Electricity Cost Calculator

What this calculator answers

What it costs to run an appliance, or a whole list of them, for an hour of use, a day, a month and a year, at the price you pay per kilowatt-hour. Enter each appliance’s power and how long it runs, or the kWh figure from its EnergyGuide label or a plug-in meter. The result shows which appliance takes the biggest share of the cost, how much standby power adds, and how the total changes if your price goes up or down.

How to use it

• Name (optional): shown in the tables and the chart.
• Enter as: Watts and hours when you know the power, or kWh from a label when you have an energy figure from an EnergyGuide label or a meter.
• Power: the rated watts from the label or nameplate, in W or kW. The line under the box shows the other unit (1,500 W = 1.5 kW), so a W/kW slip stands out.
• Quantity (optional): how many identical units, such as 8 bulbs. Blank counts as 1.
• Use each day: hours and minutes it is on, from 0 to 24 hours. A kettle used for 10 minutes is 0 h 10 min.
• Days used: every day, some days a week, some days a month or some days a year. Use some days a year for a heater or an air conditioner that runs only in its season.
• Energy use and Energy use per (kWh entry): the figure and whether it is per year, month or day.
• Add average draw or standby power: opens three optional boxes. Average draw is the share of the rated power the appliance uses on average (blank means 100%). Standby power is what it draws while switched off, in watts. Standby hours a day is blank for “every hour it isn’t on”.
• Price per kWh and Price unit: the energy charge from your bill, in dollars ($0.16) or cents (16¢). See where to find it below.
• Fixed monthly charge (optional): a charge that doesn’t change with use. It is added to the totals, never to an appliance.

The results update as you type. The Try buttons load a space heater, a kettle and a refrigerator label, all worked through below. To compare an old appliance with a new one, or two ways of using the same one, press Save for comparison, change the inputs and save again: each scenario shows how much its costs differ from the first.

How to calculate the cost of running an appliance

Convert the power to kilowatts, multiply by the hours it runs to get kilowatt-hours (kWh), and multiply the kWh by your price per kWh. Watts become kWh by multiplying by the hours and dividing by 1,000: a 1,500 W heater on for 8 hours uses 1,500 × 8 ÷ 1,000 = 12 kWh. Over a year:

$$E = \frac{W \times n \times d \times h \times D}{1000} \qquad C = E \times p$$

• $E$ is the energy in kWh a year.
• $W$ is the rated power of one unit in watts (1 kW = 1,000 W), and $n$ the number of units.
• $d$ is the average draw as a decimal (1 when the appliance runs at its full rating).
• $h$ is the hours it is on each day it is used, and $D$ the days it is used in a year.
• $p$ is the price per kWh in dollars, and $C$ the cost a year.

Standby adds its own watts times the hours spent in standby, divided by 1,000. With a kWh figure from a label, $E$ is that figure turned into a yearly amount (× 12 for a month, × 365.25 for a day). A month is then $E \div 12$ and a day $E \div 365.25$.

Worked example: five appliances at $0.16 per kWh

The calculator opens with one home’s list, priced at $0.16 per kWh with a $12.00 fixed monthly charge (example values, not current prices):

1. Refrigerator, from its EnergyGuide label: 420 kWh a year.
2. Gaming PC with a 650 W power supply that averages half of that while in use: 650 W × 50% = 325 W = 0.325 kW. For 3 hours a day that is 0.975 kWh, and × 365.25 days = 356.12 kWh a year.
3. TV, 120 W for 3 h 30 min a day: 0.12 kW × 3.5 h = 0.42 kWh a day, 153.41 kWh a year. Its 1 W standby runs for the other 20.5 hours of every day: 1 W × 20.5 h × 365.25 ÷ 1,000 = 7.49 kWh, so 160.89 kWh a year in all.
4. Clothes dryer, 5 kW for 50 minutes on 3 days a week: 5 kW × 50/60 h = 4.1667 kWh a load. Three days a week is 3 × 365.25 ÷ 7 = 156.54 days a year, so 652.23 kWh.
5. LED bulbs, 8 × 9 W for 5 hours a day: 0.072 kW × 5 h = 0.36 kWh a day, 131.49 kWh a year.

Together: 420 + 356.12 + 160.89 + 652.23 + 131.49 = 1,720.73 kWh a year, or 1,720.73 ÷ 12 = 143.39 kWh a month. At $0.16 per kWh that is $22.94 a month ($275.32 a year, $0.75 on an average day). With the $12.00 fixed charge the monthly total is $34.94, or $419.32 a year.

The dryer is the largest share at 37.9%, $8.70 a month, even though it runs only 2 h 30 min a week (3 loads of 50 minutes): 5 kW costs $0.80 an hour, so each 50-minute load is $0.67. The TV’s standby adds only $1.20 a year. The five rows round to $5.60, $4.75, $2.15, $8.70 and $1.75, which add to $22.95: one cent more than the total, because each row is rounded on its own.

How much does it cost to run a space heater?

A 1,500 W space heater running at full power costs 1.5 kW × your price per kWh for every hour. At $0.16 per kWh that is $0.24 an hour. Eight hours a day is 12 kWh, $1.92 a day, or $58.44 a month of 30.4375 days (the Space heater, 8 h button loads this case). The table of other hours shows 4 hours a day at $29.22 a month and 12 hours at $87.66.

Two things lower the real figure. A heater with a thermostat can switch off once the room is warm: if a plug-in meter shows it running at half its rating on average, enter an average draw of 50% and the month drops to $29.22. And heating is seasonal, so the year figure, $701.28, assumes a whole year of winter days. For a 90-day season, set Days used to some days a year and enter 90: the year becomes $1.92 × 90 = $172.80 (1,080 kWh), and the month figure, $14.40, is that season’s cost spread over 12 months. The assumptions under the result keep the other number in view: a month of use every day still costs $58.44.

Short uses work the same way in minutes. A 1,500 W kettle on for 10 minutes a day uses 1.5 kW × 1/6 h = 0.25 kWh, which is $0.04 a day, $1.22 a month and $14.61 a year at $0.16 per kWh.

Where to find your price per kWh

Look on your bill for the energy (or supply) charge per kWh, written like $0.16/kWh or 16¢ per kWh, and choose the matching price unit. If your bill splits supply and delivery into two per-kWh charges, add them. Prices vary from place to place, by season and sometimes by time of day, so use your own figure rather than a national average.

Dividing a bill’s total by the kWh it lists gives an average that already includes the fixed charges and taxes. That average works as a rough price, but then leave the fixed monthly charge blank, or it is counted twice.

If you type a dollar price of 1 or more, or a cent price below 1, the calculator asks whether you meant the other unit: $16 per kWh is far more likely to be 16¢.

Using the kWh from an EnergyGuide label or a meter

The yellow EnergyGuide label on refrigerators, freezers, dishwashers, clothes washers, water heaters, room air conditioners, TVs and some other appliances shows how much energy the model uses in typical use. Its dollar figure is based on a national average price, so enter the kWh figure with your own price instead. Choose kWh from a label, type the figure and pick year, month or day.

A refrigerator labeled 420 kWh a year uses 35 kWh a month, which costs $5.60 a month or $67.20 a year at $0.16 per kWh (the Fridge, 420 kWh button). An older fridge that uses 700 kWh a year would cost $112.00, so replacing it saves $44.80 a year at that price. Save the first result for comparison, change 420 to 700, and the change column shows the difference. Dividing what the new model costs you by that yearly saving gives a rough payback time: $448 would take $448 ÷ $44.80 = 10 years at an unchanged price.

A plug-in electricity usage monitor measures the actual kWh of a 120-volt device over any period, which is especially useful for appliances that don’t run constantly, such as a fridge. It can’t measure large appliances wired for 220 volts or more, such as electric clothes dryers and central air conditioners.

Why real use can differ from the wattage label

The wattage on a nameplate is the most the appliance draws. Many run below it most of the time: a refrigerator switches on and off as needed to hold its temperature, and appliances with settings, such as a fan on low, draw less than the maximum. The Energy Saver method estimates a refrigerator’s running time at full power as a third of the time it is plugged in, which is an average draw of about 33%.

Use Average draw for these. The example’s gaming PC is entered at 50% of its 650 W power supply rating, which halves its cost compared with the rating. A meter reading or a label kWh figure is better than a guessed percentage whenever you can get one.

Standby power: devices that use electricity when “off”

A device keeps drawing a little power while it is switched off if it has a remote control, a display or light that stays on (even one LED), a wall adapter or other external power supply, a built-in battery, or a network connection that is always on. Each draws only a little, but it draws it every hour of the year: 1 W drawn all day, every day, is 24 × 365.25 ÷ 1,000 = 8.77 kWh a year, which is $1.40 at $0.16 per kWh. Ten watts of standby spread across the house add $14.03 a year.

Enter standby watts under Add average draw or standby power. Leave Standby hours a day blank and the calculator counts every hour the appliance isn’t in use, on the days it is used and on the days it isn’t. A plug-in meter shows the standby watts; readings below about 1 W are less accurate.

How the calculator counts days, months and years

A year is 365.25 days, the average over a four-year leap-year cycle, and a month is one twelfth of it, 30.4375 days. A week is 7 days. Everything is worked out per year first and then divided, so the day, week, month and year figures always agree with each other.

• Every day: 365.25 days of use a year.
• Some days a week: days × 365.25 ÷ 7 a year, so 3 days a week is 156.54 days.
• Some days a month: days × 12 a year (up to 30; choose every day for a full month).
• Some days a year: the days you enter, up to 365. It matches the Energy Saver method’s number of days used a year and suits seasonal appliances. The month figure is then the yearly cost divided by 12, an average rather than a month in season.

A billing period runs from one meter reading to the next and is rarely exactly 30.4375 days, so a single bill can differ from the monthly figure even when every input is right.

Reading the result

• Cost a month is the energy cost of everything on the list, with the fixed charge added in the line below it.
• The three tiles show a day and a year, plus the cost of an hour of use for a single appliance (a week for a single kWh figure) or the largest share for a list.
• Where the energy cost goes ranks the appliances by monthly cost; with more than six, the smallest are grouped as “Other”.
• Cost by appliance (the Cost tab) lists each one’s cost a month, a year and on an average day, its share, and the cost of an hour of use (all units on) and of a day it is used (one load, one evening). The Energy (kWh) tab gives kWh a day, a month and a year, with a standby column when any appliance has standby power. Cost by period gives the kWh, energy cost, fixed charge and total for a day, week, month and year. Each table downloads as CSV.
• Cost at other prices per kWh repeats the calculation 2¢ and 4¢ either side of your price. In the example every 2¢ adds $2.87 a month. For a single appliance, Cost at other hours of use does the same for its hours.

Assumptions and limitations

• Each appliance draws its rated power times its average draw the whole time it is on. Real draw varies with settings, load, age and room temperature.
• One flat price per kWh. Tiered rates, time-of-use rates, taxes and separate delivery charges are not modeled unless you fold them into the price.
• Every day, some days a week and some days a month assume the same use all year; for a seasonal appliance choose some days a year.
• The fixed charge belongs to the whole bill and is never split across appliances.
• Charger and battery losses (phones, laptops, electric cars) are not added; enter the kWh drawn from the wall if you have it.
• Results are rounded for display only. This is an educational estimate, not a quote or a bill from your utility.

Common mistakes

• Typing cents in the dollar box. 16 in dollars is $16 per kWh; either type 0.16 or switch the unit to ¢ per kWh.
• Typing kilowatts as watts. A dryer rated 5 kW is 5,000 W; 5 with the unit set to W is a 5-watt night light.
• Putting minutes in the hours box. Ten minutes of kettle use goes in the minutes box; 10 in the hours box is 10 hours a day.
• Using the nameplate for a cycling appliance. A fridge counted at its full rating all day comes out about three times the Energy Saver estimate, which counts a third of that time; use an average draw, the label kWh or a meter reading.
• Forgetting the quantity. Eight 9 W bulbs use eight times the energy of one.
• Counting the fixed charge twice. If your price is the bill total divided by kWh, it already includes the fixed charge.
• Treating a winter month as a year. A heater’s year is the days it actually runs, not 365; enter them under some days a year.