Answer
Solar acronyms are short names for sunlight data, equipment, output and finance terms. GHI, DNI and POA measure sunlight. MPPT and BESS describe equipment. PR and P90 describe output. LCOE, NPV and IRR describe money. This glossary defines 83 of them from A to Z in plain words.
Last updated: November 2026
Solar datasheets, simulation reports and permit packs run on shorthand. This glossary lists 83 acronyms in alphabetical order, each with its full name, a group, a plain meaning and a link to a longer glossary page where one exists.
SurgePV publishes this page and sells solar design software. PVsyst and SAM are other tools named in the list.
Find your term below. Use the letters to jump to a section.
A · B · C · D · E · F · G · I · K · L · M · N · O · P · R · S · T · V · W · Y
Turn these terms into a project design
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Solar Acronyms A to Z
The 83 terms below fall into 11 groups. Electrical and finance terms are the largest groups.
- Electrical13
- Finance11
- Sunlight9
- Batteries8
- Projects8
- Panels8
- Inverters6
- Software6
- Performance6
- Grid5
- Compliance3
Source: count of the entries in the A to Z tables on this page, checked November 2026.
Terms Starting with A
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| AC | Alternating Current | Electrical | Power that changes direction many times a second. The grid and most home appliances use it. | AC page |
| AFCI | Arc-Fault Circuit Interrupter | Inverters | A device that shuts off a circuit when it detects a dangerous electrical arc. | AFCI page |
| AHJ | Authority Having Jurisdiction | Compliance | The local office or person who approves your permit and inspects your work, such as a building department. | AHJ page |
| AM | Air Mass | Sunlight | How much air the sunlight passes through. AM1.5 is the standard value used for panel ratings. | |
| Anti-islanding | Anti-islanding | Inverters | A safety rule that makes an inverter stop sending power when the grid goes down. | Anti-islanding page |
| AOI | Angle of Incidence | Sunlight | The angle between the sun’s rays and a line straight out from the panel surface. | AOI page |
Terms Starting with B
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| BESS | Battery Energy Storage System | Batteries | A full battery installation: cells, battery management system, inverter and safety equipment. | BESS page |
| BIM | Building Information Modeling | Software | A digital 3D model of a building that holds data about its parts. | |
| BIPV | Building-Integrated Photovoltaics | Projects | Solar cells built into the building itself, such as roof tiles or glass, instead of added on top. | BIPV page |
| BMS | Battery Management System | Batteries | The electronics that watch and protect a battery. It tracks charge, temperature and safe limits. | BMS page |
| BOM | Bill of Materials | Projects | A list of every part needed for a system, with quantities. | BOM page |
| BoS | Balance of System | Projects | Everything in a solar system except the panels: wiring, mounts, inverters and other parts. | BoS page |
| BTM | Behind the Meter | Grid | Equipment on the customer’s side of the utility meter, such as rooftop solar. | BTM page |
Terms Starting with C
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| CAD | Computer-Aided Design | Software | Software for drawing plans and layouts. | |
| COD | Commercial Operation Date | Projects | The date a power plant starts to run as a working, selling asset. | |
| C-rate | Charge rate | Batteries | How fast a battery charges or discharges compared with its size. 1C means a full charge or discharge in one hour. | C-rate page |
| CUF | Capacity Utilization Factor | Performance | Energy made in a year divided by the energy the plant would make at full rated power all year. Check whether a contract uses AC or DC capacity. | CUF page |
Terms Starting with D
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| DC | Direct Current | Electrical | Power that flows in one direction. Solar panels and batteries make and store DC. | DC page |
| DC/AC ratio (ILR) | Inverter Loading Ratio | Electrical | Panel DC size divided by inverter AC size. A ratio of 1.2 means the panels are 20% larger than the inverter. | DC/AC ratio (ILR) page |
| DER | Distributed Energy Resource | Grid | A small power source or storage unit connected near where power is used, such as rooftop solar or a home battery. | DER page |
| DHI | Diffuse Horizontal Irradiance | Sunlight | Sunlight scattered by the sky and clouds, measured on a level surface. | DHI page |
| DNI | Direct Normal Irradiance | Sunlight | Sunlight that comes straight from the sun, measured on a surface that faces the sun. | DNI page |
| DoD | Depth of Discharge | Batteries | The share of a battery’s capacity that you have used. 80% DoD means 80% is used and 20% is left. | DoD page |
Terms Starting with E
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| EPC | Engineering, Procurement and Construction | Projects | A company that designs, buys equipment for and builds a project. | EPC page |
Terms Starting with F
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| FIT | Feed-in Tariff | Finance | A fixed price paid for each unit of power you send to the grid, set for a set period. | FIT page |
| FTM | Front of the Meter | Grid | Equipment on the utility side of the meter, such as a solar farm that sells to the grid. | FTM page |
Terms Starting with G
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| GFDI | Ground-Fault Detection and Interruption | Inverters | A safety device that finds a ground fault and stops the current. | |
| GHI | Global Horizontal Irradiance | Sunlight | All the sunlight that lands on a flat, level surface. It includes direct and scattered light. | GHI page |
| GIS | Geographic Information System | Software | Software for maps and location data, used to study sites. | GIS page |
Terms Starting with I
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| IEEE 1547 | IEEE Standard 1547 | Grid | The IEEE standard for connecting distributed energy resources, such as solar and batteries, to the utility grid. | IEEE 1547 page |
| Imp | Current at Maximum Power | Panels | The current a panel makes when it is delivering its maximum power. | |
| IRR | Internal Rate of Return | Finance | The yearly return rate that makes a project’s NPV equal zero. | |
| Isc | Short-Circuit Current | Panels | The highest current a panel can make when its two terminals are joined together. | Isc page |
| ITC | Investment Tax Credit | Finance | A US federal tax credit for energy property. The IRS says the investment credit includes an energy credit, claimed on Form 3468. Rates and rules depend on current law. | ITC page |
Terms Starting with K
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| kV | Kilovolt | Electrical | 1,000 volts. A unit of electrical pressure. | kV page |
| kVA | Kilovolt-ampere | Electrical | A unit of apparent power: volts times amps, divided by 1,000. | |
| kW | Kilowatt | Electrical | 1,000 watts. A unit of power, which is how fast energy is used or made. | kW page |
| kWh | Kilowatt-hour | Electrical | The energy used or made by 1 kW running for 1 hour. Your power bill uses this unit. | kWh page |
| kWp | Kilowatt-peak | Electrical | The DC power rating of a solar array under standard test conditions. | kWp page |
Terms Starting with L
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| LCOE | Levelized Cost of Energy | Finance | The average cost of each unit of energy over a system’s life, found by dividing lifetime cost by lifetime energy. | LCOE page |
| LFP | Lithium Iron Phosphate | Batteries | A lithium-ion battery chemistry that uses iron phosphate in the cathode. | LFP page |
Terms Starting with M
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| MACRS | Modified Accelerated Cost Recovery System | Finance | The main US tax depreciation system for business property placed in service after 1986, according to IRS Publication 946. | MACRS page |
| MLPE | Module-Level Power Electronics | Inverters | Devices fitted to each panel, such as microinverters and power optimizers. | MLPE page |
| MPPT | Maximum Power Point Tracking | Inverters | A control method that keeps panels working at the voltage that gives the most power. | MPPT page |
| MW | Megawatt | Electrical | 1,000 kilowatts, or 1 million watts. | MW page |
Terms Starting with N
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| NABCEP | North American Board of Certified Energy Practitioners | Compliance | A US certification body for solar and storage professionals. Its credentials include PV Installation Professional and Energy Storage Installation Professional. | NABCEP page |
| NEC | National Electrical Code (NFPA 70) | Compliance | The US electrical code published by NFPA as NFPA 70. | NEC page |
| NEM | Net Energy Metering | Grid | A billing method that credits you for power you send to the grid. The credit rate depends on your utility’s rules. | NEM page |
| NMC | Nickel Manganese Cobalt | Batteries | A lithium-ion battery chemistry that uses nickel, manganese and cobalt in the cathode. | NMC page |
| NOCT / NMOT | Nominal Operating Cell Temperature / Nominal Module Operating Temperature | Panels | How hot a panel gets in typical sun. IEA-PVPS describes NMOT at 800 W/m² of sunlight and 20°C air temperature. | NOCT / NMOT page |
| NPV | Net Present Value | Finance | Future cash flows, reduced to today’s value, minus the money you spend now. A positive NPV means the project earns more than the rate you used. | NPV page |
| NSRDB | National Solar Radiation Database | Sunlight | A database of sunlight and weather data, mainly for the US. The National Laboratory of the Rockies (NLR, formerly NREL) runs it. |
Terms Starting with O
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| OCPD | Overcurrent Protection Device | Electrical | A fuse or breaker that opens a circuit when current is too high. | OCPD page |
| O&M | Operations and Maintenance | Projects | The work of running a plant and keeping it in good condition. | O&M page |
Terms Starting with P
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| P50 | 50th percentile energy | Performance | The yearly energy figure that the system should beat in half of all years. | P50 page |
| P90 | 90th percentile energy | Performance | The yearly energy figure that the system should beat in nine years out of ten. It is lower than P50. See the P90 section below. | P90 page |
| P95 | 95th percentile energy | Performance | The yearly energy figure that the system should beat in 19 years out of 20. It is lower than P90. | P95 page |
| Payback | Payback Period | Finance | The time it takes for savings or income to repay the cost of a system. | Payback page |
| Pmax | Maximum Power | Panels | The highest power a panel can deliver under test conditions. Datasheets list it in watts. | Pmax page |
| POA | Plane of Array | Sunlight | The sunlight that reaches the tilted face of the panels. This is the light the panels use. | POA page |
| PPA | Power Purchase Agreement | Finance | A contract to buy the power from a solar system at an agreed price per unit. | PPA page |
| PR | Performance Ratio | Performance | How much of the energy the panels could make in the sunlight they received is actually delivered. Heat, dirt, wiring and the inverter lower it. | PR page |
| PSH | Peak Sun Hours | Sunlight | A way to state daily sunlight as hours of full-strength sun. | PSH page |
| PTO | Permission to Operate | Projects | The utility’s approval to switch on a finished system and connect it to the grid. | PTO page |
| PV | Photovoltaic | Projects | Making electricity directly from light with solar cells. | PV page |
| PVsyst | PVsyst | Software | A software package that simulates how much energy a PV system will make. |
Terms Starting with R
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| REC | Renewable Energy Certificate | Finance | The EPA says a REC represents the property rights to the environmental, social and other non-power attributes of renewable electricity. One REC is issued per megawatt-hour. | REC page |
| ROI | Return on Investment | Finance | Your gain divided by your cost, shown as a percentage. | ROI page |
| RSD | Rapid Shutdown | Inverters | A safety feature that quickly lowers the voltage on rooftop wiring so firefighters can work safely. | RSD page |
| RTE | Round-Trip Efficiency | Batteries | The share of energy you get back out of a battery for each unit you put in. | RTE page |
Terms Starting with S
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| SAM | System Advisor Model | Software | A free desktop program for studying the technical and financial side of power projects. The National Laboratory of the Rockies (NLR) makes it. | |
| SCADA | Supervisory Control and Data Acquisition | Software | A system that monitors and controls equipment from a central screen. | SCADA page |
| SoC | State of Charge | Batteries | How full a battery is, as a percentage. | SoC page |
| SREC | Solar Renewable Energy Certificate | Finance | A renewable energy certificate issued for solar power. | SREC page |
| STC | Standard Test Conditions | Panels | The lab settings for a panel’s rated power: 25°C cell temperature, 1,000 W/m² of sunlight and an AM1.5 spectrum (IEA-PVPS). | STC page |
Terms Starting with T
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| TC | Temperature Coefficient | Panels | How much a panel’s power falls for each degree of heat above the test temperature. | TC page |
| TMY | Typical Meteorological Year | Sunlight | One year of hourly weather data built to stand for a typical year at a site. |
Terms Starting with V
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| V | Volt | Electrical | The unit of electrical pressure. | V page |
| VAr | Volt-Ampere reactive | Electrical | The unit of reactive power, which moves back and forth in the circuit and does no useful work. | VAr page |
| Vmp | Voltage at Maximum Power | Panels | The voltage a panel makes when it is delivering its maximum power. | |
| Voc | Open-Circuit Voltage | Panels | The highest voltage a panel makes when nothing is connected to it. | Voc page |
Terms Starting with W
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| W / Wp | Watt / Watt-peak | Electrical | The base unit of power. Wp is a panel’s rating under standard test conditions. | W / Wp page |
Terms Starting with Y
| Acronym | Full name | Group | Meaning | Learn more |
|---|---|---|---|---|
| Yield | Specific Yield | Performance | Annual energy divided by system size, shown as kWh per kWp. | Yield page |
See P50 and P90 on your own site
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What Does P90 Mean in Solar?
P90 is a yield expected to be met or exceeded in 90% of years (P50: 50%). It is lower than P50, the median.
PVsyst’s documentation explains it this way. The tool assumes yearly energy follows a normal (bell-shaped) curve around your simulation result. For that curve, P90 sits 1.28 standard deviations (σ) below the mean. See PVsyst P50-P90 evaluations. The size of σ comes from the weather data and other uncertainties, which you must set yourself.
Here is a made-up example. Say the simulated yearly energy (P50) is 1,000 MWh. Say σ is 5% of that, or 50 MWh. Then P90 = 1,000 − (1.28 × 50) = 1,000 − 64 = 936 MWh.
Always say whether a figure is P50 or P90.
Key Takeaway
In the example, P90 is 64 MWh (6.4%) below P50. The wider the spread, the bigger the gap, so a lender that asks for P90 will see a lower number than your P50 simulation.
Acronyms People Mix Up
| Pair | The difference |
|---|---|
| kW and kWh | kW is power, how fast energy is used or made. kWh is energy, the amount used or made over time. |
| kW and kWp | kW is any power rating. kWp is the DC rating of a panel array under standard test conditions. |
| STC and NOCT / NMOT | STC is a lab test setting. NOCT / NMOT describes how hot a panel gets in typical sun. |
| PR and CUF | PR compares output with the sunlight received. CUF compares output with full rated power all year. |
| Voc and Vmp | Voc is the voltage with nothing connected. Vmp is the voltage at maximum power. Vmp is lower. |
| NEM and FIT | NEM credits you for power sent to the grid under your utility’s rules. A FIT pays a fixed price per unit for a set period. |
Pro Tip
When a report gives a yield, ask two questions: is it P50 or P90, and is the capacity in AC or DC? A mismatch on either one changes the number you compare.
Keep Learning
For sunlight terms in more depth, read the solar irradiance guide for GHI, DNI and DHI and the solar insolation reference. For design basics, see solar design principles for installers. For general sunlight terms, you can also open the solar irradiance glossary page. To see how these terms show up in a real design, read about energy yield and financial modeling in SurgePV or book a demo.
Sources used on this page: IEA-PVPS Task 13 report on climatic rating of PV modules, PVsyst documentation, NSRDB, System Advisor Model, EPA on RECs, IRS Form 3468, IRS Publication 946, IEEE 1547-2018 and NABCEP.
Frequently Asked Questions
What is the difference between GHI, DNI, DHI, and POA in solar?
GHI (Global Horizontal Irradiance) is all the sunlight on a flat, level surface. DNI (Direct Normal Irradiance) is the sunlight that comes straight from the sun. DHI (Diffuse Horizontal Irradiance) is the sunlight scattered by the sky. POA (Plane of Array) is the sunlight that reaches the tilted panels, which is the light the panels use.
What does P90 mean in solar?
P90 is the yearly energy figure that a solar system should beat in 9 years out of 10. PVsyst documentation says annual production has a 90% chance of exceeding the P90 value. P90 is lower than P50, which is the median.
What does PR mean in solar?
PR (Performance Ratio) shows how much of the energy the panels could make in the sunlight they received is actually delivered. Heat, dirt, wiring and inverter losses lower it. A lower PR than expected is a sign to check for shade, dirt, clipping or wiring problems.
What is LCOE in solar and how is it calculated?
LCOE (Levelized Cost of Energy) is the average cost of each unit of energy over a system’s life. You divide the total lifetime cost, in today’s money, by the total lifetime energy, also adjusted the same way. Use the same method for every option you compare.
What is the difference between IRR and NPV in solar finance?
IRR (Internal Rate of Return) is a yearly return rate, such as 12%. NPV (Net Present Value) is an amount of money, such as $18,000, found by reducing future cash flows to today’s value and subtracting what you spend now. IRR gives you a rate. NPV gives you an amount.
What does STC stand for in solar panel ratings?
STC stands for Standard Test Conditions. The IEA-PVPS Task 13 report on module ratings gives them as 25°C cell temperature, 1,000 W/m² of sunlight and an AM1.5 spectrum. Datasheet ratings are given at STC. Real panels often work in different conditions, so real output differs.
What is the difference between kW and kWp in solar?
A kW (kilowatt) is a unit of power. A kWp (kilowatt-peak) is the DC power rating of a solar array under standard test conditions. A 10 kWp array has 10,000 watts of panel rating at STC. Its real output is lower or higher at other times because of sunlight, heat and losses.
Where this fits
This article is part of SurgePV's Solar Design hub, which works through the topic from first principles to the decisions a project team actually has to make.
