Electricity Cost Calculator

A power rating in watts describes how fast an appliance uses energy at a given moment, but a utility bill charges for energy, not power — specifically, for kilowatt-hours (kWh), where one kilowatt-hour is the energy used by a 1,000-watt device running for one hour. A 100-watt device would need to run for ten hours to use that same one kWh. This distinction is why a powerful appliance used briefly, like a kettle, can cost less over a day than a much weaker one left running constantly, like a router or a fridge.

Utilities meter the cumulative kWh a household or business consumes and multiply it by a rate, typically quoted in cents or pence per kWh, to produce the energy portion of a bill. That per-unit rate is often just one line item among several — standing charges, delivery fees and taxes are commonly added on top and are not part of what a wattage-and-usage estimate like this one covers.

The wattage printed on an appliance’s rating plate is its maximum rated draw, not necessarily what it uses on average. Devices with a thermostat or compressor — refrigerators, space heaters, air conditioners, water heaters — cycle on and off to maintain a target temperature, so their real average consumption is often well below the nameplate figure. Devices with a simple, constant load — an incandescent bulb, a resistive heater running flat out, a motor under steady load — track their nameplate rating much more closely.

The most accurate way to know what something actually costs to run is a plug-in energy monitor, which measures real consumption directly rather than assuming full-rated power for every hour of use. A wattage-based estimate like this one is a reasonable planning tool, but it tends to overstate the cost of anything that cycles.

Many electronic devices draw a small amount of power even when switched "off," to keep a clock running, stay responsive to a remote control, or maintain a network connection — commonly a few watts, sometimes more for older equipment. Because that draw runs for every hour of the year the device is not in active use, rather than just the hours it is actually being used, a small constant load compounds into a surprisingly large annual total.

A handful of standby devices spread around a home — set-top boxes, game consoles, chargers left plugged in, smart-home hubs — can collectively use as much energy over a year as one or two appliances that run for a few hours a day. That is the reasoning behind reporting standby draw separately in this calculator’s results, rather than folding it silently into the total.

This calculator assumes a single flat rate per kWh, which keeps the maths transparent but is not how every household is billed. Many utilities now offer or require time-of-use tariffs, which charge more during peak demand hours — typically weekday evenings — and less overnight, or tiered tariffs, where the rate per kWh rises once consumption passes a threshold within a billing period.

Under either structure, the same appliance can cost noticeably more or less to run depending on when it is used, not just how much it is used — running a dishwasher or an EV charger overnight instead of during a peak window can meaningfully cut its real cost even though the kWh figure stays identical. Check your own bill or tariff plan for the structure that actually applies to you.