Kilowatt-hour facts for kids
Quick facts for kids kilowatt-hour |
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Residential electricity meter located in Canada
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| General information | |
| Unit system | Non-SI metric |
| Unit of | Energy |
| Symbol | kW⋅h or kW h |
| Conversions | |
| 1 kW⋅h in ... | ... is equal to ... |
| SI units | 3.6 MJ |
| CGS units | 3.6×1013 erg |
| English Engineering units | ≈ 2,655,224 ft⋅lbf |
| British Gravitational units | ≈ 85,429,300 ft⋅pdl |
The kilowatt-hour (often written as kWh) is a way to measure energy. It tells you how much energy is used over a period of time. Imagine a light bulb that uses 1000 watts (which is 1 kilowatt). If you leave that light bulb on for one hour, it uses one kilowatt-hour of energy. This unit is super important because it's how we measure the electricity we use in our homes and schools.
Contents
What is a Kilowatt-Hour?
A kilowatt-hour (kWh) is a unit of energy. It combines two things: power (measured in kilowatts) and time (measured in hours). Think of it like this:
- A kilowatt (kW) is a measure of power. It's how fast energy is being used or produced. One kilowatt is 1,000 watts.
- An hour (h) is a measure of time.
So, a kilowatt-hour is simply 1 kilowatt of power used for 1 hour. It's equal to 3,600,000 joules (or 3.6 megajoules), which is the standard unit of energy in science.
How is kWh Written?
You'll most often see kilowatt-hour written as "kWh". This is the most common way, especially on your electricity bill. Sometimes, you might see it written as "kW⋅h" or "kW h". These ways are more technically correct in science, but "kWh" is widely accepted and understood. It's important not to confuse "kWh" with "kW/h". "kW/h" would mean kilowatts per hour, which is a different idea. It would describe how fast the power itself is changing, not the total energy used.
How Electricity is Sold
Electricity companies usually sell electricity to people and businesses in kilowatt-hours. When you get your electricity bill, it shows how many kWh you've used. To figure out how much it costs to run an electrical device, you multiply:
- The device's power (in kilowatts)
- The time it runs (in hours)
- The price per kilowatt-hour (what the electricity company charges)
For example, if you have a device that uses 1 kilowatt of power and you run it for 1 hour, you've used 1 kWh. If the electricity costs 15 cents per kWh, then running that device for an hour costs 15 cents.
Examples of kWh Use
Let's look at some examples of how much energy different things use:
- A large electric heater that uses 1000 watts (1 kilowatt) will use 1 kWh if it runs for one hour.
- A television that uses 100 watts will use 1 kWh if it runs continuously for 10 hours.
- A 40-watt light bulb will use 1 kWh if it stays on for 25 hours.
- A healthy adult doing hard manual work for eight hours uses about half a kilowatt-hour of energy.
Converting Energy Units
Energy can be measured in different units. The table below shows how kilowatt-hours relate to other energy units like joules and calories. To change a value from a unit in the left column to a unit in the top row, you multiply it by the number in the box where they meet.
| 1 J = 1 kg⋅m2⋅s−2 = | 1 | 2.77778 × 10−4 | 2.77778 × 10−7 | 6.241 × 1018 | 0.239 |
|---|---|---|---|---|---|
| 1 Wh = | 3.6 × 103 | 1 | 0.001 | 2.247 × 1022 | 859.8 |
| 1 kWh = | 3.6 × 106 | 1,000 | 1 | 2.247 × 1025 | 8.598 × 105 |
| 1 eV = | 1.602 × 10−19 | 4.45 × 10−23 | 4.45 × 10−26 | 1 | 3.827 × 10−20 |
| 1 cal = | 4.184 | 1.162 × 10−3 | 1.162 × 10−6 | 2.612 × 1019 | 1 |
| Joule | Watt-hour | Kilowatt-hour | Electronvolt | Calorie |
Larger Energy Units
Just like we have kilograms for 1,000 grams, we have bigger versions of the watt-hour for measuring very large amounts of energy:
- Megawatt-hour (MWh): This is 1,000 kilowatt-hours (1 million watt-hours). It's used for bigger energy users, like factories.
- Gigawatt-hour (GWh): This is 1,000 megawatt-hours (1 billion watt-hours). Power plants often talk about the energy they produce in GWh.
- Terawatt-hour (TWh): This is 1,000 gigawatt-hours (1 trillion watt-hours). This unit is huge! It's used to talk about the total electricity used by entire countries in a year.
Energy vs. Power: What's the Difference?
It's easy to mix up energy and power, but they are different!
- Energy (measured in kilowatt-hours or joules) is the total amount of "work" that can be done or has been done. Think of it as the fuel in a car's tank.
- Power (measured in kilowatts or watts) is the rate at which energy is used or produced. Think of it as how fast the car is burning fuel.
For example, a battery stores energy. When you use a device powered by the battery, the battery delivers its stored energy at a certain power. If the device needs more power, the battery's energy will run out faster.
Other Ways Energy is Measured
Sometimes, you'll see other units related to energy:
- Kilowatt-hours per year (kWh/year): This unit is used to describe how much energy an appliance uses over a whole year. It helps compare how energy-efficient different appliances are.
- Battery Capacity: The energy stored in a battery is often shown in ampere-hours (Ah). To get the energy in watt-hours (Wh), you multiply the ampere-hour value by the battery's voltage. For example, a 2500 mAh (2.5 Ah) battery with a voltage of 3.7 V stores about 9.25 Wh of energy (2.5 Ah * 3.7 V = 9.25 Wh).
See also
In Spanish: Kilovatio hora para niños
