Power Explained: Watts vs kWh, Appliance Running Costs, and Why Data Centres Matter

Power is the rate at which energy is used — and understanding it explains your electricity bill, your data centre costs, and why some appliances are expensive to run

Energy and power are often used interchangeably, but they measure different things. Energy is the total work done or heat produced — the quantity. Power is the rate at which that energy is transferred or used — the speed. The distinction matters practically: a 100W bulb and a 10W bulb both use electricity, but the 100W bulb uses energy ten times faster, which is why it costs ten times more to run per hour.

Power and energy: the relationship

Power (Watts, W): energy transferred per unit time. 1 Watt = 1 Joule per second.

Energy (Joules, kWh): total work done. 1 kWh = 3,600,000 Joules = 1,000W running for 1 hour.

The formula: Energy (kWh) = Power (kW) × Time (hours)

A 2,400W electric oven running for 30 minutes: 2.4 × 0.5 = 1.2 kWh

At £0.25/kWh, that's £0.30 per oven use. At $0.14/kWh, that's $0.17.

The power units in everyday use

Watts (W): the SI unit of power. All consumer electronics, light bulbs, and home appliances are rated in watts.

Kilowatts (kW): 1,000 watts. Used for heating, cooling, and EV charging equipment. A 7kW home EV charger, a 3.5kW electric kettle, an 8kW electric shower.

Megawatts (MW): 1,000,000 watts. Used for power stations and industrial facilities. A large onshore wind turbine produces 2–5 MW at full capacity. A coal-fired power station runs at 500–2,000 MW.

Gigawatts (GW): 1,000 MW. Grid-scale electricity statistics. The UK total electricity demand averages around 30–40 GW; peak winter demand around 50 GW.

Horsepower (hp): legacy unit for vehicle engines and some industrial motors. 1 hp = 745.7 watts. A 200hp car engine = 149.1 kW. Electric motors are often rated in kW; some manufacturers still advertise in equivalent horsepower for marketing purposes.

BTU/hour (BTU/hr): used in the US and some markets for heating and cooling equipment. 1 BTU/hr = 0.293 watts. A 12,000 BTU/hr air conditioner (1 "ton of cooling") = 3,516 watts.

Power consumption in data centres

Data centre electricity consumption is one of the most significant and growing components of global electricity use. Understanding data centre power explains cloud computing costs and why major tech companies have significant sustainability commitments.

Data centre power metrics:

PUE (Power Usage Effectiveness): total facility power divided by IT equipment power. PUE of 1.0 means all electricity goes to servers. PUE of 1.5 means 50% overhead for cooling and infrastructure.

• Average data centre PUE: ~1.5–1.6
• Hyperscale facility (Google, AWS, Meta): ~1.1–1.2
• Legacy enterprise data centres: 2.0+ common

Power density: kilowatts per server rack. Standard density: 5–10 kW/rack. High-performance computing: 20–50 kW/rack. AI GPU clusters: 50–100+ kW/rack.

AI model training power: training large language models consumes enormous amounts of electricity. Estimates for training a major LLM run from 500 MWh to several GWh — equivalent to thousands of homes' annual electricity consumption. This is why AI training is concentrated in regions with low-cost renewable electricity.

A server rack at 10 kW:

• Annual energy: 10 × 8,760 = 87,600 kWh
• At US data centre electricity prices (~$0.06/kWh commercial): ~$5,256/year in electricity
• With PUE of 1.5: effective electricity cost of $7,884/year
• This underlies cloud computing pricing — the electricity cost is a significant component of server operating cost.

Electric vehicle power: charging speeds

EV charger types are defined by their power output:

The battery accepts a maximum charge rate (limited by battery chemistry), so "350 kW" doesn't mean every EV charges at 350 kW. Most consumer EVs peak at 100–150 kW DC charging even at ultra-rapid stations.

Renewable energy power output

Understanding power ratings of renewable energy sources helps contextualise energy transition discussions:

Solar panel: a 400W residential panel produces ~1.4 kWh/day in the UK (average irradiance). A 4kW rooftop system (~10 panels) produces ~3,500 kWh/year — roughly half a typical UK home's electricity consumption.

Wind turbine:

• Onshore residential (domestic): 2–10 kW
• Commercial onshore: 2–5 MW
• Offshore: 8–15 MW

Nuclear: a typical reactor: 1,000–1,600 MW continuous output. High capacity factor (90%+) — nuclear produces power nearly continuously unlike variable renewables.

How to use the Power Converter on sadiqbd.com

1. Enter the power value and source unit — watts, kW, MW, horsepower, BTU/hr
2. Convert — see equivalents across all power units
3. Use for running cost calculations — convert to kW, multiply by hours to get kWh, multiply by electricity rate

Frequently Asked Questions

What's the difference between kW and kWh? kW is power (the rate of energy use). kWh is energy (the total amount used). A 3kW kettle (power) running for 2 minutes uses 0.1 kWh (energy). Electricity bills charge for kWh — how much energy you consumed, not just how powerful your devices are.

Why are electric motors more efficient than petrol engines? Electric motors convert 85–95% of electrical energy into mechanical work. Internal combustion engines convert only 20–40% of fuel energy into mechanical work — the rest is lost as heat. This fundamental efficiency difference is why EVs use so much less energy per km than petrol cars.

Is the Power Converter free? Yes — completely free, no sign-up required.

Power is the lens that connects your electricity bill, your data centre budget, and the energy transition. Understanding it turns abstract numbers into actionable calculations.

Try the Power Converter free at sadiqbd.com — convert between watts, kilowatts, megawatts, horsepower, and BTU/hr instantly.