Home Energy Glossary

Energy documents use compact terms that can obscure the assumptions behind a bill or equipment comparison. These definitions are general orientation, not a rating of a particular product or a promise about performance. Labels, test procedures and utility tariffs can be revised, so consult the cited DOE, EIA, NREL and ENERGY STAR resources for current technical explanations.

Electricity and heat units

Watt (W): a unit of power, or the rate of energy use. Kilowatt (kW): 1,000 watts. Kilowatt-hour (kWh): energy used by a 1 kW load over one hour; this is the common household electricity billing unit. Btu: British thermal unit, a measure of heat energy. MMBtu: one million Btu. Therm: a natural-gas billing unit equal to 100,000 Btu under the EIA conversion used by WattWise.

Heating and cooling performance

AFUE: annual fuel utilization efficiency, a seasonal measure of how much fuel energy a furnace converts to useful heat under its rating procedure. COP: coefficient of performance, the ratio of useful heat delivered to energy consumed under stated conditions; a seasonal COP represents performance across a defined period, while actual outcomes depend on climate and equipment. HSPF2: a heating seasonal performance factor under a current heat-pump test procedure. SEER2: a seasonal cooling efficiency measure under the updated test procedure. These labels use distinct test frameworks and are not interchangeable as direct guarantees of a home's energy bill.

Heat pump: equipment that transfers heat between indoor and outdoor spaces rather than relying only on direct fuel combustion or resistance heating. Resistance heat: heat made by passing electric current through a resistive element. Delivered heat: useful heat reaching the conditioned space, distinct from fuel or electricity input. DOE's heat-pump and Energy Saver resources describe these technologies and household efficiency topics.

Solar and utility billing language

PV: photovoltaic technology that converts sunlight to electricity. PVWatts: NREL's web calculator for estimating grid-connected photovoltaic energy production from system and location assumptions. Degradation: a reduction in a solar panel system's output over time; a model assumption is not the same as a warranty claim. Net metering: a utility billing arrangement that may credit customer exports against imports under a tariff. Net billing: a framework in which imported and exported electricity can be valued separately. Export credit: the value assigned to electricity sent to the grid. The utility's current tariff defines the actual treatment.

Time-of-use rate: a price schedule that varies by time period. Demand charge: a bill component based on a measured peak rate of electricity use during a defined interval, appearing in some tariffs and customer classes. Fixed customer charge: a bill amount that does not directly scale with kWh in the billing period. Effective average rate: total bill divided by billed kWh; marginal rate is the incremental cost of another unit and can differ.

Vehicle and household terms

kWh per mile: electricity consumed for each mile in an EV efficiency calculation. MPGe: a miles-per-gallon-equivalent comparison metric; it does not mean a vehicle uses gasoline. Standby load: power drawn while a device is not performing its main task but remains connected or ready. Duty cycle: the portion of elapsed time a device operates at a given power level. These distinctions help prevent nameplate wattage from being multiplied by every hour a plug is occupied.

ENERGY STAR provides household energy guidance; EIA explains energy prices and conversions; DOE explains heat pumps; and NREL hosts PVWatts. Their pages describe general terms, while a utility, manufacturer, installer or vehicle documentation supplies specifics for an individual setup. A glossary cannot establish whether a tariff, rating or equipment configuration fits a particular residence.