National average is 7-12 years. States with high electricity rates and good incentives (CA, NY, MA) see payback in 5-8 years. Systems typically last 25-30+ years, providing 15-20 years of pure savings.
Solar Irradiance: The amount of solar radiation reaching a given area, measured in watts per square meter (W/m2). Standard Test Conditions (STC) use 1,000 W/m2 as the baseline for panel ratings. Actual irradiance varies by location, time of day, season, and weather.
Solar Lease: A financing arrangement where a company owns the solar panels on your roof and you pay a fixed monthly lease payment for the electricity they produce. Requires $0 down but you do not own the system, cannot claim the tax credit, and the lease complicates home sales.
Solar Canopy: An elevated solar panel structure that provides both power generation and covered parking or outdoor space. Common in commercial parking lots. Solar canopies cost 15-30% more than rooftop installations due to structural steel requirements but utilize otherwise wasted space.
Soft Costs: Non-hardware costs of a solar installation including permitting, inspection fees, customer acquisition, financing, overhead, and installer profit margin. Soft costs account for 30-50% of total residential solar system cost in the US and are a major reason US solar is more expensive than in other countries.
Solar Access Law: State and local laws that protect a homeowner right to install solar panels and access sunlight. Solar access laws prevent HOAs from banning solar panels and protect against neighbors shading your system with new construction. Most states have some form of solar access protection.
Solar Tracker: A motorized mounting system that rotates solar panels to follow the sun across the sky. Single-axis trackers increase production by 15-25% vs fixed-tilt. Dual-axis trackers add 30-40%. Primarily used in utility-scale and large commercial installations due to cost and maintenance requirements.
Snow Load: The weight of accumulated snow on solar panels, measured in pounds per square foot. Solar racking systems must be engineered to support local snow load requirements. Panels at 15+ degrees typically self-clear snow. Northern installations may need stronger racking rated for 30-60 psf snow loads.
Shade Analysis: A site assessment that maps shading patterns on your roof throughout the year. Tools like the Solmetric SunEye or Aurora Solar use satellite imagery and 3D modeling to predict how much energy your system will produce. Even partial shading can significantly reduce output, especially with string inverters.
Solar Renewable Energy Certificate (SREC): A tradeable certificate representing the environmental value of 1 MWh of solar electricity generated. Some states (NJ, MA, MD, PA, OH, IL) require utilities to purchase SRECs, creating a market where solar owners earn $10-$300+ per SREC depending on supply and demand in each state market.

T

Tilt Angle: The angle of solar panels relative to horizontal ground. Optimal tilt approximately equals your latitude (e.g., 30 degrees in Houston, 40 degrees in New York). Most residential panels follow roof pitch. Adjustable tilt racks on ground mounts can increase production by 5-15%.
Time-of-Use (TOU) Rates: An electricity pricing structure where the cost per kWh varies by time of day. Peak hours (afternoon/evening) cost more; off-peak hours (night) cost less. Solar owners on TOU plans benefit from battery storage to shift solar production to high-rate periods.
TOPCon: Tunnel Oxide Passivated Contact -- an advanced solar cell technology that adds a thin oxide layer and doped polysilicon to the rear of the cell. TOPCon cells achieve 22-25% efficiency, higher than standard PERC cells. Used by Trina, JinkoSolar, LONGi, and Silfab.

U

Utility-Scale Solar: Large solar power plants (typically 1 MW to 500+ MW) that sell electricity directly to the utility grid. Utility-scale solar costs $0.70-$1.20/watt -- far less than residential -- due to economies of scale. These projects power hundreds of thousands of homes.

V

Virtual Net Metering: A billing arrangement that allows solar credits from one location to be applied to electric bills at a different location. Used in community solar programs where subscribers receive credits on their utility bills from a shared solar farm without installing panels on their own property.