Electricity Cost Calculator

Every appliance has a power rating printed on it somewhere. Enter that wattage, say how many hours a day the thing actually runs, and add what you pay per kilowatt-hour. The calculator turns those three numbers into a cost per day, month and year, updating as you type. It is the fastest way to settle whether the space heater or the old chest freezer is the real money pit.

Where to find the wattage

Look on the rating label, usually a silver sticker on the back or base of the unit or moulded into the plug on kettles and hair dryers. It shows either watts (W) directly or a voltage and a current, in which case watts is volts × amps. A UK kettle rated 230 V and 13 A draws about 2,990 W. If only a kilowatt figure appears, multiply by 1,000, so a 2.4 kW heater is 2,400 W.

The formula

kWh/day = watts × hours ÷ 1000

The division by 1,000 converts watt-hours into kilowatt-hours, the unit your supplier meters and prices. Watts times hours gives watt-hours; a thousand watt-hours make one kilowatt-hour. The running cost is that daily energy multiplied by your unit price, then scaled to a 30-day month and a 365-day year. With the defaults, 1,500 W for 3.5 hours at 0.28 per kWh, the arithmetic is 1,500 × 3.5 ÷ 1000 = 5.25 kWh a day. At 0.28 that is 1.47 a day, 44.10 across the month and 536.55 over the year.

Two simplifications are built in. The month uses a flat 30 days rather than the calendar length, and the year uses 365, so a leap year is fractionally higher. The rate is treated as a single flat number, which suits most households but not tariffs where the price changes by time of day. If your appliance cycles on and off, like a fridge or a thermostat-controlled heater, enter its averaged running hours or its averaged wattage rather than the peak figure on the label.

From the watt to the kilowatt-hour

The unit doing the work here carries the name of James Watt, the Scottish engineer whose separate condenser, patented in 1769, and partnership with Matthew Boulton from 1775 made mechanical power something you could buy, measure and compare. Watt himself never saw an electrical unit named after him; he died in 1819. The name was proposed much later, in August 1882, when C. William Siemens, in his president's address to the fifty-second meeting of the British Association for the Advancement of Science, suggested that a unit of power might fittingly be called the watt and defined it as the power conveyed by a current of one ampere across a potential difference of one volt. Siemens's definition was adopted as the international watt at the International Conference on Electric Units and Standards held in London in October 1908, and the 11th General Conference on Weights and Measures brought the absolute watt into the International System of Units in 1960 as a derived unit equal to one joule per second, the joule itself named for the English physicist James Prescott Joule.

A watt measures a rate of energy flow, not a quantity, which is why a bill cannot be written in watts alone. Multiply a rate of power by a length of time and you get energy: run one kilowatt for one hour and you have used one kilowatt-hour, equal to 3.6 million joules. The kilowatt-hour survived as the billing unit precisely because it maps onto how people use appliances, in hours rather than seconds, and in the thousands of watts a home draws rather than raw joules.

How electricity came to be billed by the unit

When Thomas Edison's Pearl Street Station switched on at 255-257 Pearl Street on 4 September 1882, it carried an initial load of 400 lamps for 82 customers in the surrounding blocks of lower Manhattan, and no cheap, reliable way existed to measure how much each of them had drawn. Edison's answer was an electrochemical, or electrolytic, meter: a cell wired into the supply in which a measured fraction of the customer's current slowly plated metal out of solution onto a plate. A meter reader took the plates out periodically and weighed them, and the change in weight stood in for consumption. Because the supply voltage was held steady, totalling current this way came close enough to totalling energy to bill on. It worked, but it was slow, labour-intensive and disliked by customers who could not read it for themselves.

The device that made per-unit billing routine was the spinning-disc induction meter. In the autumn of 1889 the Hungarian engineer Ottó Bláthy patented an alternating-current watt-hour meter, and the Ganz Works showed the first specimen of it at the Frankfurt fair that same autumn. In the United States, Oliver Shallenberger of Westinghouse had watched a spring drop into an experimental lamp in 1888 and start turning in the alternating field; he worked that effect into an induction ampere-hour meter the same year. In 1894 he applied the induction principle to a watt-hour meter, using a disc whose speed of rotation was made proportional to the power in the circuit, and that form went on to dominate homes for a century. The faster the disc turned, the more power was flowing, and a geared register counted the accumulated kilowatt-hours behind the glass.

The pricing model on a modern bill was shaped a little later by Samuel Insull, Edison's former private secretary, who built Chicago Edison into a mass enterprise. Visiting Brighton, England, over Christmas 1894, he found a demand-metered billing system already in use there, and by 1897 he was offering Chicago customers a two-part tariff that separated a fixed charge, reflecting the cost of standing ready to supply them, from a per-kilowatt-hour charge for the energy actually used. That split is still visible today as the standing charge and the unit rate. This calculator prices only the unit-rate side, the part that moves with how much you run an appliance; the fixed daily charge sits outside the formula because it does not change with usage.

Typical appliance draw

Power ratings tell you far less than you would think, because what matters is watts multiplied by hours.

A kettle pulls a huge 3,000 W but only for the moment it boils. A fridge is the reverse: rated higher, yet averaging around 40 W because it cycles, and it never switches off.

Standby and phantom loads

Devices left in standby still draw power. A console in rest mode, a TV showing its little red light, a charger left plugged in with nothing attached: each is only 1 to 10 W, but a dozen of them running around the clock can add 30 to 60 kWh a month. At 0.28 that is roughly 8 to 17 for electricity doing nothing. A switched power strip is the cheapest saving in the house.

What a unit costs by market

Prices per kWh move constantly, so treat these as ballparks. The US residential average was 17.3 cents across 2025 on the Energy Information Administration's annual figures, with summer months running higher than the yearly mean. In Great Britain the Ofgem price cap has held the electricity unit rate between roughly 24 and 27 pence through recent quarters, resetting every three months. Germany sits among Europe's highest: the industry body BDEW puts the 2026 household average near 37 cents per kWh, after several years closer to 40. Canada and Australia vary widely by province and state. Your own bill is the figure to trust: find the per-kWh unit rate, which is separate from the fixed daily standing charge this calculator does not include.

Results are estimates based on a flat rate and steady use, not a substitute for your metered bill. See the site disclaimer.

Frequently asked questions

How do I calculate the cost of running an appliance?

Multiply the appliance wattage by the hours you run it, divide by 1,000 to get kilowatt-hours, then multiply by your price per kWh. A 1,500 W heater for 3.5 hours is 1,500 × 3.5 ÷ 1000 = 5.25 kWh, and at 0.28 per kWh that is 1.47 a day. Over a 30-day month it adds up to 44.10.

How much does it cost to run a 1500W heater?

At the default 3.5 hours a day and 0.28 per kWh, a 1,500 W heater costs 1.47 a day, 44.10 a month and 536.55 a year. Halve the running time and every figure halves too. It is usually the single most expensive plug-in device in a home during winter.

How much electricity does something on standby use?

Standby loads are small individually, typically 1 to 10 watts for a TV, console or charger left plugged in. But a dozen of them drawing power 24 hours a day can total 30 to 60 kWh a month, which is 8 to 17 at a 0.28 rate. Switching devices off at the wall removes that draw entirely.

What is the average price per kWh?

As rough ballparks, US residential electricity averages near 0.17 USD per kWh, the UK sits around 0.25 GBP under the price cap, and Germany is among Europe's highest at roughly 0.40 EUR. Rates move with the seasons and vary widely by region, so the unit rate on your own bill is the number to enter.

Why is my fridge cheap to run if it draws hundreds of watts?

A fridge compressor is rated a few hundred watts, but it only runs when it needs to cool, cycling on and off through the day. Averaged out it draws roughly 40 W, so at 24 hours it uses about 0.96 kWh a day and costs around 0.38 at a 0.40 rate. Use the averaged figure, not the peak rating on the label.