If you want to know how much a toaster, TV, heater, or laptop costs to run, the basic method is simple: convert watts to kWh, then multiply by your electricity tariff. In formula form, that is kWh = watts / 1000 x hours, and cost = kWh x tariff.

The same method works for short uses, daily use, or standby power. If you know the appliance’s wattage and how long it runs, you can make a useful estimate without guessing.

The basic formula

Electricity bills are usually based on energy used, measured in kilowatt-hours, or kWh. A kilowatt-hour is a unit of energy, and you can read more about it in the Wikipedia explanation of kilowatt-hour.

To calculate the energy used by an appliance:

  • Convert watts to kilowatts by dividing by 1000
  • Multiply by the number of hours the appliance runs
  • Multiply the result by your electricity tariff

So the full calculation is:

kWh = watts / 1000 x hours

cost = kWh x tariff

If your appliance label shows amps instead of watts, you will need to convert first using voltage. For most household appliances, watts are usually easier to work with because the rating plate often lists them directly.

What the watt rating means

The watt rating tells you how much power the appliance draws while it is running. A higher watt rating usually means higher electricity use, but the actual cost also depends on how long you use it.

For example:

  • A small device with a low watt rating can still cost more than expected if it runs for many hours
  • A high-watt appliance can be cheap to use if it runs only briefly
  • The same appliance can cost very different amounts depending on settings, temperature, and cycle length

If you are estimating a household device, look for the label on the appliance, the user manual, or the manufacturer’s website. If you want a rough estimate for a specific period, our hours calculator can help you keep the time part straight before you plug it into the electricity formula.

How to use your electricity tariff

Your tariff is the price you pay per kWh. It may appear on your bill as a rate per unit of electricity. If your bill has more than one rate, use the rate that matches the time and usage you are calculating.

To estimate cost:

  1. Find the appliance wattage
  2. Estimate the number of hours used
  3. Convert watts to kW
  4. Multiply by hours to get kWh
  5. Multiply by your tariff

If your plan includes separate peak and off-peak rates, keep the calculations separate for each time period. That matters for appliances you can run overnight, such as dishwashers or laundry machines.

Worked example with a hypothetical tariff

Here is a simple example using made-up numbers.

Suppose you have a 1500-watt appliance that runs for 2 hours, and your tariff is $0.20 per kWh.

First, convert watts to kilowatts:

1.5 kW

Then calculate the energy used:

3 kWh

Now multiply by the tariff:

0.6 cost in dollars

So this appliance costs 0.6 to run for 2 hours at that tariff.

You can use the same method for almost any appliance. If the wattage is lower, the cost falls. If the run time is longer, the cost rises in direct proportion.

Standby power can still cost money

Some appliances use power even when they look off. This is called standby power, phantom load, or idle power. It is common in televisions, game consoles, microwave ovens, chargers, and devices with clocks or remote sensors.

The calculation is the same, but you use the standby wattage and the number of hours the device stays plugged in.

For example, if a device draws 5 watts in standby and stays plugged in for 24 hours, the energy use is:

0.12 kWh per day

Then the daily cost is:

0.02 per day

That may look small, but standby use can add up if many devices stay plugged in all the time. If you are checking where electricity is going in your home, the cost of living calculator can help you think about utility costs alongside other regular expenses.

How to estimate a bill from several appliances

To estimate the cost of a room, a home office, or part of your household, calculate each appliance separately and add the results. That is often more useful than trying to guess one total wattage.

A practical way to do it:

  • List the appliances you want to track
  • Write down each watt rating
  • Estimate daily or weekly hours of use
  • Calculate each cost separately
  • Add the results for a total

This method is especially helpful for items that do not run continuously. For example, a kettle may use a lot of power but only for a short time, while a router may use less power but run all day. If you need to compare appliance running costs with other transport or utility expenses, our trip cost calculator shows the same general idea of turning usage into money.

Reading your bill and checking your estimate

Your electricity bill usually shows how much energy you used in kWh, plus the rate or rates you were charged. To check your estimate, compare your calculated kWh with the bill’s usage figure for the same period.

Look for:

  • Total kWh used during the billing period
  • The unit rate or rates per kWh
  • Any fixed charges that are not based on usage
  • Any time-based differences, such as peak and off-peak pricing

A few things can make your estimate differ from the bill:

  • Appliances may use more or less power than the label suggests
  • Heating and cooling equipment often cycles on and off
  • Some devices use different power in different modes
  • Your bill may include standing charges or taxes separate from energy use

If your goal is to understand why the bill changed, compare one month with another and note which appliances were used more often. For a careful record, keep the calculations in a notes app or spreadsheet so you can check assumptions later.

When a simple estimate is not enough

The watt-hours formula is good for most everyday checks, but it is still an estimate. If you need a more exact figure, use a plug-in electricity meter for small appliances, or a smart meter reading if your supplier provides usage data.

You may also want extra detail when:

  • An appliance has several power modes
  • The device cycles on and off automatically
  • You are trying to separate standby use from active use
  • Your tariff changes by time of day

In those cases, the same core method still applies. You just need better input data.

Frequently asked questions

What is the formula for appliance electricity cost?
Use kWh = watts / 1000 x hours, then cost = kWh x tariff. This gives you an estimate of how much an appliance costs to run for a given time.
How do I calculate cost if I only know watts?
Watts alone are not enough to find cost. You also need the number of hours used and your electricity tariff per kWh. Once you have those, you can calculate the energy and the price.
Do I use watts or kilowatts in the formula?
Start with watts, then divide by 1000 to convert to kilowatts. That makes the result compatible with the kWh unit used on electricity bills.
Does standby power really matter?
Yes, especially for devices that stay plugged in all day. One device may not cost much on its own, but several always-on devices can add up over time.
How do I estimate electricity cost from my bill?
Check the number of kWh used and the rate per kWh on the bill. Then multiply the usage by the rate, and add any fixed charges if you want the full bill total.
Why does my estimate not match my bill exactly?
Real appliances do not always draw a constant amount of power, and bills can include extra charges beyond energy use. Time-of-use rates and standby use can also change the total.
Can I calculate the cost of multiple appliances together?
Yes. Calculate each appliance separately using its own wattage and hours, then add the costs. That is the clearest way to estimate the total electricity use for a room or household area.

About this article

Written by Yahav Atia, WSO – the digital agency (wso.co.il) that owns and founded this site, with the help of AI, and passed automated quality checks before publication. Numbers in the worked examples were computed in code, not estimated.