Power and energy are related but different things — and the confusion between watts and watt-hours causes real problems in energy planning, equipment sizing, and understanding electricity bills, because power is a rate and energy is an amount accumulated over time
The previous articles on this site covered power unit basics, electricity bills and appliance running costs, renewable energy output, human power in sport, and UPS sizing with power factor. This article addresses power vs energy in practical planning — specifically how to correctly calculate energy consumption from power ratings, and the common mistakes people make when thinking about electricity.
The fundamental distinction: rate vs amount
Power (measured in watts, kilowatts, megawatts) is a rate — how fast energy is being transferred or consumed at any given instant. Like speed (km/h), which tells you how fast you're going, power tells you how fast energy is flowing.
Energy (measured in watt-hours, kilowatt-hours, joules) is an amount — the total energy transferred or consumed over a period of time. Like distance (km), which accumulates as you travel at a given speed for a given time, energy accumulates as a device runs at a given power level for a given duration.
The relationship: Energy = Power × Time
A 2,000W kettle running for 3 minutes (0.05 hours) uses: 2,000W × 0.05h = 100 watt-hours = 0.1 kWh
A 100W lamp running for 10 hours uses: 100W × 10h = 1,000 watt-hours = 1 kWh
The kettle is 20× more powerful than the lamp, but the lamp running all day uses 10× more energy than the kettle boiling once.
Electricity bills: what you're actually paying for
Electricity bills charge for energy, not power — the unit of measurement is the kilowatt-hour (kWh), sometimes called a "unit" of electricity.
UK average electricity price (2024): approximately 24p/kWh
Appliance running cost formula:
- Running cost (pence per hour) = power rating (kW) × price per kWh (pence)
- A 2.5kW electric heater: 2.5 × 24 = 60p per hour
- A 0.01kW (10W) LED bulb: 0.01 × 24 = 0.24p per hour (about £0.002)
Annual appliance cost:
- Electric shower (8.5kW, 8 minutes daily, 365 days): 8.5kW × (8/60)h × 365 × 24p = £99/year
- Refrigerator (typically 40-80W average, 24/7): 0.06kW × 8,760h × 24p = £126/year for a 60W average
Counterintuitive findings from this calculation:
- A refrigerator (always on, low wattage) often costs more annually than a washing machine (high wattage but limited hours)
- LED lighting at 10W costs a fraction of the equivalent incandescent (60W) for the same light output
- An electric car charger at 7kW costs about £1.68/hour but typical charging sessions are 1-2 hours
Power ratings on devices: nameplate vs actual consumption
The power rating on an appliance's label is the maximum consumption — devices rarely run at their rated wattage continuously.
Reasons actual consumption differs from nameplate:
- Variable loads: a washing machine at 2,000W nameplate uses 2,000W during the heating cycle but much less during spinning and resting phases; the average across a cycle is roughly 500-700W
- Power factors: particularly for motors and some electronic devices — the apparent power (in VA) differs from real power (in W); the ratio is the power factor. A 1,000VA device with a 0.8 power factor uses 800W of real power
- Standby consumption: TVs, computers, and smart devices continue drawing power in standby mode — often 1-10W continuously, which adds up for always-on devices
- Efficiency variation: at partial loads, many devices are less efficient than at full load
For accurate energy estimates: measure with a plug-in energy monitor (a "smart plug" with energy monitoring, or a dedicated plug-in meter) rather than relying on nameplate ratings. These measure actual watt-hours consumed, showing the real cost rather than the theoretical maximum.
Grid-scale power units: MW, GW, TW
For electricity generation and grid planning, individual kilowatts are too small — the relevant units are megawatts (MW), gigawatts (GW), and terawatts (TW):
- 1 MW = 1,000 kW = 1,000,000 W
- 1 GW = 1,000 MW = 1,000,000 kW
- 1 TW = 1,000 GW
Context for scale:
- A typical large coal or gas power station: 1-3 GW capacity
- UK's total electricity demand (winter peak): approximately 50-60 GW
- Global electricity generating capacity: approximately 9 TW installed
- A single wind turbine (large offshore): 10-15 MW rated capacity
- An electric kettle: 2-3 kW (0.000003 GW)
The "capacity vs generation" distinction (covered in the renewables article): rated capacity is the maximum possible output; actual generation depends on capacity factor — how often the plant runs at full capacity.
How to use the Power Converter on sadiqbd.com
- Convert between watts and kW for appliance calculations — energy bills and cost calculations work in kW and kWh; most device labels show watts; the conversion is divide by 1,000
- Convert between kW and horsepower for motor and engine specifications — mechanical equipment often uses hp (metric or imperial); electrical equivalents use kW
- Convert between BTU/hr and watts/kW for HVAC and cooling equipment — air conditioners are frequently rated in BTU/hr in some markets and kW in others; 1 kW of cooling = approximately 3,412 BTU/hr
Frequently Asked Questions
If my electricity bill says I used 300 kWh last month, what does that actually mean? 300 kWh means you consumed 300 kilowatt-hours of energy — equivalent to running a 1kW device (like a small electric heater, or a desktop computer under load) continuously for 300 hours, or a 100W device for 3,000 hours. At 24p/kWh, your energy cost is £72 for the month (before standing charges and taxes). To understand where this consumption comes from, estimate the dominant appliances: if your always-on refrigerator uses 60W × 720 hours/month = 43.2 kWh, that's 14% of 300 kWh from one appliance alone. This is why identifying and replacing high-consumption always-on devices has more annual impact than behavioral changes to how often you boil the kettle.
Is the Power Converter free? Yes — completely free, no sign-up required.
Try the Power Converter free at sadiqbd.com — convert between watts, kilowatts, horsepower, BTU/hr, and more power units instantly.
