Reference
Glossary
Plain-English definitions for the solar, battery, HVAC, and EV terms used across the site — every term here is linked from at least one article where it comes up.
ACH50
Air Changes per Hour at 50 Pascals — the standard measurement from a blower door test, expressing how many times the entire volume of air in a home is replaced per hour when the house is pressurized to 50 Pascals. Lower is tighter: homes built after 2015 often measure around 3-7 ACH50, while an untouched older home can measure 12-20 or higher.
Aeroseal (Aerosol Duct Sealing)
A duct-sealing technique where a fine sealant is injected into a pressurized duct system and automatically builds up at leak points from the inside, reaching gaps that are physically inaccessible for manual sealing. Aeroseal is a specific branded process; the general technique is also called aerosol duct sealing.
Annual Limit
A cap on how much of a given tax credit you can claim in a single tax year, even if your qualifying spending was higher. Some home-efficiency credits reset their annual limit each year, which can make it worth spreading a multi-project upgrade (insulation one year, a heat pump the next) across tax years.
Attic Ventilation
The passive or powered airflow through an attic space, meant to reduce summer heat buildup and control moisture. Good ventilation matters most when the attic floor isn't well air-sealed; a well-sealed, well-insulated attic floor reduces how much ventilation actually affects the living space below.
Backup Heat (Auxiliary Heat)
A secondary heat source — usually electric resistance strips, though sometimes a furnace in a dual-fuel setup — that kicks in when a heat pump alone can't keep up with demand, typically during extreme cold. Backup heat is far less efficient than the heat pump itself, so a system that relies on it too often erodes the savings a heat pump is supposed to deliver.
Basis (Tax Basis)
The dollar amount used to calculate a tax credit or deduction — typically the total qualifying cost of the equipment and installation. For the federal solar credit, basis is generally the full contracted cost of the system, though certain rebates and incentives can reduce it.
Battery Thermal Management
The system (liquid loops, heaters, or air channels) an EV uses to keep its battery pack within its efficient operating temperature range. It's the main reason cold-weather range loss varies so much between EV models — a well-managed pack conditions the battery before a trip instead of just tolerating the cold.
Blower Door Test
A diagnostic test where a calibrated fan is sealed into a home's doorway and used to pressurize or depressurize the house, letting an energy auditor measure exactly how much air is leaking and pinpoint where. It's the standard, quantifiable way to measure a home's air-sealing performance rather than guessing from a walkthrough.
Capacity Retention
The flip side of degradation rate: the percentage of a battery's original usable capacity still available after a given number of years or cycles. A warranty promising "70% capacity retention at 10 years" is the same claim as "30% degradation by year 10."
Carryforward (Tax Credit)
The ability to apply an unused portion of a tax credit to a future tax year if it exceeds your tax liability in the year you claim it. The Residential Clean Energy Credit allowed carryforward before it expired; check current IRS guidance for whether a specific credit you're claiming permits it.
Census Tract
A small, roughly permanent statistical subdivision of a county used by the U.S. Census Bureau, and referenced by some energy incentive programs (such as low-income or energy-community bonus credits) to determine geographic eligibility. You can look up your specific census tract using the Census Bureau's online geocoder.
CFM (Cubic Feet per Minute)
A measurement of airflow volume over time, used to rate fans, ductwork capacity, and duct leakage. A whole-house fan might move 4,000-7,000 CFM, while a leaky duct system might lose several hundred CFM of conditioned air before it reaches its intended room.
Charge Cycle
One full discharge-then-recharge of a battery, from 100% down to 0% and back (two 50% swings count as one cycle). Battery warranties are usually written against a cycle count as much as a calendar term, so how you use the battery affects how fast you use up its warranty.
Clean Vehicle Credit
The federal tax credit for new and used electric vehicles under Internal Revenue Code Section 30D/25E. Eligibility rules (income caps, vehicle price caps, battery-sourcing requirements) and the credit's availability have changed more than once in recent years, so treat any specific dollar figure as something to re-verify against current IRS guidance before you rely on it.
Climate Zone
One of the numbered zones (1 through 8, per the U.S. Department of Energy/IECC map) used to set recommended insulation levels, HVAC sizing, and building code requirements for a given region. Colder zones (higher numbers) call for higher R-values and different equipment choices than milder ones.
Cold-Climate Heat Pump
A heat pump specifically engineered (via variable-speed compressors and enhanced refrigerant cycles) to maintain meaningful heating output and efficiency at outdoor temperatures well below 0°F, where standard heat pump models lose most of their capacity.
COP
Shorthand for Coefficient of Performance — see that entry for the full explanation. COP typically drops as outdoor temperature drops, which is why cold-climate heat pump models are engineered to hold a usable COP at much lower temperatures than standard models.
COP (Coefficient of Performance)
The ratio of heating or cooling output to electricity input for a heat pump — a COP of 3 means it delivers 3 units of heat for every 1 unit of electricity consumed. Unlike a furnace's efficiency rating, COP can exceed 1 because a heat pump moves heat rather than generating it from fuel.
Cost-Effectiveness Ratio
A way of comparing efficiency upgrades by their savings per dollar spent, rather than by total savings or total cost alone. A $300 upgrade that saves $150/year is more cost-effective than a $3,000 upgrade that saves $600/year, even though the second saves more in absolute terms — the first pays back in 2 years, the second in 5.
Degradation Rate
The rate at which a battery (or solar panel) loses usable capacity each year through normal chemical or material aging. It's usually expressed as a percentage of original capacity lost per year, and it's cumulative — a 2%/year rate means roughly 20% less capacity after a decade.
Demand Charge
A separate line-item charge (common on commercial bills, rarer but not unheard of on residential ones) based on the single highest rate of power draw (in kW) during a billing period, not on total energy used. It rewards spreading out big loads instead of running everything at once.
Duct Leakage
Conditioned air lost from the duct system before it reaches a room's register, typically through gaps at joints, boots, and connections. Measured with a duct blaster test; a typical home loses 20-30% of its conditioned air this way, according to ENERGY STAR and DOE research.
Ductless Mini-Split
A heat pump system that conditions individual rooms or zones directly through wall- or ceiling-mounted indoor units, connected to an outdoor compressor by refrigerant lines instead of ductwork. It's the common choice for homes without existing ducts, additions, or single-room retrofits.
eGallon
A comparison metric (popularized by the U.S. Department of Energy) showing what it would cost to drive the same distance an equivalent gas car could go on one gallon of gasoline, but using electricity instead. It's a quick way to compare fuel cost per mile across a gas car and an EV without doing the unit conversion yourself.
Electrical Panel Capacity
The total amperage a home's main electrical panel can safely distribute across all its circuits. Adding a Level 2 charger (or a heat pump, or a battery) can push an older or already-full panel past its capacity, which is why panel upgrades are one of the most common surprise costs in electrification projects.
EnergyGuide Label
The yellow label required by the FTC on major appliances, showing estimated annual energy use and cost compared to similar models. It's a regulated, standardized figure — unlike marketing terms like "energy saver," which aren't independently verified.
Export Rate
The per-kWh price a utility pays for solar power you send back to the grid under a net billing arrangement. It's usually set by a regulatory formula tied to wholesale power costs and is commonly well below the retail rate you pay for the electricity you use.
HSPF2
The heating-mode counterpart to SEER2 — Heating Seasonal Performance Factor, updated to the same 2023 testing standard — measuring a heat pump's heating output over a season divided by electricity used.
Inverter
The component that converts the DC (direct current) electricity solar panels produce into the AC (alternating current) electricity a home's wiring and the grid actually use. String inverters, microinverters, and hybrid inverters (which also manage battery storage) differ in cost, per-panel monitoring, and shading tolerance.
kW (Kilowatt)
A unit of power equal to 1,000 watts, used to describe how fast something can deliver or draw energy at a given moment — a charger's kW rating, not its kWh, is what determines how quickly it fills a battery.
kWh (Kilowatt-Hour)
The standard unit your utility bills you in — one kilowatt of power sustained for one hour. It's a measure of energy used over time, not instantaneous power, which is why it's the right unit for battery capacity and monthly usage but the wrong one for describing how fast something can charge or discharge.
Level 1 Charging
Charging an EV from a standard 120-volt household outlet using the cable that typically ships with the car. It adds only a few miles of range per hour, which is fine for a plug-in hybrid or a low-mileage commuter but usually too slow for daily use with a longer-range EV.
Level 2 Charger
A 240-volt EV charging unit, typically hardwired or plugged into a dedicated high-amperage outlet, that charges roughly 5–10x faster than a standard household outlet. It's the standard choice for home charging and usually the piece of hardware an installer is quoting when they price out an "EV charger install."
Level 2 Charging
Charging at 240 volts rather than the standard household 120 volts, delivering roughly 15–50+ miles of range per hour depending on the vehicle and the charger's amperage. This is the speed tier most home EV charger installations are built around.
Levelized Billing (Budget Billing)
A utility program that averages your estimated annual energy cost into equal monthly payments, rather than billing the actual amount used each month. It smooths out seasonal spikes (like a high summer AC bill) but doesn't reduce your total annual cost — utilities typically true-up the account periodically to correct for over- or under-estimation.
Low-E Coating
A microscopically thin, transparent metallic coating applied to window glass that reflects infrared heat while letting visible light through. It reduces heat loss in winter and heat gain in summer without noticeably darkening the glass, and is standard on most quality replacement windows today.
Lumen
The unit of visible brightness a light source produces, as distinct from watts (which measure power drawn, not light output). LED bulbs produce far more lumens per watt than incandescent bulbs, which is the entire basis of their energy savings — comparing bulbs by wattage alone is comparing the wrong number.
MAGI (Modified Adjusted Gross Income)
A version of your adjusted gross income with certain deductions added back, used by the IRS to determine eligibility for various credits and deductions, including income caps on some clean energy incentives. It's not the same number as your gross income or your taxable income — check the specific IRS worksheet for the credit in question to calculate it correctly.
Marginal Tax Rate
The tax rate applied to your next dollar of income, based on which federal tax bracket that dollar falls into — not the rate applied to your entire income. It matters for energy tax credits because a nonrefundable credit's value is capped by your tax liability, which is calculated using your full bracket structure, not just your marginal rate.
Mastic Sealant
A thick, paintable sealant used to seal duct joints and connections — the standard professional alternative to duct tape, which dries out and fails within a few years despite its name. Mastic stays flexible and adhered for the life of the duct system.
Modified Energy Factor (MEF)
An older efficiency rating for clothes washers, measuring energy used per cubic foot of capacity per cycle. Largely superseded by the Integrated Modified Energy Factor (IMEF) in current ENERGY STAR certifications, but still referenced in some older product documentation.
Net Billing
A billing arrangement where exported solar power is credited at a separate, typically lower "export rate" rather than the retail rate you pay for imported power. It generally makes battery storage more valuable, since self-consuming your own solar (via a battery) beats exporting it at a discounted rate.
Net Metering
A billing arrangement where excess solar power you export to the grid earns a credit at (or close to) the same retail rate you pay for power you import, effectively using the grid as free storage. It's more favorable to solar owners than net billing, which is why utilities in many states have been shifting away from it.
Nonrefundable Credit
A tax credit that can reduce your tax liability to zero, but won't generate a refund beyond that — if the credit is worth more than you owe, you don't get the difference back as cash (though some nonrefundable credits, like the former Residential Clean Energy Credit, allow you to carry the unused portion forward to a future year). This is different from a refundable credit, which can generate a refund even below zero tax owed.
Payback Period
The time it takes for the cumulative savings from an upgrade to equal its upfront cost — after which every additional year of savings is net positive. A shorter payback period means lower financial risk, but it doesn't account for savings that continue well beyond the payback point.
Peak Demand
The highest level of electricity draw — either your household's or the whole grid's — during a given period. Utilities design time-of-use rates and demand charges around peak demand because that's the moment their generation and infrastructure are most stressed.
Performance Warranty
A manufacturer's guarantee that a solar panel (or battery) will still produce at least a specified percentage of its original rated output after a given number of years, distinct from the product warranty that only covers hardware defects. This is the warranty that matters most for long-term degradation-rate claims.
Performance-Based Incentive (PBI)
An incentive paid out based on a system's actual measured energy production over time (e.g., cents per kWh generated over several years) rather than as a single upfront rebate tied to installation. PBIs reward systems that keep performing well, but they take longer to pay out in full than an upfront rebate.
Phantom Load (Standby Power)
The electricity a device draws while switched "off" but still plugged in — powering a clock, a remote sensor, or a standby light. Individually small, but added up across a household's chargers, TVs, and game consoles, it can be a meaningful and easily avoidable share of a monthly bill.
Placed in Service
The IRS's technical term for the date equipment is installed and ready for its intended use — not the date you purchased it or signed a contract. This date determines which tax year's rules apply, which became critical after the 2025 law that ended several energy credits based on placed-in-service dates rather than purchase dates.
Price per Watt
The standard way to compare solar quotes on equal footing — total system cost divided by system size in watts. It lets you compare a small premium-panel system against a larger budget-panel system without total price alone being misleading.
Product Warranty (Equipment Warranty)
A manufacturer's guarantee against physical defects or workmanship failures in solar panels, batteries, or inverters — separate from a performance warranty, which guarantees an output level rather than just defect-free hardware.
R-Value
A measure of how well an insulation material resists heat flow — higher R-value means better insulation. It's additive (stacking two R-13 layers gives roughly R-26) but only tells part of the story, since gaps, compression, and air leaks can undercut a material's rated performance in practice.
Range Anxiety
The worry that an EV won't have enough charge to reach its destination or the next charger. It's a real behavioral factor in EV adoption, but it's also measurably reduced by home charging (most trips start at 100%) and by cold-weather range loss being predictable enough to plan around.
Rate Base
The total value of a utility's infrastructure investments (power plants, transmission lines, equipment) that regulators allow it to earn a return on through customer rates. It's a core concept in how utility rates are set, though most homeowners never need to interact with it directly — it mainly shows up in rate-case proceedings.
Rate Case
A formal regulatory proceeding where a utility requests permission to change its rates, reviewed and approved (or rejected/modified) by a state public utility commission. Rate cases are where most electricity price increases are decided, and many states allow public comment during the process.
Rate Lock
A fixed-rate plan, typically offered in deregulated electricity markets, that guarantees a set price per kWh for a defined contract period regardless of market price swings. It protects against rate spikes but can mean paying more than the variable market rate if prices drop during your contract term.
Rebate
A direct payment — as a discount at purchase, a mailed check, or a bill credit — for buying qualifying equipment, independent of your tax situation. State and utility rebate programs for heat pumps, batteries, and efficiency upgrades often have limited annual funding, so timing can matter as much as eligibility.
Retail Choice (Deregulated Electricity)
A market structure, available in some U.S. states, where customers can choose their electricity supplier separately from the utility that delivers the power over the wires. Where available, it allows shopping between competing rate plans; in traditional regulated states, the utility is the only option for both supply and delivery.
SEER2
The updated (as of 2023) U.S. testing standard for Seasonal Energy Efficiency Ratio, measuring cooling output over a season divided by energy used, tested under conditions that better reflect real-world duct static pressure than the older SEER standard. SEER2 numbers run slightly lower than old SEER numbers for a comparably efficient unit, so don't compare the two ratings directly.
Setback Schedule
A programmed thermostat schedule that deliberately lets indoor temperature drift a few degrees during hours when no one's home or everyone's asleep, then recovers to a comfortable setpoint before it matters. It's the mechanism behind most of a smart thermostat's claimed savings.
Smart Thermostat Setback
A setback schedule that a smart thermostat manages automatically — using occupancy sensing, geofencing, or learned routines — rather than a schedule the homeowner has to program and remember to adjust manually.
Solar Reflectance
The fraction of incoming solar energy a roofing material reflects rather than absorbs, expressed as a value between 0 (absorbs everything) and 1 (reflects everything). Cool roof products are rated on solar reflectance alongside thermal emittance by the Cool Roof Rating Council.
Stack Effect
The tendency of warm air to rise and escape through a home's upper leaks (attic bypasses), pulling cold outside air in through lower leaks (basement, crawlspace) to replace it — like a slow chimney running through the whole house. It's the main reason attic and rim-joist sealing rank as high-priority air-sealing targets.
Tax Credit
A dollar-for-dollar reduction in the income tax you owe, as opposed to a deduction (which only reduces the income your tax is calculated on). A $1,000 tax credit cuts your tax bill by $1,000; a $1,000 deduction saves you $1,000 times your marginal tax rate.
Tax Credit vs. Rebate
A tax credit reduces the tax you owe when you file, so its value depends on having enough tax liability to use it. A rebate is a direct cash-back payment (often point-of-sale or mailed after purchase) that doesn't depend on your tax situation. The two can usually be combined, but the order in which you apply them can affect how much of your basis remains for other incentives.
Taxable Basis
The qualifying cost amount left after certain rebates and incentives are subtracted, used to calculate a tax credit. Because some state rebates reduce your federal taxable basis and others don't, stacking multiple incentives on one project requires checking each program's specific rule rather than assuming they're all additive.
Thermal Emittance
A material's ability to release absorbed heat rather than retain it, rated on a 0-to-1 scale alongside solar reflectance for cool roofing products. A roof can have high thermal emittance even with moderate reflectance, which still helps it shed absorbed heat faster than a low-emittance material.
Thermal Imaging (Infrared Scan)
Using an infrared camera during a home energy audit to visualize temperature differences across walls, ceilings, and windows — revealing missing insulation, air leaks, or moisture intrusion that aren't visible to the naked eye.
Tiered Rate
A pricing structure where the cost per kWh increases once your usage crosses certain thresholds within a billing period — the more you use, the higher the rate on those additional kWh. It's a different structure from time-of-use rates, which vary by time of day rather than total volume used.
Time-of-Use (TOU) Rate
An electricity pricing plan where the price per kWh changes depending on the time of day, typically rising during a late-afternoon/early-evening "peak" window when grid demand is highest. TOU rates are what make battery arbitrage (charging cheap, discharging during the peak) financially worthwhile.
True-Up
A periodic reconciliation where a utility compares estimated charges (like on a levelized billing plan) against actual usage and adjusts your account to correct the difference. It also describes the annual reconciliation many net-metering programs use to settle the value of exported solar energy against energy consumed.
U-Factor
The rate of heat transfer through a window or door — the inverse of R-value, expressed per square foot per degree of temperature difference. Lower U-factor means better insulation; it's the number ENERGY STAR uses to set window performance thresholds by climate zone, rather than R-value.
Urban Heat Island
The tendency of cities to run measurably hotter than surrounding rural areas, largely due to dark pavement and roofing absorbing and re-radiating solar heat. Cool roofing is one of the few individual-home upgrades that also contributes, in aggregate, to reducing this effect at a city scale.
Watt (W)
The base unit of power — the rate at which energy is used or produced at a given instant. A 100W bulb draws 100 watts continuously; multiply by hours used to get the energy consumed in watt-hours (or divide by 1,000 for kWh).
Zone Control
Heating or cooling different areas of a home independently and to different setpoints, rather than treating the whole house as one thermostat zone. Ductless mini-splits offer zone control natively; ducted systems need zoning dampers and multiple thermostats to achieve the same thing.