A sinusoidal quantity having a frequency that is an integral multiple of the frequency of a periodic quantity to which it is related.


A unit of pressure for a fluid, commonly used in water pumping and hydro power to express height a pump must lift water, or the distance water falls. Total head accounts for friction head losses, etc.


A form of thermal energy resulting from combustion, chemical reaction, friction, or movement of electricity. As a thermodynamic condition, heat, at a constant pressure, is equal to internal or intrinsic energy plus pressure times volume.

Heat Absorbing Window Glass

A type of window glass that contains special tints that cause the window to absorb as much as 45% of incoming solar energy, to reduce heat gain in an interior space. Part of the absorbed heat will continue to be passed through the window by conduction and reradiation.

Heat Balance

Energy output from a system that equals energy input.

Heat Content

The amount of heat in a quantity of matter at a specific temperature and pressure.

Heat Engine

A device that produces mechanical energy directly from two heat reservoirs of different temperatures. A machine that converts thermal energy to mechanical energy, such as a steam engine or turbine.

Heat Exchanger

A device used to transfer heat from a fluid (liquid or gas) to another fluid where the two fluids are physically separated.

Heat Gain

The amount of heat introduced to a space from all heat producing sources, such as building occupants, lights, appliances, and from the environment, mainly solar energy.

Heating Capacity (Also specific heat)

The quantity of heat necessary to raise the temperature of a specific mass of a substance by one degree.

Heating Degree Day(s) (HDD)

The number of degrees per day that the daily average temperature (the mean of the maximum and minimum recorded temperatures) is below a base temperature, usually 65 degrees Fahrenheit, unless otherwise specified; used to determine indoor space heating requirements and heating system sizing. Total HDD is the cumulative total for the year/heating season. The higher the HDD for a location, the colder the daily average temperature(s).

Heating Fuels

Any gaseous, liquid, or solid fuel used for indoor space heating.

Heating Fuel Units

Standardized weights or volumes for heating fuels.

Heating Load

The rate of heat flow required to maintain a specific indoor temperature; usually measured in Btu per hour.

Heating Season

The coldest months of the year; months where average daily temperatures fall below 65 degrees Fahrenheit creating demand for indoor space heating.

Heating Seasonal Performance Factor (HSPF)

The measure of seasonal or annual efficiency of a heat pump operating in the heating mode. It takes into account the variations in temperature that can occur within a season and is the average number of Btu of heat delivered for every watt-hour of electricity used by the heat pump over a heating season.

Heating Value

The amount of heat produced from the complete combustion of a unit of fuel. The higher (or gross) heating value is that when all products of combustion are cooled to the pre-combustion temperature, water vapor formed during combustion is condensed, and necessary corrections have been made. Lower (or net) heating value is obtained by subtracting from the gross heating value the latent heat of vaporization of the water vapor formed by the combustion of the hydrogen in the fuel.

Heating, Ventilation, and Air-Conditioning (HVAC) System

All the components of the appliance used to condition interior air of a building.

Heat Loss

The heat that flows from the building interior, through the building envelope to the outside environment.

Heat Pipe

A device that transfers heat by the continuous evaporation and condensation of an internal fluid.

Heat Pump

An electricity powered device that extracts available heat from one area (the heat source) and transfers it to another (the heat sink) to either heat or cool an interior space or to extract heat energy from a fluid.

Heat Pump Water Heaters

A water heater that uses electricity to move heat from one place to another instead of generating heat directly.

Heat Rate

The ratio of fuel energy input as heat per unit of net work output; a measure of a power plant thermal efficiency, generally expressed as Btu per net kilowatt-hour.

Heat Recovery Ventilator

A device that captures the heat from the exhaust air from a building and transfers it to the supply/fresh air entering the building to preheat the air and increase overall heating efficiency.

Heat Register

The grilled opening into a room by which the amount of warm air from a furnace can be directed or controlled; may include a damper.

Heat Sink

A structure or media that absorbs heat.

Heat Source

A structure or media from which heat can be absorbed or extracted.

Heat Storage

A device or media that absorbs heat for storage for later use.

Heat Storage Capacity

The amount of heat that a material can absorb and store.

Heat Transfer

The flow of heat from one area to another by conduction, convection, and/or radiation. Heat flows naturally from a warmer to a cooler material or space.

Heat Transfer Fluid

A gas or liquid used to move heat energy from one place to another; a refrigerant.

Heat Transmission Coefficient

Any coefficient used to calculate heat transmission by conduction, convection, or radiation through materials or structures.

Heliochemical Process

The utilization of solar energy through photosynthesis.


A device used to simulate the angle of the sun for assessing shading potentials of building structures or landscape features.


A device that tracks the movement of the sun; used to orient solar concentrating systems.


Any process that uses solar radiation to produce useful heat.


Site planning that accounts for natural solar heating and cooling processes and their relationship to building shape, orientation, and siting.


An instrument for measuring solar radiation.


Any device (or plant) that follows the sun’s apparent movement across the sky.

Hemispherical Bowl Technology

A solar energy concentrating technology that uses a linear receiver that tracks the focal area of a reflector or array of reflectors.


A measure of the number of cycles or wavelengths of electrical energy per second; U.S. electricity supply has a standard frequency of 60 hertz.


A region of electrical contact between two different materials.

Higher Heating Value (HHV)

The maximum heating value of a fuel sample, which includes the calorific value of the fuel (bone dry) and the latent heat of vaporization of the water in the fuel. (See moisture content and net (lower) heating value, below.)

High-Intensity Discharge Lamp

A lamp that consists of a sealed arc tube inside a glass envelope, or outer jacket. The inner arc tube is filled with elements that emit light when ionized by electric current. A ballast is required to provide the proper starting voltage and to regulate current during operation.

High-Pressure Sodium Lamp

A type of High-Intensity Discharge (HID) lamp that uses sodium under high pressure as the primary light-producing element. These high efficiency lights produce a golden white color and are used for interior industrial applications, such as in warehouses and manufacturing, and for security, street, and area lighting.


The vacancy where an electron would normally exist in a solid; behaves like a positively charged particle.

Home Energy Rating Systems (HERS)

A nationally recognized energy rating program that gives builders, mortgage lenders, secondary lending markets, homeowners, sellers, and buyers a precise evaluation of energy losing deficiencies in homes. Builders can use this system to gauge the energy quality in their home and also to have a star rating on their home to compare to other similarly built homes.


The region between an n-layer and a p-layer in a single material, photovoltaic cell.

Horizontal-Axis Wind Turbines

Turbines in which the axis of the rotor’s rotation is parallel to the wind stream and the ground.

Horizontal Ground Loop

In this type of closed-loop geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are laid out in a plane parallel to the ground surface. The most common layouts either use two pipes, one buried at six feet, and the other at four feet, or two pipes placed side-by-side at five feet in the ground in a two-foot wide trench. The trenches must be at least four feet deep. Horizontal ground loops are generally most cost-effective for residential installations, particularly for new construction where sufficient land is available. Also see closed-loop geothermal heat pump systems.

Horsepower (hp)

A unit of rate of operation. Electrical hp: a measure of time rate of mechanical energy output; usually applied to electric motors as the maximum output; 1 electrical hp is equal to 0.746 kilowatts or 2,545 Btu per hour. Shaft hp: a measure of the actual mechanical energy per unit time delivered to a turning shaft; 1 shaft Hp is equal to 1 electrical Hp or 550 foot pounds per second. Boiler Hp: a measure to the maximum rate to heat output of a steam generator; 1 boiler Hp is equal to 33,480 Btu per hour steam output.

Horsepower Hour (hph)

One horsepower provided over one hour; equal to 0.745 kilowatt-hour or 2,545 Btu.

Hot Air Furnace

A heating unit where heat is distributed by means of convection or fans.

Hot Dry Rock

A geothermal energy resource that consists of high temperature rocks above 300 F (150 C) that may be fractured and have little or no water. To extract the heat, the rock must first be fractured, then water is injected into the rock and pumped out to extract the heat. In the western United States, as much as 95,000 square miles (246,050 square km) have hot dry rock potential.

Hot Water Heating Systems

(See Hydronic)

Hub Height

The height above the ground that a horizontal axis wind turbine’s hub is located.


A device used to maintain a specified humidity in a conditioned space.


A measure of the moisture content of air; may be expressed as absolute, mixing ratio, saturation deficit, relative, or specific.

Hybrid System

A renewable energy system that includes two different types of technologies that produce the same type of energy; for e.g., a wind turbine and a solar photovoltaic array combined to meet a power demand.

Hydroelectric Power Plant

A power plant that produces electricity by the force of water falling through a hydro turbine that spins a generator.


A chemical element that can be used as a fuel since it has a very high energy content.

Hydrogenated Amorphous Silicon

Amorphous silicon with a small amount of incorporated hydrogen. The hydrogen neutralizes dangling bonds in the amorphous silicon, allowing charge carriers to flow more freely.

Hydronic Heating Systems

A type of heating system where water is heated in a boiler and either moves by natural convection or is pumped to heat exchangers or radiators in rooms; radiant floor systems have a grid of tubing laid out in the floor for distributing heat. The temperature in each room is controlled by regulating the flow of hot water through the radiators or tubing.

Hydrothermal fluids

These fluids can be either water or steam trapped in fractured or porous rocks; they are found from several hundred feet to several miles below the Earth’s surface. The temperatures vary from about 90 F to 680 F (32 C to 360 C) but roughly 2/3 range in temperature from 150 F to 250 F (65.5 C to 121.1 C). The latter are the easiest to access and, therefore, the only forms being used commercially.