Energy Cost Calculator
Electricity running cost from power, hours, and per-kWh rate
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What this calculator computes
The energy-cost calculator turns a power rating, an operating duration, and a per-kWh tariff into the total electricity cost of running an appliance, expressed in the local currency. The math is straightforward — cost = (W × h × rate) / 1000 — but the practical value comes from the ability to compare alternatives (LED vs incandescent, electric vs gas, peak vs off-peak charging) in cash terms rather than abstract energy units. UK retail electricity tariffs in 2025 average about 27 pence per kWh on standard variable rates, with Economy 7 dual-rate plans charging roughly 35 p/kWh during peak hours and 12 p/kWh during the seven overnight off-peak hours. US average residential rates are about 16 ¢/kWh nationally, ranging from 11 ¢ in low-cost states like Idaho and Washington to 33 ¢ in Hawaii and California. EU rates vary even more widely, from 9 ¢/kWh in Hungary to 46 ¢/kWh in Germany after the 2022 gas crisis. This calculator accepts the power in watts, the duration in hours (with optional repetition for daily or monthly totals), and the per-kWh rate in the user's currency, and returns the total cost. It is also useful for diagnosing high bills: working back from a known monthly bill total and a list of major loads often reveals which always-on or duty-cycled load is driving the cost. The calculation assumes a flat per-kWh rate; for time-of-use tariffs, run the calculation separately for peak and off-peak hours and sum the results.
Calculator
The formula
Formula
cost = (W × h × rate) / 1000
Worked example
When to use this calculator
Use this calculator whenever you need a quick cash estimate of running cost for any electric appliance, or to compare two alternatives in monetary rather than energy terms. The most common scenarios are residential bill diagnosis (matching the monthly bill total to the major load contributors), appliance replacement decisions (justifying an LED-bulb upgrade or a heat-pump conversion in payback-period terms), tariff comparison and time-of-use optimisation (calculating the savings from shifting a load to off-peak hours), and EV-charging cost estimation (computing the per-charge cost at home vs at a public charger). The calculator is most accurate for constant-power loads and flat-rate tariffs; for variable loads, use measured kWh from a smart plug or utility bill rather than nameplate wattage, and for time-of-use tariffs run the peak and off-peak portions separately. UK households on Economy 7 should split their always-on load by clock-hour position to reflect the actual peak/off-peak split, since the calculator assumes a single flat rate per run.
Common input mistakes
- Using a stale tariff figure. UK retail electricity rates moved from about 19 p/kWh pre-crisis (2021) to 34 p/kWh during the 2023 peak, settling at 27 p/kWh after the 2024 price-cap revisions. A cost calculation based on a years-old rate can under-state running cost by 50% or more. Pull the current rate off your most recent bill, not from memory or generic averages.
- Forgetting standing charges. UK electricity bills include a daily standing charge (typically 50–60 p/day in 2025) on top of the per-kWh rate, regardless of consumption. The standing charge does not depend on the appliance and so is excluded from per-appliance cost calculations, but it must be added back when comparing total annual electricity cost figures.
Frequently asked questions
How do I calculate the running cost of an appliance?
Multiply the wattage by the hours of operation and by the per-kWh tariff, then divide by 1000 to scale watts into kilowatts. cost = (W × h × rate) / 1000. A 1500 W heater running 4 hours a day at 27 p/kWh costs (1500 × 4 × 0.27) / 1000 = £1.62 per day, or £48.60 per month if used every day at that schedule.
What is the average electricity rate in the UK?
UK Ofgem price-cap retail electricity rates as of late 2024 average about 27 p/kWh on standard variable tariffs, having fallen from a peak of 34 p/kWh during the 2023 energy crisis. Economy 7 dual-rate tariffs charge approximately 35 p/kWh during peak hours and 12 p/kWh during the seven overnight off-peak hours, with the exact split varying by supplier. Always check your most recent bill for the current rate; the price cap is reviewed quarterly.
How much does it cost to charge an EV at home?
A typical UK EV with a 60 kWh battery charged from 20% to 80% adds 36 kWh, which at the standard 27 p/kWh costs £9.72 per charge. On Economy 7 off-peak at 12 p/kWh the same charge costs £4.32, less than half. EV-specific tariffs (Octopus Go, Intelligent Octopus) push the off-peak rate down to 7–9 p/kWh, dropping the per-charge cost to about £2.50–£3.25 — comparable to the energy cost of a single pint of beer.
Does this include standing charges?
No — the calculator returns the marginal cost of running a specific load and excludes the daily standing charge that all UK customers pay regardless of consumption. Standing charges in 2025 average 50–60 p/day for electricity and 30–35 p/day for gas, totalling about £290 per year. They cover utility infrastructure costs and are not avoidable by reducing per-appliance usage; only switching tariffs or suppliers changes them.
How do I compare peak-hour and off-peak-hour cost?
Run the calculation separately for the hours spent on each tariff segment and sum the results. A heat-pump running 4 hours during off-peak Economy 7 (12 p/kWh) and 6 hours during peak (35 p/kWh) at 3 kW averages (3000 × 4 × 0.12 + 3000 × 6 × 0.35) / 1000 = £1.44 + £6.30 = £7.74/day, compared with £8.40/day on a flat 28 p tariff. The off-peak shift saves only £0.66/day in this case because the bulk of the demand falls in peak hours; running the same heat pump entirely on off-peak hours via a smart programme would save much more.