Resources

A solar terms glossary for people reading the fine print

Solar proposals, utility tariffs, tax rules, and emissions reports each have their own vocabulary. This glossary translates the terms you're most likely to meet, with a note on why each one matters.

Book a free fit call Free tools

A library card-catalog drawer whose index cards are tiny solar cells — The idea
Half of a solar proposal is vocabulary. Learn the words and the numbers start talking.

This solar terms glossary covers the words that show up in proposals, lease offers, utility bills, monitoring dashboards, tax discussions, and sustainability questionnaires. Each definition is written in plain English and, where it helps, ends with the question worth asking.

Where the costly mix-ups happen

Some of the most expensive misunderstandings start with two similar terms. A kilowatt is not a kilowatt-hour. A price per watt quoted on DC size is not comparable to one quoted on AC size. A lease and a PPA can produce the same monthly bill and very different 25-year totals. The entries below are cross-linked so you can follow those threads.

How to use this solar terms glossary

Policy terms, including tax credits, net metering riders, and program rules, reflect conditions as of November 2026 and change often. Treat them as general information, confirm current details with the source, and take tax and legal questions to a qualified professional. Our scope of services explains where our work stops and theirs begins.

If a term in your own documents isn’t here, send it to us. When you’re ready to apply these terms to a real decision, our sample reports show how they appear in practice, and the FAQ answers the questions people ask most.

#

25D (Residential Clean Energy Credit): The federal income tax credit homeowners could claim for solar, battery storage, and certain other equipment at their homes. The 2025 budget reconciliation law ended it for expenditures made after December 31, 2025, and the IRS treats an expenditure as made when installation is complete. Anyone who claimed it for 2025 should confirm the claim and any carryforward with a tax professional. See also: ITC (investment tax credit), 48E (Clean Electricity Investment Credit). Related service or guide.

45Y (Clean Electricity Production Credit): A federal tax credit paid per kilowatt-hour of electricity a qualified zero-emission facility generates, generally for its first 10 years in service. It is the production-based alternative to 48E, and a project claims one or the other. For wind and solar, the 2025 budget law added begin-construction and placed-in-service deadlines plus foreign-entity restrictions. Eligibility is a question for a tax professional. See also: 48E (Clean Electricity Investment Credit), Begin construction, Placed in service, FEOC (foreign entity of concern).

48E (Clean Electricity Investment Credit): The federal investment tax credit for qualified clean electricity facilities and energy storage placed in service after 2024, calculated as a percentage of eligible cost. The base rate is 6%, rising to 30% for projects under 1 megawatt (AC) or projects that meet prevailing wage and apprenticeship rules, with possible bonus amounts. Under the 2025 budget law, solar and wind projects that begin construction after July 4, 2026 must be placed in service by December 31, 2027. Energy storage follows a longer schedule. See also: 45Y (Clean Electricity Production Credit), Begin construction, Placed in service, Prevailing wage and apprenticeship, Domestic content bonus. Related service or guide.

A

AHJ (authority having jurisdiction): The office responsible for enforcing building, electrical, and fire codes for a project, such as a city or county building department or a state inspection office. The AHJ issues permits and arranges inspections. We help assemble and track permit paperwork; plans and inspections are handled by licensed parties and by the AHJ itself. See also: Interconnection, PTO (permission to operate). Related service or guide.

Availability: The share of time a solar system, or a component such as an inverter, is able to produce power while the sun is up. An inverter that sat offline for three weeks in June might still show about 94% availability for the year, yet the lost energy is a larger share than that, because June days are long. Monitoring data reveals gaps that monthly bills hide. See also: Performance ratio, Monitoring portal.

Avoided cost: The cost a utility avoids by not generating or buying an additional unit of electricity itself. Regulators use avoided-cost components to price some customer-owned generation, and the Kentucky PSC relied on them when it set export credit rates for newer LG&E and KU net metering customers. See also: Export credit, NMS-2, Public Service Commission (PSC).

B

Backup load panel: A separate electrical panel holding only the circuits a battery will power during an outage, such as the refrigerator, well pump, furnace fan, and a few lights. Also called a critical loads panel. A smaller backup panel lets a smaller battery last longer. Which circuits move, and how, is decided and wired by a licensed electrician. See also: Depth of discharge, Round-trip efficiency. Related service or guide.

Begin construction: The tax-law test for when a clean energy project has started, which matters because credit deadlines depend on it. For wind and solar, Treasury's August 2025 guidance (Notice 2025-42) emphasized physical work of a significant nature followed by continuous progress and narrowed the 5% cost safe harbor to small solar facilities. Guidance in this area has changed more than once, so whether a project has begun construction is a determination for a tax professional working from the current rules. See also: Placed in service, 48E (Clean Electricity Investment Credit), 45Y (Clean Electricity Production Credit). Related service or guide.

Bid leveling: Adjusting differently formatted installer bids so they compare on the same basis: same scope, excluded items priced in, warranty terms side by side, and price per watt calculated the same way. The lowest quote and the lowest leveled price are often different bids. See also: Price per watt. Related service or guide.

Bonus depreciation: A federal tax provision that lets a business deduct a large share of a qualifying asset's cost in the first year instead of spreading it out. The 2025 budget law restored a 100% first-year allowance for certain property acquired and placed in service after January 19, 2025. Claiming an investment tax credit generally reduces the depreciable basis, so ask your CPA how the two interact. See also: MACRS, ITC (investment tax credit).

C

Capacity factor: The energy a system actually produced over a period divided by what it would have produced running at full nameplate output every hour of that period. Solar capacity factors are modest because the sun sets, so the figure is most useful for comparing a system with itself, or with similar systems nearby, over time. See also: Specific yield, Performance ratio.

Clipping: Energy lost when the panels could produce more DC power than the inverter can convert, so the inverter caps its output. A little clipping during the brightest hours is a normal trade-off when the DC/AC ratio is above 1. Heavy clipping can mean the inverter is undersized for the array. See also: DC/AC ratio, Inverter.

D

DC/AC ratio: The array's DC nameplate capacity divided by the inverter's AC rating. A 9.6 kWdc array on a 7.6 kWac inverter has a ratio of about 1.26. Ratios somewhat above 1 are common because panels rarely reach full nameplate output; very high ratios increase clipping. See also: kWdc and kWac, Clipping.

Dealer fee: A fee a lender charges the installer for offering a loan, often to buy down an advertised interest rate. It is usually built into the system price, so a financed customer can pay more than a cash buyer for the same equipment. Ask for the cash price and the financed price side by side; the gap is where the fee shows up. See also: Price per watt, Escalator. Related service or guide.

Decommissioning bond: Financial security, such as a surety bond or letter of credit, set aside so money is available to remove a solar facility and restore the land at the end of its life. Kentucky law requires decommissioning plans and bonding for merchant solar facilities approved by the siting board. For a landowner, what the lease itself requires, who holds the security, and when it is posted are questions for your attorney. See also: Siting board, Option period. Related service or guide.

Degradation: The gradual decline in a solar panel's output over time. Module warranties typically specify a maximum yearly loss after a larger first-year drop. A financial model that assumes zero degradation overstates later-year savings; one that assumes too much understates them. See also: Production estimate, Performance ratio.

Demand charge: A charge based on your highest rate of electricity use, measured in kilowatts, during a billing period, often the highest 15-minute average. Larger commercial and industrial rate classes typically include one. Solar can cut energy charges without touching the demand charge if your peak comes at 6 p.m. or on a cloudy afternoon. See also: Kilowatt (kW), Load factor, Interval data.

Depth of discharge: How much of a battery's stored energy is used before it is recharged, expressed as a percentage of capacity. Usable capacity and some warranty terms depend on depth-of-discharge limits, so compare batteries on usable kilowatt-hours, not just nameplate capacity. See also: Round-trip efficiency, Backup load panel.

Discount rate: The annual rate used to convert future dollars into today's dollars in a net present value calculation. It reflects what your money could otherwise earn, or what borrowing costs you. A higher discount rate makes savings 20 years away count for less, which can turn a marginal project from positive to negative. See also: NPV (net present value), IRR (internal rate of return).

Domestic content bonus: An additional federal credit amount for 48E and 45Y projects that meet thresholds for U.S.-made steel, iron, and manufactured products. Qualifying takes supplier documentation, and certain entities using elective pay can see their credit reduced if the requirements aren't met. We track supplier paperwork; whether a project qualifies is for your tax adviser. See also: 48E (Clean Electricity Investment Credit), Elective pay, FEOC (foreign entity of concern). Related service or guide.

E

eGRID: The U.S. Environmental Protection Agency's Emissions & Generation Resource Integrated Database, which publishes emission rates for power plants, states, and grid subregions. Its subregion output emission rates are a common source for location-based Scope 2 estimates. eGRID data lags by about two years, so match the data year as closely as you can to your inventory year. See also: Emission factor, Location-based method, Scope 2 emissions.

Elective pay: A provision of the tax code (section 6417) that lets tax-exempt organizations, state and local governments, Indian tribal governments, rural electric cooperatives, and certain other entities receive eligible clean energy credits as a payment from the IRS. It requires pre-filing registration and follows the same eligibility rules and deadlines as the underlying credit. Also called direct pay. See also: Transferability, 48E (Clean Electricity Investment Credit). Related service or guide.

Emission factor: A number that converts an activity into emissions, such as metric tons of carbon dioxide equivalent per megawatt-hour of electricity. The factor you choose often moves a Scope 2 result more than the electricity data does, so a credible report names its source and year. See also: eGRID, Location-based method, Market-based method.

Escalator: A fixed annual percentage increase in a lease or PPA payment. A 2.9% escalator sounds small, but over 25 years it raises the last payment to about double the first. Compare the escalator with a realistic utility rate path, not the best-case one in a sales presentation. See also: Lease (solar lease), PPA (power purchase agreement). Related service or guide.

Export credit: The value a utility credits for solar electricity you send to the grid instead of using on site. In Kentucky, newer net metering customers at LG&E, KU, and several other utilities receive export credits set by the PSC below the retail rate, which makes using solar power on site more valuable. See also: Net metering, Net billing, Self-consumption, NMS-2. Related service or guide.

F

FEOC (foreign entity of concern): Common shorthand for the 2025 budget law's restrictions on prohibited foreign entities, including entities tied to certain countries such as China. Projects that begin construction after December 31, 2025, including wind, solar, and storage, must keep the share of costs attributable to such entities below set thresholds, a material assistance test, to claim 48E or 45Y. Treasury issued interim guidance in early 2026. Your tax adviser makes the determination. See also: Domestic content bonus, 48E (Clean Electricity Investment Credit), 45Y (Clean Electricity Production Credit). Related service or guide.

G

GHG Protocol: The most widely used set of standards for corporate greenhouse gas accounting, developed by the World Resources Institute and the World Business Council for Sustainable Development. Its Scope 2 Guidance (2015) calls for reporting electricity emissions both location-based and market-based. Proposed revisions to that guidance, including hourly matching, went to public consultation in late 2025. See also: Scope 1 emissions, Scope 2 emissions, Scope 3 emissions. Related service or guide.

Green Tariff: LG&E and KU's program that lets business customers support renewable energy through renewable energy certificates, a solar array built at the customer's facility (Business Solar), or, for the largest customers, Renewable Power Agreements tied to new renewable projects. Each option has different costs and a different effect on market-based Scope 2. See also: REC (renewable energy certificate), Market-based method. Related service or guide.

I

Interconnection: The utility's process and agreement for connecting a generator, such as rooftop solar or a battery, to its grid. It typically involves an application, utility review, an interconnection agreement, inspection sign-off, and finally permission to operate. We coordinate the paperwork; technical studies and electrical work are done by licensed parties and the utility. See also: PTO (permission to operate), One-line diagram, AHJ (authority having jurisdiction). Related service or guide.

Interval data: Meter readings recorded every 15 minutes or every hour instead of once a month. Interval data shows when you use power, which determines how much solar you'll use on site, how a battery would cycle, and what sets your demand charge. Utilities with smart meters often provide it through your online account or on request. See also: Self-consumption, Demand charge, Load factor.

Inverter: The device that converts a solar array's direct current (DC) into the alternating current (AC) your building uses. The main types are string inverters, microinverters, and string inverters paired with optimizers; hybrid inverters also manage a battery. Inverters usually carry shorter warranties than panels, so a long-term model should include a replacement. See also: String inverter, Microinverter, Optimizer (power optimizer), Clipping.

IRR (internal rate of return): The discount rate at which a project's net present value equals zero, expressed as an annual percentage. It lets you compare a solar investment with other uses of money. IRR can mislead when cash flows switch between positive and negative more than once, as with a mid-life inverter replacement, so read it alongside NPV. See also: NPV (net present value), Discount rate, Simple payback.

Irradiance: The power of sunlight striking a surface, measured in watts per square meter. Production estimates start with irradiance data for your location, then account for panel tilt, orientation, shading, temperature, and system losses. See also: Plane of array (POA) irradiance, TMY (typical meteorological year).

ITC (investment tax credit): A federal tax credit equal to a percentage of an energy project's eligible cost, claimed for the year the project is placed in service. For commercial projects placed in service after 2024, the technology-neutral 48E credit took over from the older section 48 energy credit. The homeowner equivalent was the separate 25D credit, which ended after 2025. See also: 48E (Clean Electricity Investment Credit), 25D (Residential Clean Energy Credit), Bonus depreciation.

K

Kilowatt (kW): A unit of power: the rate at which electricity is produced or used at a given moment. Solar system size is stated in kilowatts, and so are commercial demand charges. If kilowatt-hours are the distance a car travels, kilowatts are its speed. See also: Kilowatt-hour (kWh), kWdc and kWac, Demand charge.

Kilowatt-hour (kWh): A unit of energy: one kilowatt sustained for one hour. Your electric bill charges for kilowatt-hours, and a solar system's yearly production is stated in them. A 1 kW load running for 10 hours uses 10 kWh. See also: Kilowatt (kW), Specific yield.

kWdc and kWac: Two ways to state a solar system's size. kWdc is the panels' combined direct-current nameplate rating; kWac is the inverter's alternating-current output limit. The same system might be described as 9.6 kWdc or 7.6 kWac, so check which one a price per watt is based on before comparing quotes. See also: DC/AC ratio, Price per watt, Kilowatt (kW).

L

LCOE (levelized cost of energy): A system's lifetime cost, including purchase, financing, maintenance, and replacements, divided by its lifetime energy production, usually with both discounted to present value. It produces a cost per kilowatt-hour you can compare with your utility rate, and it's only as good as the production and cost assumptions underneath it. See also: NPV (net present value), Discount rate, Production estimate.

Lease (solar lease): An arrangement in which a company owns the solar system on your property and you pay a fixed monthly amount, often with an annual escalator, typically for 20–25 years. The owner usually handles maintenance and keeps any tax benefits. Read what happens if you sell the house, and ask how a system you don't own fits your utility's net metering rules. See also: PPA (power purchase agreement), Escalator, Net metering. Related service or guide.

Load factor: Average demand divided by peak demand over a period. A low load factor means short, sharp peaks, which is where demand charges bite and where solar alone may not help much. A plant running steadily around the clock has a high load factor. See also: Demand charge, Interval data.

Location-based method: One of the two Scope 2 methods in the GHG Protocol. It multiplies the electricity you use by the average emission rate of the grid where you use it, such as an eGRID subregion rate. Buying RECs or joining a green tariff doesn't change a location-based result. See also: Market-based method, eGRID, Scope 2 emissions. Related service or guide.

M

MACRS: The Modified Accelerated Cost Recovery System, the federal depreciation system for business property. Solar equipment was long treated as five-year property. The 2025 budget law removed solar and wind energy property from that five-year class for property beginning construction after December 31, 2024, so the recovery period for a newer project needs your CPA's review. See also: Bonus depreciation, Begin construction.

Market-based method: The second Scope 2 method in the GHG Protocol. It reflects the electricity you've contracted for, using instruments such as RECs, green tariff purchases, or supplier-specific emission rates, with a residual mix for the rest. In a coal-heavy service area, a market-based result can come out higher than the location-based one if you hold no renewable instruments. See also: Location-based method, REC (renewable energy certificate), Green Tariff. Related service or guide.

Microinverter: A small inverter mounted behind one or two panels that converts power to AC right at the panel. Shade or a fault on one panel affects only that panel, and monitoring can show each panel's output. There are more devices on the roof, so each failure is small, but finding replacements after an installer closes can take some work. See also: Inverter, String inverter, Optimizer (power optimizer).

Monitoring portal: The website or app where a solar system's production data appears, usually run by the inverter manufacturer. Many accounts are set up under the installer's name. If the installer closes, owners can lose access, and with it the data a warranty claim needs. Ask who owns the account before the system is finished. See also: Orphaned system, Performance ratio. Related service or guide.

N

Net billing: A compensation structure that credits exported solar electricity at a set rate, often below retail, while you pay the full retail rate for electricity you draw from the grid. Kentucky's newer net metering tariffs, such as LG&E and KU's NMS-2, work this way, which rewards using solar power on site. See also: Net metering, Export credit, NMS-2, Self-consumption.

Net metering: A billing arrangement for customer-owned generation. In Kentucky it applies to eligible systems of up to 45 kilowatts. In its original form, each kilowatt-hour exported offsets one used. Under Senate Bill 100 (2019), the PSC sets compensation for new customers' exports, and Kentucky law keeps a customer's net metering terms in effect at that premises for 25 years. See also: NMS-1, NMS-2, Net billing, Export credit. Related service or guide.

NMS-1: LG&E and KU's original net metering rider, for customers who began net metering service before September 24, 2021. Excess generation earns kilowatt-hour credits that offset usage in later billing periods. Under Kentucky law those terms stay at the premises for 25 years, but adding capacity or modifying the system moves a customer to NMS-2. See also: NMS-2, Net metering. Related service or guide.

NMS-2: LG&E and KU's net metering rider for customers who began service on or after September 24, 2021. Energy sent to the grid earns a dollar-denominated bill credit at a per-kWh rate set by the PSC, which is lower than the retail rate. Credits offset energy charges and carry forward, but they don't pay out in cash or offset basic service charges. Check the current tariff for the rate. See also: NMS-1, Net billing, Export credit, Avoided cost. Related service or guide.

NPV (net present value): The sum of a project's future cash flows, each discounted to today's dollars, minus the upfront cost. A positive NPV means the project beats your chosen discount rate. It is more informative than payback because it counts every year, not just the years until you break even. See also: Discount rate, IRR (internal rate of return), Simple payback.

O

One-line diagram: A simplified electrical drawing showing how a system's major components connect, from panels and inverters to disconnects, the meter, and the grid. Utilities and permit offices usually require one. It is prepared by the installer or a licensed professional; we don't prepare one-line diagrams, though we track that one has been filed. See also: Interconnection, AHJ (authority having jurisdiction).

Optimizer (power optimizer): A module-level device that conditions each panel's DC output before it travels to a central string inverter. Like microinverters, optimizers limit the effect of shade or a weak panel and allow panel-level monitoring, while the conversion to AC still happens in one inverter. See also: String inverter, Microinverter, Inverter.

Option period: The early phase of a land lease during which a developer holds the right, but not the obligation, to build. Option payments are usually far smaller than operating rent, and extensions can tie up land for years before construction. Compare how long the option lasts, what it pays, and what you may do with the land meanwhile. See also: Decommissioning bond, Siting board. Related service or guide.

Orphaned system: A solar system whose installer or financing company has gone out of business, leaving the owner without a service contact, monitoring access, or a clear warranty path. Manufacturer warranties often survive the installer, but claims usually need production data and paperwork. Several large residential installers and financiers failed in 2024–2026. See also: Monitoring portal, Performance ratio. Related service or guide.

P

Performance ratio: Actual energy produced divided by the energy the system should have produced given the sunlight it actually received. It separates weather from equipment: a low-output year with a normal performance ratio was cloudy, while a low performance ratio points to something in the system. See also: Plane of array (POA) irradiance, Availability, Specific yield. Related service or guide.

Placed in service: For tax purposes, the date a system is ready and available for its intended use, which generally means it can operate as designed with the needed approvals in place. Many credit rules key off this date, including the end-of-2027 deadline for some wind and solar projects. Your tax adviser determines the date for your project. See also: Begin construction, PTO (permission to operate), 48E (Clean Electricity Investment Credit).

Plane of array (POA) irradiance: Sunlight measured on the same tilt and orientation as the panels, rather than on flat ground. Performance analysis compares production with POA irradiance because that is the sunlight the panels actually received. See also: Irradiance, Performance ratio.

PPA (power purchase agreement): A contract to buy the electricity a system produces at a set price per kilowatt-hour, usually with an annual escalator, from a company that owns the system. You pay for output, not equipment. Whether a third-party PPA is permitted for your site, and how it interacts with your utility's net metering rules, are questions for your attorney and utility. See also: Lease (solar lease), Escalator. Related service or guide.

Prevailing wage and apprenticeship: Labor requirements in the federal clean electricity credits. Projects of 1 megawatt (AC) or more generally must pay prevailing wages and use qualified apprentices to earn the full 30% 48E rate instead of the 6% base rate. Smaller projects are exempt from these requirements. See also: 48E (Clean Electricity Investment Credit), 45Y (Clean Electricity Production Credit).

Price per watt: A system's total price divided by its size in watts, the usual yardstick for comparing quotes. Check whether it's based on DC or AC size; whether it includes dealer fees, batteries, roof work, and permit fees; and whether it's stated before or after incentives. See also: kWdc and kWac, Dealer fee, Bid leveling. Related service or guide.

Production estimate: A projection of how many kilowatt-hours a system will produce in a typical year, usually from modeling software using local weather data, panel orientation, shading, and loss assumptions. Ask what loss and degradation figures it uses, and compare it with an independent model such as PVWatts. See also: PVWatts, TMY (typical meteorological year), Specific yield, Degradation.

PTO (permission to operate): The utility's written approval to switch on a grid-connected system after installation, inspection, and any meter change. Running the system before PTO can violate the interconnection agreement. Delays between installation and PTO are common, so it pays to track every open item. See also: Interconnection, AHJ (authority having jurisdiction), Placed in service. Related service or guide.

Public Service Commission (PSC): The Kentucky agency that regulates the rates and service of the state's investor-owned utilities and many of its electric cooperatives. It approves tariffs, including net metering riders and export credit rates, and its case records are public. Municipal utilities and some TVA-served systems follow different rules. See also: Net metering, NMS-2, Siting board.

PVWatts: A free online calculator that estimates a grid-connected solar system's production from its location, size, tilt, orientation, and losses, using typical-year weather data. It comes from the U.S. Department of Energy's national laboratory in Golden, Colorado, formerly NREL, which was renamed the National Laboratory of the Rockies in December 2025. It's a good first check on an installer's production estimate. See also: Production estimate, TMY (typical meteorological year).

R

REC (renewable energy certificate): A tradable certificate representing the environmental attributes of one megawatt-hour of renewable electricity. Whoever holds and retires the REC can claim that electricity as renewable. If you sell or sign away the RECs from your own array, you generally can't also claim its output as renewable in market-based Scope 2. See also: Market-based method, Green Tariff. Related service or guide.

Round-trip efficiency: The share of energy put into a battery that comes back out, after charging and discharging losses. Spec sheets for lithium-ion home batteries commonly list roughly 85% to 90%. When exports earn less than retail, storing surplus solar can pay, but only if the value gap is bigger than those losses. See also: Depth of discharge, Self-consumption, Backup load panel. Related service or guide.

S

Scope 1 emissions: Direct greenhouse gas emissions from sources a company owns or controls, such as boilers, furnaces, company vehicles, and on-site generators. Burning natural gas for heat is Scope 1; the electricity you buy is Scope 2. See also: Scope 2 emissions, Scope 3 emissions, GHG Protocol.

Scope 2 emissions: Indirect emissions from generating the electricity, steam, heat, or cooling a company buys. They occur at the power plant, but they count in your inventory because your use drives them. The GHG Protocol calls for reporting them two ways, location-based and market-based. See also: Location-based method, Market-based method, Scope 1 emissions, Scope 3 emissions. Related service or guide.

Scope 3 emissions: All other indirect emissions in a company's value chain, from purchased goods and shipping to how customers use and dispose of products. Your Scope 1 and Scope 2 emissions are part of your customers' Scope 3, which is why manufacturers now receive supplier sustainability questionnaires. See also: Scope 1 emissions, Scope 2 emissions, GHG Protocol. Related service or guide.

Self-consumption: The share of solar output used on site as it's produced rather than exported. When exports earn less than retail, each self-consumed kilowatt-hour is worth more than each exported one, so matching system size to your loads matters more than it did under one-to-one net metering. See also: Export credit, Net billing, Interval data.

Simple payback: Net cost divided by first-year savings, giving the number of years to break even. It's easy to grasp but ignores rate changes, degradation, replacement costs, financing, and everything after breakeven. Use it as a first screen, then look at NPV and lifetime cash flow. See also: NPV (net present value), IRR (internal rate of return). Related service or guide.

Siting board: Kentucky's State Board on Electric Generation and Transmission Siting, which reviews applications for merchant (non-utility) generating facilities above a size threshold set in state law, including large solar farms. It can attach conditions such as setbacks, mitigation, and decommissioning requirements. Landowners weighing a lease can follow a project's case in the board's public records. See also: Decommissioning bond, Option period, Public Service Commission (PSC). Related service or guide.

Soiling: Dust, pollen, bird droppings, and debris that block sunlight on panels. Regular rain usually keeps soiling losses modest in Kentucky, but low-tilt panels near gravel roads, grain handling, or poultry house exhaust fans can collect enough to matter. Monitoring data helps you decide whether cleaning is worth paying for. See also: Performance ratio, Degradation.

Specific yield: Annual energy produced per kilowatt of installed DC capacity, in kWh per kWdc. It normalizes for size, so you can compare a 6 kW home system with a 400 kW rooftop, or check whether a proposal's production estimate is plausible for your area. See also: Capacity factor, Production estimate, kWdc and kWac.

String: A group of solar panels wired in series so their voltages add up before reaching an inverter. Without module-level electronics, shade or a fault on one panel can pull down the whole string. On a two-string system, output that suddenly drops to about half of expected can point to a string problem for a qualified electrician to check. See also: String inverter, Optimizer (power optimizer), Microinverter.

String inverter: A central inverter that takes DC power from one or more strings of panels and converts it to AC. It is a single device to monitor and replace, usually at lower cost than module-level electronics, but shading on part of a string reduces that string's output. See also: Inverter, String, Optimizer (power optimizer).

T

Time-of-use rate: A rate design that charges different prices depending on the time of day, and sometimes the season. Under time-of-use pricing, when solar produces and when you use power matter as much as how much. Compare your interval data with the rate's peak hours before choosing one. See also: Interval data, Self-consumption.

TMY (typical meteorological year): A year of hourly weather data assembled from many years of records to represent typical conditions at a location. Production models use TMY data, so actual output in any given year will run above or below the estimate. A cloudy year isn't a broken system. See also: PVWatts, Production estimate, Irradiance.

Transferability: A provision of the tax code (section 6418) that lets a business sell all or part of certain clean energy tax credits to an unrelated buyer for cash. It helps owners who can't use the full credit themselves. Entities eligible for elective pay generally use that route instead. See also: Elective pay, 48E (Clean Electricity Investment Credit).