Power & Energy Calculator

Electrical Energy Guide

What should I know about Understanding kWh?

A kilowatt-hour (kWh) is the standard unit of electrical energy on your bill. One kWh = 1,000 watts running for 1 hour (or 100 watts for 10 hours, etc.). Energy = Power (kW) × Time (hours). A 2kW kettle used for 5 minutes uses 0.17 kWh. At 24.5p/kWh, that is 4p per boil. UK electricity price as of October 2024 is approximately 24.5p/kWh (Ofgem price cap). Annual household electricity usage averages 2,700–4,100 kWh depending on household size.

What do I need to know about Most Expensive Appliances?

Annual running costs at 24.5p/kWh and typical usage: electric storage heater (1kW × 8h = £714/year), immersion heater (3kW × 2h/day = £536), tumble dryer (2.5kW × 2h × 3 days/week = £191), electric oven (2kW × 1h/day = £179), dishwasher (1.2kW × 1h/day = £107), washing machine (1kW × 1h/day = £89), fridge-freezer (30W average = £64), TV (80W × 4h/day = £28), LED lights (10W × 5h/day per bulb = £4.50). Replacing a 3-hour-per-day incandescent bulb (60W) with LED (8W) saves approximately £14/year per bulb, which adds up meaningfully across a whole house of multiple light fittings.

What do I need to know about CO₂ and Carbon Intensity?

The UK electricity grid's carbon intensity averages approximately 225g CO₂/kWh (2024), down from over 500g in 2000 as coal is replaced by renewables. At 225g/kWh, a 2kW tumble dryer used for 2 hours emits approximately 900g CO₂ per use. Annual household electricity use of 3,500 kWh produces approximately 790kg CO₂. Carbon intensity varies significantly by time of day — typically lower when wind generation is high (often at night or in stormy weather). Apps like Electricity Maps or the National Grid's own carbon intensity tracker let you check the current grid mix in real time, so energy-hungry tasks can be timed for lower-carbon periods if it matters to you.

What's the key thing to understand about Energy Efficiency Ratings?

UK appliance energy labels use an A–G scale (A+++ to G for older products). An A-rated fridge uses approximately 30% less energy than a D-rated equivalent. The EU/UK energy label shows estimated annual kWh consumption — divide by 365 for daily consumption and multiply by your unit rate. Heat pump tumble dryers (A-rated) typically use 1.5–2kWh per cycle vs 5kWh for a vented dryer — a significant running cost difference for heavy users. Induction hobs are approximately 85% efficient at transferring energy into the pan, compared to around 70% for gas, since gas loses more heat to the surrounding air rather than the cookware itself.

What should I know about Converting Between Power and Energy?

Power (watts, W) is the RATE of energy use, while energy (kilowatt-hours, kWh, or joules) is the TOTAL amount used over time: Energy = Power × Time. A 2kW kettle run for 6 minutes (0.1 hours) uses 2×0.1=0.2 kWh. A 60W lightbulb left on for 10 hours uses 60×10=600 Wh=0.6kWh. One kWh equals 3,600,000 joules (since 1kW=1000W and 1 hour=3600 seconds, so 1kWh=1000×3600 J). This distinction between power (an instantaneous rate, like speed) and energy (an accumulated total, like distance travelled) is exactly the same relationship as speed and distance, and confusing the two — quoting an appliance's wattage as if it were the energy used, without accounting for how long it ran — is the most common mistake when estimating electricity costs.

What do I need to know about Calculating Running Costs?

Running cost = Energy (kWh) × unit rate (pence per kWh). At a typical UK rate of around 25p/kWh (2026), a 3kW electric shower used for 10 minutes (1/6 hour) costs 3×(1/6)×25 = 12.5p per shower. An electric vehicle with a 60kWh battery charged fully from empty costs roughly 60×25=1500p=£15, though off-peak tariffs can be substantially cheaper. Standing charges (a fixed daily fee regardless of usage) are added separately and aren't part of the per-kWh calculation at all — a frequent source of confusion when comparing a calculated usage cost against an actual bill, since the bill includes both the variable usage charge and this fixed standing charge, meaning a usage-only calculation will always come in below the final total on the bill.

What do I need to know about Phantom (Standby) Load?

Many devices draw a small amount of power even while switched 'off' or idle — game consoles, chargers left plugged in, TVs waiting for a remote signal — known as phantom or standby load. Individually tiny, these loads commonly add up to 5–10% of a typical household's total electricity use when summed across every always-plugged-in device. Because this power draws continuously, 24 hours a day, even a modest 5 W phantom load (0.005 kW × 8,760 hours/year ≈ 44 kWh/year) is worth more over a year than the same device drawing far higher power for just a few minutes.

Worked Example: Running Cost of a Space Heater

A 2 kW electric space heater runs for 5 hours on a tariff of 28p per kWh. Energy used: E = P × t = 2 kW × 5 h = 10 kWh Cost = 10 kWh × £0.28/kWh = £2.80 for that one session. Run it every evening for a 30-day month and the cost becomes 30 × £2.80 = £84 — a useful reminder of why electric resistance heating is one of the most expensive ways to heat a room compared to a heat pump of similar output (which might use a third of the electricity for the same heat, thanks to a coefficient of performance around 3). If the same heater instead ran at a lower 1 kW setting for 8 hours, energy used would be 1 × 8 = 8 kWh, costing £2.24 — lower power for longer isn't always cheaper, so comparing total kWh (not just the power rating) is the only reliable way to compare running costs between settings or appliances.

Typical Power Draw by Appliance

Notice that peak power (the wattage on the nameplate) and daily energy use (kWh) tell very different stories: a kettle's power rating is enormous but it only runs for minutes at a time, while a fridge's power rating looks modest but it accumulates usage around the clock. When budgeting electricity costs, always multiply power by realistic hours of actual use, not just compare wattage ratings side by side.

Worked Example: Annual Cost of Standby (Phantom) Load

A television left on standby draws a small but continuous 5 W, every hour of every day, all year. Annual energy use: E = P × t = 5 W × (24 × 365) hours = 5 × 8,760 = 43,800 Wh = 43.8 kWh At 28p per kWh, the annual cost is 43.8 × £0.28 ≈ £12.26 — for a device that's technically 'off'. Multiply that across a typical household with 8-10 standby devices (set-top boxes, chargers, games consoles, smart speakers) and phantom load alone can add £80-120 a year to an electricity bill, entirely invisible in day-to-day use since no single device looks expensive on its own. This is precisely why energy audits emphasise standby power separately from active-use power: unlike a kettle or heater, standby load runs 24/7 with no visible benefit, so even a small wattage figure compounds into a meaningful annual cost purely through the sheer number of hours involved (8,760 per year, versus perhaps a few hundred hours of actual active appliance use).

Converting Between Power Units

Mixing unit systems is a common source of error when comparing an engine's horsepower rating to an electric motor's kW rating — a 100 kW electric motor is roughly 100,000/745.7 ≈ 134 horsepower, a conversion worth doing explicitly rather than assuming the numbers are directly comparable.

Energy vs. Power: The Core Distinction

Power (measured in watts) is the RATE of energy use at an instant, while energy (measured in kWh, joules, or similar) is the TOTAL amount consumed over a period of time. Confusing the two is extremely common in everyday language — saying an appliance 'uses a lot of power' when what's actually meant is that it uses a lot of energy over the time it's left running. A hairdryer (2000 W) has far higher power than a fridge (150 W), yet the fridge, running continuously for 24 hours, can easily use more total energy in a day (3.6 kWh) than a hairdryer used for 10 minutes (0.33 kWh) — power tells you the appliance's instantaneous demand, while energy (power × time) tells you what actually shows up on the electricity bill.