BTU (British Thermal Unit)
A BTU is a unit of heat energy — specifically, the energy needed to raise the temperature of one pound of water by 1°F. It's the standard rating for the heating or cooling capacity of an appliance, not its energy consumption.
Where you'll see it: Air conditioner sizing (5,000–60,000 BTU for residential units), furnace output ratings, space heater capacity, and gas dryer heat output. A larger BTU rating means more heating/cooling power delivered per hour — but it doesn't tell you the electricity cost on its own. To get cost, BTU has to be combined with efficiency rating (SEER, AFUE, HSPF) and your local energy rate.
Quick conversion: 1 BTU = 0.293 watt-hours. 12,000 BTU = 1 "ton" of cooling capacity (a common central AC sizing unit). Use our AC Running Cost Calculator or Space Heater Cost Calculator to convert BTU + efficiency into an actual dollar cost.
SEER (Seasonal Energy Efficiency Ratio)
SEER measures how efficiently an air conditioner converts electricity into cooling over a typical season — calculated as total cooling output (BTU) divided by total energy input (watt-hours). Higher SEER means the unit delivers the same cooling for less electricity.
Where you'll see it: Central AC and heat pump nameplates. As of 2023, federal minimums are 14 SEER in northern states and 15 SEER in the South/Southwest. Units from before 2006 are often 8–10 SEER — using 40–75% more electricity than a modern 16 SEER unit for the same cooling output.
Why it matters for cost: Upgrading from 10 SEER to 20 SEER roughly cuts cooling electricity use in half. The IRA provides a 30% tax credit (up to $600) for qualifying high-SEER AC units. See the full breakdown in our AC Running Cost Calculator or compare directly in Central AC vs Window Unit.
kWh (Kilowatt-Hour)
A kilowatt-hour is the unit your electric utility bills you on. It represents the energy used by a 1,000-watt (1 kW) device running for one hour — or equivalently, a 100-watt device running for 10 hours. Your electric bill is simply: kWh used × your rate per kWh.
Where you'll see it: Every line of every appliance cost calculation on this site starts here. A 1,500W space heater running 1 hour uses 1.5 kWh; at the U.S. average rate of ~16¢/kWh, that's $0.24.
Typical U.S. rates: Idaho/Pacific Northwest ~9¢/kWh, national average ~16¢/kWh, California ~25¢/kWh, Hawaii ~40¢/kWh. Your exact rate is on your utility bill. Every calculator on this site — from Appliance Energy Cost to Home Energy Cost — starts from this unit.
R-Value
R-value measures a material's resistance to heat flow — essentially, how well it insulates. Higher R-value means better insulating performance and slower heat transfer through walls, attics, floors, and windows.
Where you'll see it: Insulation product labels (R-13 to R-49 for common batts and blown-in materials), and implicitly in window ratings (a single-pane window is about R-1; a good triple-pane window can reach R-5 to R-8).
Why it matters: Recommended attic insulation in most U.S. climates is R-38 to R-60; many older homes have only R-11 to R-19, losing significant heat through the roof in winter and gaining heat in summer. See our Insulation Savings Calculator and the Window Insulation vs Replacement comparison for cost-effective upgrade paths.
CO2e (Carbon Dioxide Equivalent)
CO2e is a standardized way to express the climate impact of different greenhouse gases on one scale. Methane, nitrous oxide, and other gases trap heat far more effectively than CO2 per molecule — CO2e converts their impact into "how much CO2 would cause the same warming."
Where you'll see it: Every carbon footprint calculator on this site — flight emissions, driving emissions, diet carbon, and home energy carbon — reports results in kg or tonnes of CO2e, not just CO2, because it captures the full climate impact including methane from sources like livestock and natural gas leaks.
Example: Methane has roughly 28–36x the warming potential of CO2 over 100 years. A diet with significant beef consumption has a much higher CO2e footprint than its direct CO2 alone would suggest, because of methane from cattle digestion. See our Diet Carbon Calculator and Carbon Footprint Calculator.
Therm
A therm is a unit of natural gas energy equal to 100,000 BTU. Gas utilities bill residential customers per therm used for heating, water heating, and cooking — similar to how electric utilities bill per kWh.
Where you'll see it: Gas furnace and gas dryer cost calculations. At a typical U.S. price of $1.00–1.30 per therm, gas heat costs significantly less per unit of energy than electricity — roughly 3–4x cheaper per BTU-equivalent in most markets.
Why it matters: This price gap is why electric space heaters rarely save money compared to gas central heat unless combined with a significant thermostat setback. See Heating Cost by Fuel Type and Central Heat vs Space Heater for the full comparison.
ENERGY STAR
ENERGY STAR is a certification program run by the U.S. EPA and Department of Energy. Products earning the label meet efficiency standards that typically use 10–50% less energy than standard equivalents, depending on the category.
Where you'll see it: Refrigerators, dishwashers, washing machines, dryers, water heaters, windows, and HVAC equipment. Many ENERGY STAR products also qualify for federal tax credits under the Inflation Reduction Act (IRA) — up to 30% for heat pumps, heat pump water heaters, and heat pump dryers, and up to $600/year for qualifying windows and AC units.
Why it matters: When comparing an older appliance to a new ENERGY STAR model, the certification is a reliable shortcut for "this will use meaningfully less energy" without needing to compare spec sheets line by line. See Refrigerator Energy Cost and Appliance Energy Cost Calculator.
COP (Coefficient of Performance)
COP measures heat pump efficiency as a ratio: units of heating or cooling delivered per unit of electricity consumed. A COP of 3 means the heat pump delivers 3 kWh of heat for every 1 kWh of electricity it uses — moving heat rather than generating it from scratch.
Where you'll see it: Heat pump specifications, heat pump water heaters, and heat pump dryers. Standard electric resistance heating (baseboards, space heaters) has a COP of exactly 1 — one unit of heat per unit of electricity. Modern heat pumps typically run COP 2.5–4 in moderate climates, meaning they're 2.5–4x more efficient than resistance heating for the same warmth.
Why it matters: This is the core reason heat pumps are the recommended electric heating upgrade over baseboards or space heaters — same fuel (electricity), far more heat delivered per dollar. See Heat Pump vs Furnace and Gas Dryer vs Heat Pump Dryer.
MPGe (Miles Per Gallon Equivalent)
MPGe converts an electric vehicle's energy consumption into a gasoline-equivalent fuel economy figure, using the EPA's conversion factor of 33.7 kWh = 1 gallon of gasoline energy-equivalent. It allows EVs and gas cars to be compared on the same "miles per unit of fuel" scale.
Where you'll see it: EV window stickers and fuel economy comparisons. A typical EV rated at 100 MPGe uses about 34 kWh to travel 100 miles — vastly more energy-efficient per mile than even the best hybrid, though the "fuel" (electricity) source and cost differ from gasoline.
Why it matters for cost, not just efficiency: MPGe alone doesn't tell you the dollar cost — that depends on your electricity rate. See our Car vs EV Carbon Calculator and Gas Car vs Hybrid Carbon comparison for full cost and emissions context.
HSPF (Heating Seasonal Performance Factor)
HSPF measures heat pump heating efficiency over a typical heating season — total heat output (BTU) divided by total electricity consumed (watt-hours). It's the heating-mode counterpart to SEER (which measures cooling). Higher HSPF means lower heating costs for the same heat pump.
Where you'll see it: Heat pump specification sheets, usually listed alongside SEER since most heat pumps provide both heating and cooling. Federal minimum as of 2023 is HSPF2 7.5 for most regions. Cold-climate heat pumps designed for sub-freezing performance are rated separately and can maintain efficiency down to -15°F or lower.
Why it matters: A heat pump with a higher HSPF costs less to run through a full winter than a lower-rated unit of the same capacity. Compare options in Heat Pump vs Furnace.
AFUE (Annual Fuel Utilization Efficiency)
AFUE is the percentage of fuel energy a furnace or boiler converts into usable heat over a year, accounting for start-up/shutdown losses and standby heat loss. An AFUE of 95% means 95 cents of every fuel dollar becomes heat in your home; the remaining 5% is lost, mostly as exhaust heat.
Where you'll see it: Gas and oil furnace ratings. Federal minimum is 80% AFUE for most non-weatherized furnaces (90%+ for the Northern climate zone). High-efficiency "condensing" furnaces reach 95–98.5% AFUE by recapturing heat that older units simply vent outside.
Why it matters: Upgrading from an 80% AFUE furnace to a 96% AFUE furnace cuts gas usage by roughly 17% for the same heat output. See Heating Cost by Fuel Type for the full cost comparison across furnace types.
Watt vs Kilowatt
A watt is the base unit of electrical power — the rate at which a device uses energy at any given instant. A kilowatt (kW) equals 1,000 watts. Wattage tells you the rate of energy use; multiplying by time in hours gives you kWh, the actual energy consumed and what you're billed for.
Where you'll see it: Every appliance's nameplate or spec sheet — a hair dryer might be 1,500W, a refrigerator 150W (but running intermittently), a central AC 3,000–5,000W (3–5 kW). Formula: kWh = Watts ÷ 1,000 × Hours used.
Why it matters: This is the base formula behind every calculator on this site. See Appliance Energy Cost Calculator to apply it to any device in your home.
Time-of-Use (TOU) Rate
A time-of-use rate structure charges different prices for electricity depending on when you use it. Peak hours (typically afternoon through evening, when overall grid demand is highest) cost more per kWh; off-peak hours (typically overnight and early morning) cost less — sometimes 30–50% less.
Where you'll see it: Increasingly common in states with high renewable penetration (California, much of the Northeast) as utilities try to shift demand away from peak hours. Not all utilities offer TOU pricing — check your bill or utility website.
Why it matters: Shifting high-energy activities (laundry, dishwashing, EV charging, dryer use) to off-peak hours can meaningfully cut costs with no change in usage amount. See Dryer Cost Guide for an example of the savings.
Embodied Carbon
Embodied carbon is the total CO2 emitted during a product's manufacturing, material extraction, transport, and installation — distinct from "operational carbon," which is the emissions produced while using the product (e.g., electricity for a device, fuel for a car).
Where it matters: Solar panels, EVs, and heat pumps all have meaningful embodied carbon from manufacturing — but for products used over many years, the operational carbon savings versus a less-efficient alternative typically outweigh the embodied carbon within 1–3 years of use. For solar panels specifically, embodied carbon is usually offset within 1–4 years of clean electricity generation, out of a 25–30 year lifespan.
Why it matters for decision-making: "It takes more energy to make than it saves" is rarely true for modern efficient products, but embodied carbon is a legitimate factor in the full lifecycle picture. See our Solar CO2 Offset Calculator for a payback-based view.