Solar Glossary

AC Power: see Alternating Current

Alternating Current

Alternating current (AC) is electric current that reverses direction many times a second; in the US it completes 60 cycles per second (60 Hz).

Ampere: see Current

Avoided Cost

Avoided cost is the cost a utility would have paid to generate or buy electricity itself, and does not pay because it buys the power from a small producer instead.

Ballasted Solar Mount

A ballasted solar mount is a racking system for flat or low-slope roofs that holds solar panels in place with weight, usually concrete blocks in trays, instead of bolting through the roof.

Battery Backup

Battery backup is a home battery's ability to keep power flowing to some or all of your circuits when the utility grid goes down.

Battery Backup Duration

Battery backup duration is how long a charged home battery can keep your chosen loads running when the grid is down.

Battery Capacity

Battery capacity is the amount of energy a battery can store, given for home batteries in kilowatt-hours (kWh).

Battery Demand Management: see Peak Shaving

Battery Energy Storage System

A battery energy storage system (BESS) is a complete installation that stores electricity in batteries and releases it on demand: the battery cells plus the inverter, battery management system, thermal controls and safety equipment that make them usable on a home or the grid.

Battery Management System

A battery management system (BMS) is the electronics and software inside a battery pack that measure each cell's voltage, current and temperature, estimate how full and how healthy the battery is, keep the cells balanced, and disconnect the battery before any limit is exceeded.

Battery Power Rating

A battery power rating is the most electricity a battery can deliver at one moment, measured in kilowatts (kW) or kilovolt-amperes (kVA).

Battery Round-Trip Efficiency

Battery round-trip efficiency is the percentage of the electricity put into a battery that you can take back out.

Battery State of Charge

Battery state of charge (SoC) is how full a battery is right now, shown as a percent of the energy it can hold.

Battery Storage: see Solar Battery

BESS: see Battery Energy Storage System

Bifacial Solar Panel

A bifacial solar panel produces electricity from light hitting both its front and its back, using cells that are active on both sides behind a glass or clear rear cover.

Central Inverter

A central inverter is one large inverter that turns the direct current (DC) from hundreds or thousands of solar panels into alternating current (AC) for the grid, all inside one cabinet.

Cost per Watt

A system's installed price divided by its DC size in watts; the standard way to compare solar quotes of different sizes.

DC Power: see Direct Current

DC-to-AC Ratio

The DC-to-AC ratio, also called the inverter loading ratio (ILR), is a solar array’s total panel rating in DC watts divided by the inverter’s AC output rating.

Demand Charge

A demand charge is the part of an electric bill priced on your highest rate of power use, measured in kilowatts, rather than on the total energy you used in kilowatt-hours.

Depth of Discharge

Depth of discharge (DoD) is the share of a battery's capacity that has been drawn out, as a percentage: a full battery is at 0% DoD and an empty one at 100%.

Direct Current

Direct current (DC) is electric current that flows in one direction only, at a steady polarity, the kind produced by solar cells and stored in batteries.

Disconnect Switch

A solar disconnect switch is a manually operated switch or circuit breaker that separates a solar power system, or one piece of its equipment, from every source of power feeding it, so the system can be safely isolated by utility crews, inspectors, firefighters or electricians.

Distributed Energy Resource

A distributed energy resource (DER) is a small power source or storage device connected to the local distribution grid or installed behind a customer’s meter, such as rooftop solar, a home battery, an EV charger or a smart thermostat in a demand-response programme.

Distributed Generation

Distributed generation (DG) is electricity made by small power sources close to where it is used, such as rooftop solar on a home, instead of by a large power plant far away on the transmission grid.

Electric Current

Electric current is the rate at which electric charge flows through a conductor, measured in amperes (amps, A).

Electric Grid

The electric grid is the connected network of power plants, high-voltage transmission lines, substations, transformers and local distribution lines that carries electricity from where it is generated to the homes and businesses that use it.

Electrical Load

An electrical load is any device or circuit that draws electric power, from a light bulb to a heat pump; a home's load is the total of them at any moment.

Electrical Panel

An electrical panel, also called the main service panel, breaker box or load center, is the metal cabinet where utility power enters a home and is split into branch circuits, each protected by a circuit breaker.

Electricity Rate

An electricity rate is the approved price schedule your utility uses to bill you: a fixed monthly charge plus a price for each kilowatt-hour you use, and on some schedules a charge for your peak demand.

Energy Arbitrage

Energy arbitrage is storing electricity when it is cheap and using or selling it when it is expensive, earning the price difference minus the energy lost in storage.

Excess Solar Generation: see Export Compensation

Export Compensation

Export compensation is the credit or payment a utility gives for electricity a solar system sends to the grid, the excess generation the home does not use at that moment.

Grid Connection: see Grid-Tied Solar

Grid Outage

A grid outage is a loss of electricity from the utility grid to homes and businesses, from a brief flicker to days without power.

Grid Resilience

Grid resilience is the power system’s ability to withstand a major shock, such as a hurricane, ice storm or cyber attack, keep critical service going, and recover quickly afterwards.

Grid-Tied Inverter

A grid-tied inverter is an inverter built to run in step with the utility grid.

Grid-Tied Solar

Grid-tied solar is a solar system connected to the utility grid through your home’s electrical panel and meter, so the house uses solar power first, draws from the grid when solar falls short, and can send surplus back to the utility.

Ground-Mounted Solar

Solar panels set on a frame anchored in the ground rather than on a roof, used where the roof is shaded, small or facing the wrong way.

HJT Solar Panel

An HJT solar panel (heterojunction) is a panel built from cells made of an n-type crystalline silicon wafer coated on both faces with ultra-thin layers of amorphous silicon, joining two different forms of silicon in one cell.

Hybrid Inverter

A hybrid inverter is a single inverter that takes DC from solar panels, charges and discharges a home battery, and supplies AC to the house and grid, so solar and storage share one box instead of two.

Hybrid Solar System

A hybrid solar system is a grid-connected solar system with a battery, able to store surplus solar, use it later, export to the grid when allowed, and keep chosen circuits running when the grid fails.

IBC Solar Panel

An IBC solar panel (interdigitated back contact) is a panel built from cells whose positive and negative contacts are both on the back, laid out in interlocking fingers, so no metal gridlines shade the front of the cell.

Interconnection Agreement

The contract between a customer and the utility that sets the terms for connecting and running a solar system, including insurance, disconnection rights and the export tariff.

Interconnection Application

The request, usually filed by the installer, asking the utility to approve a system's connection; it lists the equipment, size and single-line diagram.

Interconnection Study

A deeper engineering review the utility runs when a system fails the quick screen, usually only for large systems or crowded circuits.

Inverter Clipping

Inverter clipping is the energy a solar system loses when the panels could produce more DC power than the inverter’s maximum AC rating, so the inverter caps its output at that limit.

Inverter Efficiency

Inverter efficiency is the percentage of the DC power entering a solar inverter that leaves it as usable AC power; the rest becomes heat.

Inverter Sizing

Inverter sizing is the choice of how many kilowatts of AC output a solar inverter should have, based on the panel array, your utility’s size limits and, if you add a battery, the loads you want to run in an outage.

Kilowatt

A kilowatt (kW) is a unit of power equal to 1,000 watts: it measures how fast electricity is being produced or used at a given moment.

Kilowatt-Hour

A kilowatt-hour (kWh) is a unit of energy equal to one kilowatt of power used or produced for one hour, or 1,000 watt-hours.

Levelized Cost of Energy

The levelized cost of energy (LCOE) is the average cost of each kilowatt-hour a power source produces over its life: everything it costs to build, finance, run and maintain, divided by all the electricity it generates, with both usually discounted to today's dollars.

LFP Battery

An LFP battery is a type of lithium-ion battery whose cathode is made of lithium iron phosphate (LiFePO4).

Lithium-Ion Battery

A lithium-ion battery is a rechargeable battery that stores energy by moving lithium ions from the cathode to the anode when charging and back again when discharging.

Main Service Panel: see Electrical Panel

Megawatt

A megawatt (MW) is a unit of power equal to 1,000 kilowatts or one million watts.

Megawatt-Hour

A megawatt-hour (MWh) is a unit of energy equal to 1,000 kilowatt-hours.

Microinverter

A microinverter is a small inverter mounted behind a single solar panel that converts that panel’s DC output to grid-ready AC on the roof, so each panel works independently of the others.

Monocrystalline Solar Panel

A monocrystalline solar panel is built from cells sliced out of a single, continuous silicon crystal, which lets electrons move with fewer obstacles than in silicon made of many fused crystals.

Monofacial Solar Panel: see Bifacial Solar Panel

MPPT

MPPT, or maximum power point tracking, is the control method inverters and charge controllers use to keep solar panels at the voltage and current combination where they produce the most power, adjusting it continuously as sunlight and temperature change.

Net Billing

Net billing is a solar compensation method in which the utility bills every kWh you import at its retail rate and credits every kWh you export at a separate, usually lower, export rate, with the two settled in dollars rather than netted in kilowatt-hours.

Net Metering

Net metering is a utility billing arrangement in which the electricity your solar system sends to the grid is subtracted from the electricity you take from it, so you are billed only for the net amount over a set period.

Net Metering Credits: see Net Metering

Off-Grid Solar

A solar system with no utility connection, which must store enough energy in batteries (often with a generator) to carry the home through nights and cloudy stretches.

Peak Shaving

Peak shaving is discharging a battery, or cutting load, during the moments a building draws the most power from the grid, so the highest kilowatt reading in the billing period comes out lower.

Peak Sun Hours

Peak sun hours are the number of hours a day that sunlight would need to shine at 1,000 watts per square metre to deliver the same energy the site actually receives.

PERC Solar Panel

A PERC solar panel is a panel built from passivated emitter and rear cells: standard p-type silicon cells with an extra insulating layer on the back that bounces unused light back through the cell and stops electrons being lost at the rear surface.

Permission to Operate

The utility's written approval to switch a new solar system on and start exporting. Running the system before it arrives can breach the interconnection agreement.

Photovoltaic

Photovoltaic (PV) refers to any material or device that converts light directly into electricity, with no moving parts and no heat engine in between.

Photovoltaic Cell

A photovoltaic cell, or solar cell, is a thin piece of semiconductor, usually silicon, that produces direct-current electricity when light falls on it.

Polycrystalline Solar Panel

A polycrystalline solar panel (also called multicrystalline) is a panel built from cells cut from a block of silicon that cooled into many small crystals, which gives the cells their blue, speckled look and makes them a little less efficient than single-crystal cells.

Power Factor

Power factor is the ratio of real power, measured in watts, to apparent power, measured in volt-amperes, on an AC circuit.

Power Optimizer

A power optimizer is a small DC-to-DC converter fitted behind each solar panel that tracks that panel’s maximum power point and adjusts its voltage, then sends DC on to a central string inverter for conversion to AC.

PTO Solar

Short for permission to operate.

Rapid Shutdown

Rapid shutdown is a safety function required by the National Electrical Code (NEC 690.12) that drops the voltage on a building-mounted solar array's wiring to a controlled level within 30 seconds of someone starting shutdown, so firefighters are not exposed to live DC wires on the roof.

Roof Mount: see Solar Mounting System

Roof Penetration

A roof penetration is any hole made through the roof covering, and on a solar install it usually means each point where a mount is bolted through the shingles into a rafter to hold the racking.

Roof Solar Potential

Roof solar potential is an estimate of how much solar electricity a roof could produce in a year, based on its usable area, direction, tilt, shade and local sunlight, before money, wiring or utility rules are taken into account.

Rooftop Solar

Solar panels mounted on a building's roof, the most common way homes install solar because the roof is space the owner already has.

Solar Access

Solar access is the ability to install a solar energy system and keep sunlight reaching it.

Solar Array

A solar array is the complete set of solar panels in one installation, wired together so their combined direct-current output feeds the inverter as a single source.

Solar Azimuth

Solar azimuth is a direction measured in degrees clockwise from true north.

Solar Battery

A solar battery is a rechargeable battery, usually lithium-ion, that stores electricity your solar panels make but your home does not use straight away, so you can use it after dark, during a power cut, or when grid power costs most.

Solar Battery Backup: see Battery Backup

Solar Billing

How a utility bills a home with solar: the energy you import, the energy you export, the credit for exports and the fixed charges that solar cannot offset.

Solar Break-Even Point: see Solar Payback Period

Solar Buyback Rate

A solar buyback rate is the price, in cents or dollars per kilowatt-hour, that a utility pays or credits for surplus electricity a customer’s solar system sends to the grid under a program where exports are bought rather than netted at the full retail price.

Solar Cable

Solar cable is single-conductor wire built to carry DC power from solar panels while sitting outdoors in sun, heat and rain for decades.

Solar Capacity Factor

Solar capacity factor is the ratio of the electricity a solar system actually produces over a period to what it would produce running at full rated power every hour of that period.

Solar Carport

A parking canopy whose roof is a solar array, used where a building roof cannot carry or fit the system.

Solar Charge Controller

A solar charge controller is a device between solar panels and a battery bank that regulates the voltage and current going into the batteries, so they charge fully without being overcharged.

Solar Combiner Box

A solar combiner box is a weatherproof enclosure where several strings of solar panels are joined into one larger circuit, usually with a fuse on each string, before the power travels on to the inverter.

Solar Cost

The installed price of a solar system: equipment, labour, permits, interconnection and the installer's overhead and margin, before and after any incentive.

Solar Design

Solar design is the engineering step that turns a site survey into a specific system: which panels go where, how they are wired and attached, and how they connect to the home's electrical service, documented in a plan set submitted for a permit.

Solar Energy

Solar energy is the light and heat the sun gives off.

Solar Export

Solar export is the electricity a home solar system sends out to the utility grid because the house is not using all of it at that moment; it is measured by a two-way meter as energy “received” by the utility.

Solar Financing

The ways to pay for solar: cash, a loan, a lease or a power purchase agreement. Each changes who owns the system, who claims any credit and what you pay over time.

Solar Insolation

Solar insolation is the total amount of sunlight energy that lands on a surface over a period of time, usually given in kilowatt-hours per square metre per day (kWh/m²/day), and it sets how much electricity solar panels in that place can make.

Solar Inspection

A solar inspection is the check of a finished solar installation against the approved permit and the adopted codes, carried out by the local building department's inspector.

Solar Interconnection

The utility's review and approval of a solar system's connection to the grid: an application, a technical screen, an interconnection agreement and, at the end, permission to operate.

Solar Inverter

A solar inverter is the device that converts the direct current (DC) made by solar panels into the alternating current (AC) your home and the utility grid run on.

Solar Irradiance

Solar irradiance is the power of sunlight striking a surface at a given moment, measured in watts per square metre (W/m²); insolation is that irradiance added up over time, usually in kilowatt-hours per square metre per day.

Solar Junction Box

A solar junction box is the small sealed enclosure on the back of a solar panel where the cell circuits meet the output cables.

Solar Lifetime Value: see Solar Return on Investment

Solar Loan

A solar loan is money borrowed to buy a solar energy system that you then own, repaid in monthly installments with interest.

Solar Module: see Solar Panel

Solar Mounting Bracket: see Solar Mounting System

Solar Mounting System

A solar mounting system, also called racking, is the set of attachments, rails, clamps and bonding hardware that secures solar panels to a roof or the ground and holds them at a fixed angle against wind and snow.

Solar Panel

A solar panel, called a solar module in the industry, is a weatherproof assembly of photovoltaic cells wired together behind glass in a frame, which turns sunlight into direct-current electricity.

Solar Panel Degradation

The gradual loss of a panel's output over its life, expressed as a percent per year and guaranteed by the manufacturer's performance warranty.

Solar Panel Efficiency

The share of the sunlight striking a panel that it converts to electricity, measured at standard test conditions. Higher efficiency means more watts from the same roof area, not cheaper power per se.

Solar Panel Orientation

Solar panel orientation is the compass direction a panel faces, given as an azimuth angle in degrees clockwise from true north: 90° is east, 180° is south and 270° is west.

Solar Panel Power Output: see Solar Panel Wattage

Solar Panel Temperature Coefficient

A solar panel temperature coefficient (of Pmax) is the percentage of rated power it loses for every degree Celsius its cells run above 25°C, the temperature at which the wattage on the label was measured.

Solar Panel Tilt

Solar panel tilt is the angle between a panel and the horizontal ground, in degrees: 0° is lying flat and 90° is vertical.

Solar Panel Wattage

Solar panel wattage is the maximum power a panel produces under Standard Test Conditions: 1,000 watts of sunlight per square metre, a 25°C cell and a fixed light spectrum.

Solar Payback Period

The solar payback period is the number of years it takes for the money a solar system saves on electricity, plus any annual incentives, to add up to what the system cost after upfront incentives.

Solar Permit

A solar permit is the approval a local building department, the authority having jurisdiction, issues before a solar system can be installed.

Solar Permitting: see Solar Permit

Solar Power: see Solar Energy

Solar Production

Solar production is the amount of electricity a solar system generates over a period, measured in kilowatt-hours (kWh) and usually quoted per year.

Solar PV: see Photovoltaic

Solar Racking: see Solar Mounting System

Solar Rail: see Solar Mounting System

Solar Return on Investment

Solar return on investment (ROI) is the total a solar system saves over its working life, minus what it cost, expressed as a percentage of that cost.

Solar Savings

The difference between what you would have paid the utility without solar and what you pay with it, after export credits and fixed charges.

Solar Shading

Solar shading is any shadow that blocks sunlight from part or all of a solar array, from trees, chimneys, vents, nearby buildings or other panels, reducing the electricity the system produces.

Solar Shingles

Solar shingles are roofing products with solar cells built in, installed as part of the roof covering itself instead of mounted on racks above it.

Solar Site Assessment

A solar site assessment is an inspection of a property to decide whether, where and how solar panels can be installed, covering sunlight, shade, roof direction and angle, roof condition and structure, and the home's electrical service.

Solar Site Survey: see Solar Site Assessment

Solar Structural Load

The added dead load of panels and racking plus the wind and snow forces they transfer into the roof framing, which the designer must show the roof can carry.

Solar System

A home solar system is the full set of equipment that turns sunlight into electricity your house can use: solar panels, an inverter, mounting hardware, wiring and safety disconnects, and the utility meter, sometimes with a battery.

Solar System Sizing

Solar system sizing is the process of choosing how many kilowatts (kW) of solar panels a property needs, by dividing the electricity it uses in a year by what one kW of panels produces there.

Solar Wiring

Solar wiring is the full set of conductors, connectors, conduit, grounding and labels that carry power from solar panels to the inverter, the electrical panel and the grid.

String Inverter

A string inverter is a single wall-mounted inverter that converts the DC output of one or more strings of series-wired solar panels into AC for the home.

Thin-Film Solar Panel

A thin-film solar panel is a panel that makes electricity from a light-absorbing layer only a few microns thick, deposited onto glass, metal or plastic, instead of from sliced silicon wafers.

Time-of-Use Rate

An electricity rate that charges different prices at different hours, usually highest in the late afternoon and evening. It changes what a kilowatt-hour of solar or battery output is worth.

TOPCon Solar Panel

A TOPCon solar panel is a panel built from tunnel oxide passivated contact cells, n-type silicon cells whose rear contact sits on an ultra-thin oxide layer and a doped silicon film, which lets current out while blocking most of the losses at the metal contact.

Usable Battery Capacity

Usable battery capacity is the amount of stored energy, in kilowatt-hours (kWh), that a home battery will actually deliver to your house in normal use, after the maker’s built-in protective limits.

Utility

An electric utility is the organisation that delivers electricity to your home and bills you for it.

Utility Approval

Generic wording for the utility's interconnection decision; the formal step is permission to operate.

Utility Bill Credit: see Net Metering

Utility Rate: see Electricity Rate

Voltage

Voltage is the electrical pressure that pushes current through a circuit, measured in volts (V); one volt equals one watt per ampere.

Watt

A watt (W) is the SI unit of power, equal to one joule of energy transferred every second.

Watt-Hour: see Kilowatt-Hour

Whole-Home Battery Backup

Whole-home battery backup means the battery system is wired to back up your main electrical panel, so every circuit can stay live in an outage.