Solar Terms Glossary

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Solar can feel harder than it needs to be because the language gets technical fast. A sales quote might talk about kWp, DoD, STC, cycle life, hybrid inverter, and payback period in the same conversation, even though those terms belong to completely different parts of the system.

This glossary is here to slow that down.

It is not meant to be a dictionary of every term ever used in the industry. It is a practical lookup page for the terms you are most likely to see when researching, comparing quotes, or trying to understand how a solar system actually works.

Simple visual guide to kW, kWh, and kWp

The Fastest Distinction to Learn

If you are brand new, start here.

Term	Plain-English meaning	Why it matters
kW	A unit of power, meaning how fast energy is being used or produced right now	Describes system size or appliance demand
kWh	A unit of energy, meaning how much electricity was used or produced over time	Shows electricity bills and daily energy use
kWp	Kilowatt-peak, the rated maximum output of a PV system under ideal test conditions	Used to compare solar array nameplate size

This is the mistake people make all the time.

kW is like speed.

kWh is like distance traveled.

If you get that distinction early, a lot of solar documents become much easier to read.

Core Solar Terms

These are the foundation terms that show up almost everywhere.

Term	Definition
Solar energy	Energy from the sun that can be converted into electricity, heat, or useful building energy
Solar PV	Solar photovoltaic technology that converts sunlight directly into electricity
Solar thermal	Solar technology that captures sunlight as heat rather than electricity
Photovoltaic effect	The process by which certain materials generate electricity when exposed to light
PV cell	The individual semiconductor device that converts light into electricity
Module	A weather-sealed group of solar cells wired together, often casually called a panel
Panel	In everyday use, usually the same as module, though some technical sources distinguish a panel as a collection of modules
Array	The complete group of solar modules installed as one power-generating system
Irradiance	The power of sunlight falling on a surface, usually measured in watts per square meter

If you want the big-picture introduction before drilling into terms, What Is Solar Energy is the best place to start.

Electricity and System Hardware Terms

These terms describe how solar electricity moves through a real system.

Term	Definition
DC, direct current	Electricity that flows in one direction, the type produced by solar panels and stored in batteries
AC, alternating current	Electricity that changes direction periodically, the type used by most homes and grid networks
Inverter	Equipment that converts DC electricity from solar panels or batteries into AC electricity for loads or grid export
Hybrid inverter	An inverter that manages solar panels, batteries, loads, and the grid in one integrated system
Charge controller	A device that regulates power from solar panels into batteries to avoid overcharging and battery damage
String	A set of solar modules wired in series
Grid-tied system	A solar system connected to the utility grid, usually focused on reducing electricity bills
Off-grid system	A stand-alone solar system that operates without a utility connection and requires battery storage
Hybrid system	A solar system that combines solar panels, batteries, and a grid connection
Critical loads	The circuits or appliances chosen to stay powered during an outage, such as lighting, refrigeration, or communications equipment

If those system types still blur together, Types of Solar Systems and On-Grid vs Off-Grid Systems unpack them in more detail.

Example of a solar inverter used in a real installation — If the word inverter still feels abstract, this is the kind of wall-mounted equipment the term usually refers to. Image via Wikimedia Commons, CC0.

Power, Energy, and Rating Terms

This is the category where many quotes become confusing. These are the numbers that describe system size, production, and test conditions.

Term	Definition
Watt, W	A basic unit of power
Kilowatt, kW	1,000 watts, commonly used to describe solar system size or appliance demand
Megawatt, MW	1,000 kilowatts, often used for utility-scale projects
Watt-hour, Wh	A unit of energy equal to one watt used or produced for one hour
Kilowatt-hour, kWh	1,000 watt-hours, the unit most electricity bills use
Kilowatt-peak, kWp	The rated maximum output of a PV system under standard test conditions
STC, Standard Test Conditions	The lab reference conditions used to rate solar panels, typically 1,000 W per square meter of irradiance, 25 degrees Celsius cell temperature, and air mass 1.5
Nameplate capacity	The rated output listed on the equipment label under test conditions
Solar irradiance	The intensity of sunlight available to the panel surface
Efficiency	The percentage of incoming sunlight converted into usable electricity

One important reality check here. STC ratings are useful for comparison, but real-world panel output is often lower because roof temperature, angle, dirt, wiring losses, and shading are rarely ideal.

If you want the full breakdown of that topic, Panel Efficiency Explained is the next step.

Battery and Storage Terms

These terms matter most in off-grid or backup-enabled systems.

Term	Definition
Battery bank	One or more batteries connected together to store energy
Storage capacity	The amount of energy a battery can store, usually expressed in kWh
DoD, Depth of Discharge	The percentage of a battery’s total capacity that has been used
SoC, State of Charge	The percentage of energy remaining in a battery at a given moment
Cycle life	The number of charge and discharge cycles a battery can deliver before its capacity falls below a specified level
Deep-cycle battery	A battery designed to be discharged and recharged repeatedly, unlike a starter battery that mainly delivers short bursts of high current
Autonomy	The number of hours or days a battery-backed system can run without fresh solar input or grid support
Usable capacity	The amount of battery energy that can actually be used after accounting for discharge limits and protection margins

These terms tend to be discussed together because they are connected. A battery may have a large total capacity, but usable capacity depends on the allowed DoD, and long-term value depends on cycle life.

If that is the part you are sizing or comparing, Depth of Discharge Explained and Battery Sizing go deeper.

Performance and Operating Terms

These terms show up when people move from basic definitions into how well a system performs in real life.

Term	Definition
Yield	The amount of energy a solar system actually produces over a period of time
Performance ratio	A measure of how efficiently a PV system performs relative to its theoretical output under site conditions
Degradation	The gradual decline in panel output over time
Shading loss	Energy lost when part of a panel or array is shaded
Temperature coefficient	A value showing how panel output changes as cell temperature rises or falls
Self-consumption	The share of solar electricity used directly on-site rather than exported
Export	Electricity sent from the solar system to the utility grid

This category is where nameplate size meets reality. Two systems with the same kWp may not produce the same annual yield if one site runs hotter, has more shade, or faces a weaker roof orientation.

Financial Terms

Solar is not only a technical project. It is also an economic one, which is why these terms appear in proposals, calculators, and ROI discussions.

Term	Definition
CapEx	Capital expenditure, meaning the upfront cost of equipment and installation
Payback period	The time it takes for energy savings and incentives to recover the net upfront cost of the system
ROI, Return on Investment	A measure of how much financial return the system generates relative to what was spent
LCOE, Levelized Cost of Energy	The lifetime average cost of generating one unit of electricity from a system, often expressed as cost per kWh
Net metering	A billing mechanism that credits exported solar electricity against electricity imported from the grid, depending on local utility rules
Incentive	A rebate, tax credit, grant, or tariff support mechanism that reduces project cost or increases project value
Self-consumption savings	Bill reduction achieved by using your own solar electricity instead of buying it from the grid

One quick caution here. Payback period is easy to understand, which is why people love it, but it is not the same as ROI and it does not capture everything. Two systems can have similar payback periods and still have very different long-term economics depending on battery replacement, financing, electricity price escalation, and export policy.

Terms People Commonly Mix Up

This is the cheat sheet for the terms that trip people most often.

Term pair	The real difference
kW vs kWh	`kW` is power right now, `kWh` is energy over time
Module vs panel	In everyday use they usually mean the same thing, but technical literature may define a panel as an assembly of modules
DC vs AC	Solar panels and batteries operate in DC, while most homes and grids use AC
DoD vs SoC	`DoD` tells you how much of a battery has been used, `SoC` tells you how much remains
kWp vs real output	`kWp` is the rated peak output under STC, not the guaranteed everyday field output
Grid-tied vs hybrid	Grid-tied focuses on solar plus utility connection, while hybrid adds battery interaction and backup capability

How to Read Solar Language More Comfortably

If you are reading a quote, datasheet, or buying guide and the terms start stacking up, this order usually helps.

Figure out whether the number is about power, energy, storage, or money
Check whether the value is a lab rating like STC or a real-world estimate
Ask whether the term describes the panel, the battery, the inverter, or the bill

That sounds almost too simple, but it clears up a lot of confusion fast.

Good Next Reads From This Glossary

Panel Efficiency Explained Best next read if you keep seeing STC, efficiency, and output ratings in panel datasheets.

Depth of Discharge Explained Best if battery terms like DoD, SoC, and usable capacity are the main source of confusion.

Solar Payback Period Useful if the terms confusing you are financial rather than technical.

How Solar Panels Work A good next step if you want the glossary terms to connect back to the full PV process.

Key Takeaways

kW, kWh, and kWp are not interchangeable, and knowing the difference makes solar quotes much easier to understand.
Battery terms like DoD, SoC, and cycle life matter most in storage, off-grid, and backup applications.
Financial terms such as payback period, ROI, and LCOE answer different questions and should not be treated as the same metric.
STC ratings are useful for comparing equipment, but real-world output is usually lower than the lab rating.
The fastest way to get comfortable with solar language is to sort each term into one of four buckets, electricity, storage, performance, or finance.

Sources Used for This Page

This page was expanded using the research notes and source list provided for this project, especially the following references.