Electricity Cost Calculator: What It Costs to Run a Device

Enter what a device draws in watts, how long it runs, and the rate you pay. The calculator returns what it costs per hour, per day, per month and per year.

The arithmetic is simple and every calculator on the web gets it right. What almost all of them get wrong is the rate, and that single input decides the whole answer. Use the number your supplier advertises and your result can be half of the truth.

How to calculate the cost of running an appliance

Three numbers and one rate:

Cost = (watts / 1,000) x hours run x cents per kWh / 100

Dividing the watts by 1,000 converts to kilowatts, because electricity is billed in kilowatt-hours: one kilowatt drawn for one hour. Multiply by the hours it actually runs to get kilowatt-hours consumed, then by your price per kilowatt-hour to get dollars.

Two places this goes wrong before the rate even enters:

• Hours running, not hours plugged in. A fridge is switched on 24 hours a day and drawing power for perhaps 8 of them. A thermostat-controlled heater cycles. If you use switched-on hours for a device that cycles, you will overstate the cost by a wide margin.
• Watts, not amps. If the nameplate gives amps and volts, multiply them. 12 amps at 120 volts is 1,440 watts.

Why the advertised rate gives you an answer that is too low

An electricity bill has two halves, and only one of them is what suppliers advertise.

The supply half is the energy itself. It carries the rate quoted in offers, on comparison sites, and in the mail you get from retail suppliers. It is the number you can change by switching.

The delivery half is everything else: moving the electricity to you, the fixed customer charge, the meter, taxes, and whatever riders your utility currently has in effect. Delivery is set by a tariff filed with the state regulator. No supplier can change it and no amount of shopping will move it.

Here is what matters for this calculator: the per-kWh part of delivery is charged on the same kilowatt-hours your device just consumed. When a heater draws 6 kWh, you pay supply on 6 kWh and you pay delivery, taxes and riders on the same 6 kWh. Pricing that heater at the supply rate alone does not describe a cheaper heater. It describes an incomplete bill.

On one ComEd residential bill we reconciled line by line, the all-in rate came to 18.52 cents per kWh against a supply rate well under half that. Nothing was wrong with the bill. That is simply what the advertised number leaves out.

So the rate this calculator wants is your all-in rate: your bill total divided by the kilowatt-hours it delivered. If you do not have it, the kWh cost calculator computes it from a real bill in two numbers.

If you leave the rate blank here, the calculator uses 18.52 cents. That is a labelled assumption, not a measurement of your account, and the result says so. It exists so the tool answers on the first click, not so you can quote it.

A worked example

A 1,500 watt space heater, running 4 hours a day, every day.

1. Convert to kilowatts. 1,500 W / 1,000 = 1.5 kW.
2. Energy per day. 1.5 kW x 4 hours = 6 kWh.
3. Energy per year. 6 kWh x 365 days = 2,190 kWh.
4. Cost at the all-in rate. 2,190 kWh x 18.52 cents = $405.59 a year, which is $33.80 a month, $1.11 on each day it runs, and 27.8 cents for every hour it is on.

Now price the same heater at an advertised supply rate of 8.44 cents: 2,190 kWh x 8.44 cents = $184.84 a year.

Same heater, same hours, same meter. The advertised rate understates it by $220.75 a year, which is more than half the real figure. That gap is not an error on anyone's bill. It is the part of the bill the advertised rate was never describing.

What a device actually costs is mostly about hours

Watts get the attention and hours do the work. A 1,500 watt heater running 4 hours a day costs about $406 a year in the example above. A 50 watt device that never switches off, at 24 hours a day, consumes 438 kWh a year and costs about $81 at the same rate, from a nameplate thirty times smaller.

This is why the biggest line in most homes is not the loudest appliance. It is the always-on floor: everything that draws power around the clock without announcing it. A single-device calculator cannot see that floor. Reading it requires the meter's own record, which is what interval meter data is for.

What this calculator does not do

It does not estimate your bill. A whole bill carries a fixed customer charge and, for commercial accounts, demand charges priced on peak kilowatts rather than energy, and neither of those scales with one appliance.

It does not tell you whether to replace the device, and it does not project a saving from doing so. It does not recommend a supplier, and there is nothing here to buy.

It tells you one thing accurately: what that device costs to run, at the rate you actually pay.