Electrical and Power Units Glossary
Plain-English definitions of every power and electrical unit on this site: watts, kilowatts, kWh, volts, amps, kVA, kVAR, power factor, horsepower, and BTU.
Common conversions
| Input | Result |
|---|---|
| W | Power (base SI unit) |
| kW | Power, 1,000 W |
| MW | Power, 1,000 kW |
| kWh | Energy, 1 kW for 1 hour |
| V | Electrical pressure |
| A | Electric current |
| kVA | Apparent power |
| HP | Mechanical power, 745.7 W |
The math behind it
- kW = kVA × PF = 50 × 0.8
- kW = 40 kW
- A = (kW × 1000) / (√3 × V × PF) = 40,000 / (1.732 × 480 × 0.8)
- A ≈ 60.2 A
Everything you need to know
This glossary defines every power and electrical unit used across this site's calculators, in plain language, with each unit's relationship back to the kilowatt. Use it alongside the live calculator above to check what a field or result actually means.
Power and energy units
Watt (W): the base SI unit of power, equal to one joule of energy transferred per second. Every other power unit on this site is a multiple or conversion of the watt.
Kilowatt (kW): 1,000 watts. This is the standard unit for sizing residential and light-commercial electrical equipment, and the unit most of this site's calculators convert to or from.
Megawatt (MW): 1,000 kilowatts, or 1,000,000 watts. Used for utility-scale generation and large industrial loads, well beyond the range a single building typically needs.
Kilowatt-hour (kWh): a unit of energy, not power, equal to 1 kW sustained for 1 hour. It's the unit your electric utility bills you in, found by multiplying power (kW) by time (hours). See the kW vs kWh page for the full distinction.
Electrical units
Volt (V): the unit of electrical pressure, or potential difference, that pushes current through a circuit. Higher voltage moves the same power with less current, which is why long-distance transmission lines run at very high voltages.
Ampere (A): the unit of electric current, the rate of charge flow past a point in a circuit. Amps, combined with voltage, determine wire gauge and breaker size.
Volt-amp (VA) and kilovolt-amp (kVA): apparent power, the product of voltage and current without accounting for phase shift. kVA is always equal to or greater than kW, and the two are equal only at a power factor of 1.0. Generators, transformers, and UPS units are rated in kVA because that's what their windings and conductors must actually carry.
Volt-amp reactive (VAR) and kilovolt-amp reactive (kVAR): reactive power, the power that inductive and capacitive loads store and release each AC cycle without doing useful work. It doesn't appear directly on a utility bill but drives up the current a circuit must carry for a given kW.
Power factor (PF): the ratio of real power to apparent power, kW divided by kVA, always between 0 and 1. A PF of 1.0 means every watt supplied does useful work; a lower PF means more current is needed to deliver the same real power.
Mechanical and thermal units
Horsepower (HP): a unit of mechanical power. Mechanical (imperial) horsepower equals 745.7 W, electrical horsepower equals 746 W, and metric horsepower (PS) equals 735.5 W. Motors are commonly rated in HP even though their electrical input is measured in kW.
British thermal unit (BTU): a unit of heat energy, the amount needed to raise one pound of water by one degree Fahrenheit. On its own, BTU measures a quantity of heat, similar to how a joule measures a quantity of energy.
BTU per hour (BTU/hr): a rate of heat transfer, the thermal equivalent of a watt. HVAC equipment lists heating or cooling capacity in BTU/hr; 1 kW equals 3,412.14 BTU/hr.
Ton of refrigeration: a unit of cooling capacity equal to 12,000 BTU/hr, or about 3.5169 kW. One ton is the rate of heat absorption needed to melt one short ton of ice in 24 hours, and it's the standard sizing unit for residential and commercial air conditioning.
Frequency
Hertz (Hz): the unit of frequency, one cycle per second. It describes how many times AC voltage reverses direction and back each second, 60 Hz in North America and most of the Americas, 50 Hz across most of Europe, Asia, and Africa. Frequency doesn't appear directly in the power formulas on this site, but it defines what "AC" means as opposed to constant-polarity DC, and it must match between a generator and the loads it feeds.
Common applications
Nameplates list kVA, PF, voltage, and amps together. Knowing what each unit means lets you calculate the actual usable kW output instead of mistaking the kVA rating for real power capacity.
Cooling capacity appears in BTU/hr or tons, while electrical draw appears in kW or amps. Converting between them is required to compare two units from different manufacturers on the same basis.
Bills separate energy (kWh) from, on many commercial accounts, peak demand (kW). Knowing the difference explains why a bill can rise even when total kWh use stays flat, if peak demand increased.
Motor nameplates combine HP (mechanical output), amps, volts, and PF. Converting HP to electrical input kW and cross-checking it against V × A × PF confirms the nameplate numbers are internally consistent.
Common mistakes
A 100 kVA generator at PF 0.8 delivers only 80 kW. Assuming kVA equals kW at any power factor below 1.0 leads to a real-power shortfall that shows up under load.
kWh is a unit of energy, not power. Labeling an appliance's instantaneous power draw as kWh instead of kW makes the number impossible to compare against other equipment ratings.
Mechanical HP (0.7457 kW), electrical HP (0.746 kW), and metric PS (0.7355 kW) are close but not identical. Picking the wrong one introduces a small but compounding error across multi-motor installations.
Frequency (Hz) describes how fast AC reverses direction, not how much power flows. A motor rated for 50 Hz run on a 60 Hz supply can change speed and current draw, but Hz itself never appears in the basic kW formulas.
Frequently asked questions
What is the difference between W and kW?+
A kilowatt is 1,000 watts. The watt is the base SI unit of power; kW is simply a more convenient scale for household and commercial equipment, where ratings in plain watts would run into the thousands.
Is kVA the same as kW?+
No. kVA is apparent power, the total power a source must supply; kW is real power, the portion that does useful work. A 100 kVA generator at PF 0.8 delivers only 80 kW, and the two are equal only at a power factor of exactly 1.0.
What does VAR stand for?+
Volt-amp reactive, the unit of reactive power, the power inductive and capacitive loads store and release each AC cycle. kVAR is the kilo-scale version used for larger loads.
How many watts is 1 horsepower?+
745.7 W for mechanical (imperial) horsepower, 746 W for electrical horsepower, or 735.5 W for metric horsepower (PS). The three constants differ by less than 1.5%, but the correct one depends on which nameplate standard you're reading.
What is a kVAR used for?+
kVAR quantifies the reactive power a motor, transformer, or ballast draws and returns each cycle. Utilities and capacitor-bank sizing calculations use it to determine how much correction is needed to bring a facility's power factor closer to 1.0.
How many BTU/hr is 1 kW?+
3,412.14 BTU/hr. This constant converts any electrical kW rating into the thermal units HVAC equipment specs are usually written in.
What is a ton of refrigeration in kW?+
About 3.5169 kW. One ton equals 12,000 BTU/hr, the historical rate of heat needed to melt one short ton of ice in a day, still the standard sizing unit for air conditioning capacity.
What does Hz measure, and does it affect kW calculations?+
Hertz measures frequency, the number of AC cycles per second, 60 Hz in North America and 50 Hz across most of Europe and Asia. It doesn't appear directly in the kW formulas on this site, but it defines AC versus DC and must match between a power source and its connected loads.
Is 1 MW equal to 1,000 kW?+
Yes, a megawatt is exactly 1,000 kilowatts, or 1,000,000 watts. Utility-scale generation and transmission figures are usually quoted in MW because kW would produce unwieldy six- and seven-digit numbers.
What is power factor measured in?+
Power factor has no unit of its own; it's a dimensionless ratio between 0 and 1, equal to real power (kW) divided by apparent power (kVA). A PF of 0.9 means 90% of the apparent power supplied does useful work.