Otis Singleton
When it comes to electricity, there are various units of measurement, each with its own unique scale. The largest electric unit is the terawatt-hour (TWh), which is often used to express major energy production or consumption over a calendar or financial year. Other commonly used units of electrical power for appliances include watts (W), kilowatts (kW), British thermal units (BTU), horsepower (hp), and tons. The choice of units and scale can often lead to confusion, especially when big numbers are quoted inconsistently or without a sense of scale.
| Characteristics | Values |
|---|---|
| Unit of Kilowatt-hour | kWh |
| Common Representation | kW⋅h, kW h, KWh, KWH, kwh |
| Used in | Billing for delivered energy to consumers, commercial, educational, and scientific publications, media, electrical power engineering |
| Relation to Watt-hour | 1 kWh = 1000 Wh |
| Relation to Megawatt-hour | 1 kWh = 0.001 MWh |
| Relation to Milliwatt-hour | 1 kWh = 1000000 mWh |
| Relation to Terawatt-hour | 1 TWh = 1,000,000,000 kWh |
| Average Monthly Consumption in the US in 2022 | 899 kWh |
| Average Monthly Consumption in the US in 2020 | 893 kWh |
| Average to Boil 1 Litre of Water with an Electric Kettle | 0.1 kWh |
| Average for a 12-Watt LED Lamp in 24 Hours | 0.3 kWh |
| Average for a 12-Watt LED Lamp per Month | 9 kWh |
| Average Daily Output of Canada's Bruce Station | 138,000 MWh |
| Average Daily Output of India's Kaiga Atomic Power Station | 6100 MWh |
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What You'll Learn
Kilowatt-hours (kWh)
Kilowatt-hour (kWh) is a unit of energy that measures the amount of energy used over time. It is a metric unit, where 1 kilowatt-hour is equal to 1 kilowatt (1,000 watts) of power used for one hour. In other words, it is the amount of energy used by a 1,000-watt appliance running for one hour.
Kilowatt-hours are commonly used by electrical energy providers for billing purposes, as they represent the amount of energy consumed by a device over a period of time. The cost of running an electrical device can be calculated by multiplying the device's power consumption in kilowatts by the operating time in hours and the price per kilowatt-hour.
It is important to distinguish between kilowatt-hour (kWh) and kilowatt (kW). While related, they serve different purposes. A kilowatt is a measure of power, or the rate at which energy is used, while a kilowatt-hour is a measure of energy, or the capacity to do work.
To calculate the kilowatt-hours consumed by an appliance, one must first estimate the amount of time the appliance is used and note its wattage, which is usually found on the appliance's label. Then, the wattage is converted from watts (W) to kilowatts (kW) by dividing the number of watts by 1,000. Finally, the kilowatts are multiplied by the hours of daily use to obtain the kilowatt-hours per day.
For example, a 1,000-watt vacuum cleaner used for one hour will consume 1 kWh of energy. Similarly, a 12-watt LED lamp lit constantly uses about 0.3 kWh per 24 hours and about 9 kWh per month.
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Watt-hours (Wh)
Watt-hours are calculated by multiplying the power rating of an appliance in watts by the number of hours it is used. For example, a 1000-watt air conditioner used for four hours consumes 4,000 watt-hours of energy. This is because watt-hours are a measure of energy, while watts represent the rate at which energy is produced or consumed, or the electrical flow.
To understand the difference, consider a common analogy: speed and distance. Speed is the rate at which you are travelling at a given moment, whereas distance is the total length you travel over a period of time. So, if you drive at 60 miles per hour for one hour, you will have travelled 60 miles. Similarly, a 60-watt light bulb left on for one hour will have used 60 watt-hours of energy. If left on for two hours, it will have used 120 watt-hours.
Watt-hours are useful for understanding your energy consumption and can help you calculate your energy savings and monetary savings on utility bills. They are also used to discuss energy production and consumption on a larger scale, such as for business establishments or entire countries, where terawatt-hours may be used as a measurement unit.
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Megawatt-hours (MWh)
MWh are often used to measure the output of power plants and the electricity consumption of large areas, such as cities, states, or countries. For example, in 2016, the United States produced 141,181 thousand MWh of electricity. The world's largest nuclear power station, Canada's Bruce station, has a maximum capacity of 6,384 MW, and an estimated average daily output of 138,000 MWh.
MWh are also used to quantify the energy output of renewable energy sources. For instance, Iran's largest solar PV park, Jarqavieh, has a capacity of 10 MW and produces an average of 48 MWh daily. Hydropower production also varies widely in scale, from large production facilities to small "pico hydro" schemes for individual households.
A single MWh is a substantial amount of energy. It is enough to keep two refrigerators or two 60-watt light bulbs running for a year, or to drive an electric vehicle 3,600 miles. In the UK, one MWh could power around 2,000 homes for an hour, while in the US, one MWh could power an average household for 1.2 months.
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Terawatt-hours (TWh)
A terawatt-hour (TWh) is a unit of energy that is equal to one trillion watt-hours. It is also equal to 1,000,000 megawatt-hours (MWh) or 1,000,000,000 kilowatt-hours (kWh). To put it into perspective, one kilowatt-hour represents a consistent power of one kilowatt operating for one hour.
Terawatt-hours are used to measure large amounts of electrical energy to make it easier to understand in a practical context. For example, in 2022, the United States' electricity consumption was predicted to reach 4,010 terawatt-hours. This massive consumption is largely attributed to the residential sector, followed by the commercial sector, with appliances used for heating and cooling accounting for a significant share.
The energy consumption of all humans is estimated to be approximately 17.7 terawatts, with the US sharing about 17% of that figure. This equates to about 17.7 terawatt-hours of energy consumption for the entire human race, with the US consuming around 17% of that amount.
Terawatt-hours are especially useful when discussing the annual electricity generation of entire countries. They provide a standardised unit of measurement for energy data, making it easier to compare and contextualise large-scale energy production and consumption.
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BTU per hour (BTU/h)
BTU/h, or BTU per hour, is a unit of power that represents the energy transfer rate of British Thermal Units (BTU) per hour. BTU is a unit of measurement that shows how much energy a cooling unit uses to remove heat from a home within an hour. Specifically, one BTU equals the amount of heat required to raise or lower the temperature of one pound of water by one degree Fahrenheit. In the context of power, BTU/h is often used to represent the power of appliances, such as air conditioning systems.
Air conditioning systems are rated by their total BTU output, which is determined by multiplying the system's tonnage by 12,000. Tonnage refers to how much heat an AC unit can absorb. One ton of cooling is the amount of heat needed to melt one pound of ice in 24 hours. The higher the BTU rating, the larger the room or space the system can cool.
BTU per hour is also used as a benchmark to estimate the capacity of heating systems, such as gas furnaces. For example, Lennox furnaces are available in heating capacities ranging from 30,000 to 135,000 BTU/h. A furnace with a lower BTU/h range is typically designed for small multifamily homes, single-family homes, or a single floor in a two-story home. Higher-capacity furnaces are made for larger single-family homes, large single-story homes, and light commercial applications.
It's important to note that while BTU/h is a common unit of measurement, other units of power include watts (W), kilowatts (kW), horsepower (hp), and tons. Watts (W) is a unit of power used to quantify the rate of energy transfer and is defined as one joule per second. A kilowatt is a multiple of a watt, with one kilowatt (kW) equal to 1,000 watts.
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Frequently asked questions
The largest electric unit hour is the kilowatt-hour (kWh).
A kilowatt-hour is a unit of energy equal to the energy used to maintain one kilowatt of power for one hour.
A kilowatt-hour is equal to 1000 watt-hours.
Electricity is typically measured and paid for in kilowatt-hours.
The cost of running an electrical device can be calculated by multiplying the device's power consumption in kilowatts by the operating time in hours and the cost per kilowatt-hour.
