Glossary

AFUE

Annual fuel utilization efficiency: the share of a furnace’s fuel energy that becomes heat in your home over a year. A 95% AFUE gas furnace turns 95% of the gas it burns into heat.

Used in: Heat pump vs furnace

BTU

British thermal unit: the heat needed to warm one pound of water by 1°F. AC and furnace capacity is given in BTU per hour; 12,000 BTU/h is one “ton” of cooling.

Climate normals

Thirty-year averages of temperature, rainfall and frost odds for each weather station, published by NOAA. The current set covers 1991–2020.¹ Every date and temperature in our tools comes from them.

Used in: Frost dates by ZIP code

COP (coefficient of performance)

How many units of heat a heat pump delivers for each unit of electricity it uses. A COP of 3 means three times as much heat as electricity; COP falls as the outdoor air gets colder.

Used in: Heat pumps in cold weather

CYA (cyanuric acid)

Pool stabilizer that shields chlorine from sunlight. Pentair’s IntelliChlor guide recommends keeping it at 30–50 ppm.⁴

Used in: Ideal pool chemical levels, How much shock to add

Degree day

A measure of how much heating or cooling a climate needs. Each degree the daily average falls below 65°F adds one heating degree day; each degree above adds one cooling degree day.¹

Used in: Heat pump vs furnace

Derating

A cut in a machine’s output when it runs outside the conditions it was rated for, such as at altitude or in heat. Honda’s generator manual puts the loss at about 3.5% for each 1,000 feet of altitude.³

Used in: Generator size calculator

Free chlorine

The chlorine in pool water that is still available to sanitize. It is what test kits report as “FC” and what you adjust.

Frost date

The median date of the last 32°F night in spring or the first in fall, from 30 years of station records.¹ One year in two, frost comes later than the spring median.

Used in: Frost dates by ZIP code, When to cover plants for frost

Hard freeze

A night at 28°F or colder. NOAA’s normals give 28°F dates and odds alongside the 32°F frost dates.¹ Below about 28°F, covering plants typically stops working.

Used in: When to cover plants for frost, When to overseed, Frost vs freeze, When to stop mowing

Hardiness zone

USDA’s map of the average coldest winter night, in 10°F zones split into “a” and “b” halves.² It tells you which perennials survive winter, not when to plant.

Used in: When to start seeds indoors, Planting zone vs frost date, When to plant tomatoes

HSPF2

Heating seasonal performance factor: a heat pump’s heating efficiency over a season, in BTU per watt-hour. Higher is better.

Manual J

The ACCA standard for calculating a home’s heating and cooling load room by room. Contractors should run one before sizing equipment.

Used in: Disclaimer

N-P-K

The three numbers on a fertilizer bag: percent nitrogen, phosphate and potash by weight. A 50 lb bag of 24-0-6 holds 12 lb of nitrogen.

Used in: When to fertilize your lawn

Overseeding

Spreading grass seed over an existing lawn to thicken it, without tearing the lawn out first.

Used in: When to overseed, When to aerate

ppm (parts per million)

Concentration of a chemical in water. Because a US gallon of water weighs about 8.34 lb,⁵ 1 ppm in 10,000 gallons is about 1.3 oz.

Used in: Pool salt calculator

Pre-emergent herbicide

A weed preventer that stops seeds such as crabgrass from establishing. It has to be down before the weeds sprout, which is why timing matters more than the product.

Used in: When to apply pre-emergent, When to aerate

Running watts

The power an appliance draws continuously once it is running. A generator’s running (rated) watts is what it can supply continuously; Honda says not to exceed it in continuous use.³

Used in: Generator size calculator

SEER2

Seasonal energy efficiency ratio: how efficiently an air conditioner cools over a season. Higher is better; the federal minimum depends on your region and the size of the system.

Starting watts

The short burst of extra power a motor draws as it starts, above its running watts. Also called surge watts. A refrigerator, for example, may need 800–1,200 starting watts.⁶

Used in: Generator size calculator

Transfer switch

A device that connects a generator to selected home circuits and isolates them from utility lines, so power can’t flow back to the grid. It should be installed by a licensed electrician.

Used in: Portable generator safety, Generator size calculator