Watts, Kilowatt Hours and Joules: What Does It All Mean?

Your energy bill charges you in kilowatt hours (kWh). Your appliances are rated in watts (W). And energy itself is measured in joules (J) in the International System of Units (SI). These 3 units, watts, kilowatt hours, and joules, describe the same thing from 3 different angles: how fast energy flows, how much you use over time, and the raw quantity of energy transferred. Understanding the difference tells you why a 3,000W kettle costs more to run than a 10W LED bulb, and how your supplier converts your usage into a bill.

This guide covers what each unit means, how to convert joules to watts, watts to kilowatt-hours, and kilowatt-hours to joules, with worked examples and a full conversion reference table, plus how these units appear on a UK energy bill in 2026.

Energy vs Power: The Key Distinction

Energy and power are not the same thing, and the difference explains why watts and joules are separate units.

Energy is the total amount of work done, the quantity of electrical work transferred. In SI, energy is measured in joules (J). On your energy bill, energy is measured in kilowatt-hours (kWh), which is simply a more practical unit for the quantities households and businesses use.

Power is the rate at which energy is used or generated, or how fast the energy flows. Power is measured in watts (W). One watt equals one joule per second (1 W = 1 J/s). A device with a higher wattage uses energy faster than a device with a lower wattage.

A practical example: a 2,000W electric oven and a 100W light bulb both use energy. The oven uses it 20 times faster. Run both for one hour; the oven has consumed 2 kWh, while the bulb has consumed 0.1 kWh. The oven used 20 times as much energy in the same amount of time because its power rating is 20 times higher.

What Is a Watt? Definition and Examples

A watt (W) is the SI unit of power. It measures how fast energy is being used or produced at any given moment. Named after the Scottish engineer James Watt, 1 watt is defined as the transfer of 1 joule of energy per second.

In electrical terms:

Watts = Volts × Amps (W = V × A). The UK mains voltage is 230 V. A washing machine drawing 10 amps uses 230 × 10 = 2,300 W (2.3 kW). A phone charger drawing 1 amp uses 230 × 1 = 230 W at the wall, though the phone itself receives less due to conversion losses.

Watt scale reference:

• Milliwatt (mW): 0.001 W LED indicator lights, Bluetooth devices

• Watt (W): 1 W small electronics, LED bulbs

• Kilowatt (kW): 1,000 W kettles, ovens, electric heaters

• Megawatt (MW): 1,000,000 W power stations, large industrial sites

• Gigawatt (GW): 1,000,000,000 W national grid-scale generation

• Terawatt (TW): 1,000,000,000,000 W global energy consumption scale

What Is a Kilowatt and a Kilowatt Hour (kWh)?

Kilowatt (kW)

A kilowatt (kW) is simply 1,000 watts. 'Kilo' is the SI prefix for 1,000. Kilowatts and watts are both measurements of power; they describe how fast energy is being used, not how much in total. Your kettle is rated at 2–3 kW. Your boiler might be rated at 24 kW. A utility-scale wind turbine generates 2,000–5,000 kW (2–5 MW).

Kilowatt Hour (kWh)

A kilowatt hour (kWh) is a unit of energy, not power. It measures the energy consumed when a 1 kW device operates for 1 hour. This is the unit your energy supplier charges for. In 2026, UK households pay around 24–27p per kWh for electricity.

To calculate kWh from watts: divide the wattage by 1,000 to get kW, then multiply by the number of hours used.

Worked example electric kettle:

• Kettle rated at 3,000 W (3 kW)

• Used for 5 minutes per boil = 5/60 hours = 0.0833 hours

• Energy used = 3 kW × 0.0833 h = 0.25 kWh per boil

• Cost = 0.25 kWh × 27p = 6.75p per boil

Boil a kettle 4 times a day, and you spend roughly 27p on electricity for the kettle daily, or around £98/year.

What Is a Joule? Definition and Scale

A joule (J) is the SI base unit of energy. Named after the English physicist James Prescott Joule, 1 joule is the energy transferred when a force of 1 newton moves an object 1 metre in the direction of the force. In electrical terms, 1 joule is the energy transferred when 1 watt of power flows for 1 second.

Joules are the foundation of the entire energy unit system. Watts, kWh, and all other energy units connect back to joules. However, joules are tiny in the context of household energy. Lifting a small apple (approximately 100 g) by 1 metre requires roughly 1 joule of work. A single cup of tea requires around 150,000 joules (150 kJ) of energy to heat.

Joule scale reference:

• 1 J: energy to lift a 100 g object by 1 metre

• 1 kJ (kilojoule = 1,000 J): energy in a small food calorie

• 1 MJ (megajoule = 1,000,000 J): roughly the kinetic energy of a 1-tonne car at 160 km/h (100 mph)

• 3.6 MJ: equals exactly 1 kWh, the most important joule conversion for energy bills

• 1 GJ (gigajoule = 1,000,000,000 J): energy content of around 27 litres of petrol; also used to measure industrial gas consumption

The Relationship Between Joules and Watts

1 watt equals 1 joule per second. This is the fundamental link between the two units. Power (watts) is simply the rate of energy flow (joules) over time (seconds):

• Power (W) = Energy (J) ÷ Time (seconds)

• Energy (J) = Power (W) × Time (seconds)

Worked examples:

How many joules is a watt?

1 watt for 1 second = 1 joule. 1 watt for 1 minute (60 seconds) = 60 joules. 1 watt for 1 hour (3,600 seconds) = 3,600 joules.

How many watts is a joule?

A joule is not a watt it is watts multiplied by seconds. To express joules as watts, you need a time dimension: 1 joule over 1 second = 1 watt. 1 joule over 2 seconds = 0.5 watts. 3,600 joules over 3,600 seconds = 1 watt.

Convert joules to watts worked example:

A phone charges for 30 minutes (1,800 seconds) and receives 36,000 joules. What is the power? W = 36,000 J ÷ 1,800 s = 20 W. The charger delivered 20 watts.

Convert watts to joules worked example:

A 100W light bulb runs for 2 hours (7,200 seconds). How many joules does it use? J = 100 W × 7,200 s = 720,000 J (720 kJ or 0.72 MJ).

How Many Joules Are in a Kilowatt Hour?

1 kWh = 3,600,000 joules (3.6 megajoules or 3.6 MJ). This is an exact conversion. Here is where it comes from:

• 1 kWh = 1 kilowatt × 1 hour

• 1 kilowatt = 1,000 watts

• 1 hour = 3,600 seconds

Therefore: 1 kWh = 1,000 W × 3,600 s = 3,600,000 J = 3.6 MJ

To convert kWh to joules: multiply the kWh figure by 3,600,000. To convert joules to kWh: divide the joules figure by 3,600,000.

Worked example joules to kWh:

Your electric boiler transfers 54,000,000 joules (54 MJ) to heat your home. How many kWh is that? 54,000,000 ÷ 3,600,000 = 15 kWh. At 27p/kWh, that heating run cost £4.05.

Worked example kWh to joules:

A household uses 10 kWh overnight to charge an electric vehicle. How many joules? 10 × 3,600,000 = 36,000,000 J (36 MJ).

How to Convert Joules to Kilowatts (and kW to Joules)

To convert joules to kilowatts, divide the energy in joules by (time in seconds × 1,000).

Formula: kW = J ÷ (seconds × 1,000)

Worked example joules to kilowatts:

A gas boiler delivers 180,000 joules of heat over 1 minute (60 seconds). What is its power in kilowatts? kW = 180,000 ÷ (60 × 1,000) = 180,000 ÷ 60,000 = 3 kW.

To convert kilowatts to joules:

Formula: J = kW × seconds × 1,000

Worked example kilowatts to joules:

A 7 kW home EV charger runs for 30 minutes (1,800 seconds). How many joules does it transfer? J = 7 × 1,800 × 1,000 = 12,600,000 J (12.6 MJ or 3.5 kWh).

Watts, Kilowatts, Joules and kWh: Conversion Reference Table

Use this table to convert between watts (W), kilowatts (kW), joules (J), and kilowatt hours (kWh) quickly:

 Power SI Units: The International System of Units for Energy and Power

In the International System of Units (SI), the standard unit of power is the watt (W) and the standard unit of energy is the joule (J). These are part of the 7 SI base units and are universally accepted across science, engineering, and commerce.

Key SI units for energy and power:

• Joule (J): SI unit of energy J = kg⋅m²⋅s⁻²

• Watt (W): SI unit of power W = J/s = kg⋅m²⋅s⁻³

• Kilowatt hour (kWh): non-SI unit of energy, universally used for electricity billing 1 kWh = 3.6 MJ

• Ampere (A): SI unit of electric current flows through a circuit and, combined with volts, gives watts

• Volt (V): SI-derived unit of electrical potential difference UK mains = 230 V

The kWh is not strictly an SI unit (the SI unit of energy is the joule), but it is the accepted practical unit for billing because joule quantities in household consumption are unwieldy. A typical UK household uses around 2,700 kWh of electricity per year expressed in joules, that is 9,720,000,000 J (9.72 GJ).

Watts to kWh for Common UK Appliances in 2026

The table below shows the wattage, kilowatt equivalent, and hourly electricity cost at 27p/kWh (the average UK unit rate in 2026) for common household appliances:

 The electric shower illustrates why high-wattage appliances dominate energy bills. At 8,500W (8.5 kW), a 10-minute shower uses 8.5 × (10/60) = 1.42 kWh costing around 38p. Take one shower daily and that is £138/year, purely on showering electricity.

To calculate the running cost of any appliance: (wattage ÷ 1,000) × hours used × unit rate in pence = pence spent. To see how your total usage translates to a bill, compare energy tariffs at Switch to find the lowest unit rate available at your postcode.

How These Units Appear on Your Energy Bill

Your electricity bill shows usage in kWh, not watts, not joules. Your meter records cumulative kWh consumed. The supplier multiplies your kWh by the unit rate (pence per kWh) to calculate what you owe.

3 energy units on a typical 2026 UK electricity bill:

1. kWh (units consumed): the meter reading difference between your last bill and the current one, multiplied by any meter factor

2. Unit rate (p/kWh): the price per kilowatt hour, set by your tariff currently 24–27p/kWh for most UK households in 2026 under the Ofgem price cap

3. Standing charge (p/day): a fixed daily connection charge separate from usage, typically 50–65p/day for electricity

Watts and joules do not appear directly on your bill. But they determine how quickly kWh accumulate. A household running a 3 kW electric heater for 8 hours uses 24 kWh at 27p/kWh, that is £6.48 in a single day.

Frequently Asked Questions About Watts, Kilowatt Hours and Joules

How many joules is a watt?

A watt is 1 joule per second. So the number of joules a watt represents depends on time. 1 watt for 1 second = 1 joule. 1 watt for 60 seconds = 60 joules. 1 watt for 1 hour (3,600 seconds) = 3,600 joules.

How many watts is a joule?

1 joule equals 1 watt-second (1 W·s). To express joules as watts, divide by the time in seconds: W = J ÷ seconds. 1 joule delivered over 1 second = 1 watt. 1 joule delivered over 10 seconds = 0.1 watts.

How do you convert joules to watts?

Divide joules by seconds: W = J ÷ s. Example: 18,000 joules over 60 seconds = 18,000 ÷ 60 = 300 watts. You always need the time component to convert joules to watts, because joules measure energy (total) and watts measure power (rate).

How do you convert watts to joules?

Multiply watts by seconds: J = W × s. Example: a 500W microwave running for 3 minutes (180 seconds) uses 500 × 180 = 90,000 J (90 kJ). To convert watts to joules per hour: W × 3,600 = joules per hour.

How many joules are in a kilowatt hour?

1 kWh = 3,600,000 joules (3.6 MJ). This is an exact conversion: 1,000 W × 3,600 s = 3,600,000 J. To convert kWh to joules, multiply by 3,600,000. To convert joules to kWh, divide by 3,600,000.

What is the difference between watts and kilowatts?

1 kilowatt (kW) = 1,000 watts (W). Both measure the power of the rate of energy use. Watts are used for smaller devices (light bulbs, phone chargers). Kilowatts are used for larger appliances (ovens, boilers, EV chargers). To convert watts to kilowatts, divide by 1,000. To convert kilowatts to watts, multiply by 1,000.

What is the SI unit of power and energy?

The SI unit of power is the watt (W). The SI unit of energy is the joule (J). The kilowatt-hour (kWh) is not an SI unit, but it is the accepted standard for electricity billing worldwide because the quantities of energy in joules at the household scale are extremely large.

Why does my energy bill use kWh instead of joules?

Because kWh produces manageable billing numbers. A typical UK household uses around 2,700 kWh of electricity per year. In joules, that is 9,720,000,000 J (nearly 10 gigajoules). kWh is the practical billing unit because it directly reflects the meter reading, which measures kilowatt-hours consumed, not joules.