kWh Calculator

Q: How does the calculator handle days per week?

It spreads your use across the whole week. The math is (watts ÷ 1,000) × hours per day × (days ÷ 7) . So a dryer used 1 hour on 3 days a week counts as about 0.43 hours per day on average. That keeps your monthly and yearly totals correct.

kWh Calculator

Updated Aug 14, 2026 By Jehan Wadia

Introduction

A kilowatt-hour (kWh) is how power companies measure the energy you use. This kWh calculator shows how much energy your appliances use and what that energy costs you each day, month, and year.

The math is simple: watts × hours ÷ 1,000 = kWh. This tool does it for you. Pick an appliance, set how many hours a day and days a week you use it, and add your electric rate. You can add as many appliances as you want and see which ones use the most power. If you want to focus on dollars instead of energy, the Electricity Cost Calculator works from the same numbers.

There are two ways to use it:

Appliance kWh Calculator — Add each device, one at a time, and get a full energy and cost total. See also the Power Consumption Calculator for a single-device breakdown.
Whole-Home Monthly Estimator — Enter your home size, number of people, habits, and climate to estimate your monthly bill usage.

You can switch units for power (W, kW, hp, BTU/h) and energy (kWh, MWh, MJ, BTU, therms). A built-in wattage chart lists common household appliances, so you don't need to look up numbers. Every answer comes with step-by-step math and a bar chart, so you can see exactly where your electricity goes and where you can save money.

How to use our kWh Calculator

Enter what your appliances use and how long you run them, or enter facts about your home, and the calculator shows your energy use in kWh per day, month, and year plus your estimated electricity cost.

Appliance Preset: Pick an appliance from the list. Its typical wattage fills in for you. Pick "Custom" to type your own.

Appliance Name: Type a name for your device. This is optional and helps label custom items in your list.

Power Consumption: Type how much power the appliance uses. Check the label or nameplate on the device. Choose W, kW, hp, or BTU/h in the box next to it. If your label only lists amps, the Amps to Watts Calculator converts it for you.

Hours Used Per Day: Type or slide how many hours a day you run the appliance. Use any number from 0.1 to 24.

Days Used Per Week: Choose how many days each week you use it, from 1 to 7.

Add Appliance: Click this to put the appliance in your list. Add as many as you like, then remove any with the trash button.

Energy Display Unit: Choose how to show your results: kWh, MWh, MJ, BTU, or therms.

Electricity Rate: Type the price you pay for power. Find it on your electric bill. The U.S. average is about $0.17 per kWh. Pick the matching unit next to it.

Appliance Wattage Reference Table: Filter by category, sort the columns, and click "Use This" to load any appliance into the form. The Wattage Calculator helps if your device isn't listed.

Whole-Home Monthly Estimator

Home Size: Type your floor area in sq ft, m², or sq yd. It must be between 425 and 3,500 sq ft. Not sure of your area? Measure it first with the Square Footage Calculator.

Quick Size Presets: Pick a home size from the drop-down instead of typing one.

Number of Residents: Choose how many people live in the home, from 1 to 6 or more.

Energy Usage Habits: Choose how much power your household uses compared to normal, from −20% to +20%.

Climate Zone: Pick the weather where you live. Hot and cold areas use more power for heating and cooling. To size that load properly, try the BTU Calculator or the AC Tonnage Calculator.

Energy Display Unit: Choose the unit for your home results: kWh, MWh, MJ, BTU, or therms.

Click Calculate to see your totals, a bar chart, and the step-by-step math. Click Reset to start over.

What Is a kWh?

A kilowatt-hour (kWh) is how electricity is measured and billed. It means using 1,000 watts of power for one hour. A 1,000-watt space heater running for one hour uses 1 kWh. A 100-watt light bulb needs 10 hours to use that same 1 kWh.

The kWh Formula

Energy use has three parts: how much power a device pulls, how long it runs, and how often you use it.

kWh per day = (watts ÷ 1,000) × hours per day
kWh per month = kWh per day × 30
Cost = kWh × your electricity rate

Example: a 1,200-watt window air conditioner run 6 hours a day uses (1,200 ÷ 1,000) × 6 = 7.2 kWh per day. At $0.17 per kWh, that is about $1.22 a day, or roughly $37 a month.

Watts vs. Kilowatt-Hours

Watts (W) tell you how fast a device pulls power right now. Kilowatt-hours tell you how much energy it used over time. Think of watts as speed and kWh as distance traveled. Your power bill charges for kWh, not watts. The Power Calculator and the kW Calculator cover the instantaneous side of that relationship.

Where to Find an Appliance's Wattage

Look at the label or sticker on the back or bottom of the device, or check the owner's manual. If the label lists volts and amps instead, multiply them: volts × amps = watts. The Ohms Law Calculator handles that conversion along with resistance, and the Watts to Amps Calculator works in the other direction. A plug-in energy meter gives the most exact reading, since many devices do not run at full power all the time.

What Uses the Most Electricity at Home

Anything that makes heat or moves air uses the most. Electric water heaters, clothes dryers, central air conditioning, space heaters, and electric ovens are the biggest users in most homes. Small items like phone chargers, routers, and LED bulbs use very little, even if they stay on all day. Refrigerators use modest power but run 24 hours a day, so they still add up. If you are wiring a new high-draw circuit, check the Wire Size Calculator and the Voltage Drop Calculator first.

Typical Home Energy Use

The average U.S. home uses about 875 kWh a month, or a little over 10,000 kWh a year. Your number depends on home size, how many people live there, your climate, and your habits. Bigger homes and hot or very cold regions use more, because heating and cooling do most of the work. A Heat Loss Calculator shows how much of that goes out through your walls and windows.

Ways to Lower Your kWh

Move your thermostat a few degrees closer to the outside temperature.
Wash clothes in cold water and air-dry when you can.
Swap old bulbs for LEDs.
Unplug or power-strip devices that idle all day.
Run big appliances during off-peak hours if your utility offers lower rates.
Offset your usage with panels — size a system with the Solar Panel Calculator, and see how your usage affects your carbon footprint.

Other Energy Units

1 kWh equals 3.6 megajoules (MJ), 3,412 BTU, or about 0.034 therms. 1,000 kWh equals 1 megawatt-hour (MWh). These units all measure the same thing: energy. Gas bills often use therms or BTU, while electric bills use kWh. For motor ratings given in horsepower, the Horsepower Calculator ties hp back to watts, and the Generator Sizing Calculator converts your total load into the backup capacity you need.

Formulas used

Daily energy use of one appliance

E_{day} = \frac{P_{W}}{1000} \times h \times \frac{d}{7}

Total energy per day, month, and year

E_{total} = \sum_i E_{day,i}, \quad E_{month} = E_{total} \times 30, \quad E_{year} = E_{total} \times 365

Estimated running cost

C = E_{kWh} \times r_{kWh}, \quad r_{kWh} = \frac{r_{unit}}{k_{unit}}

Energy unit conversion from kWh

E_{unit} = \frac{E_{kWh}}{k_{unit}}, \quad k_{kWh}=1,\ k_{MWh}=1000,\ k_{MJ}=0.2\overline{7},\ k_{BTU}=0.00029307107,\ k_{therm}=29.30011

Appliance share of total daily energy

S_i = \frac{E_{day,i}}{E_{total}} \times 100\%

Whole-home base monthly energy

E_{base} = \left(A_{ft^2} \times 0.35\right) + \left(N_{residents} \times 45\right)

Whole-home monthly, annual, and daily energy

E_{month} = E_{base} \times \left(1 + \frac{p}{100}\right) \times m_{climate}, \quad E_{year} = E_{month} \times 12, \quad E_{day} = \frac{E_{month}}{30}

Comparison with U.S. average household

\Delta\% = \frac{E_{month} - 875}{875} \times 100\%

Frequently asked questions

How does the calculator handle days per week?

It spreads your use across the whole week. The math is (watts ÷ 1,000) × hours per day × (days ÷ 7).

So a dryer used 1 hour on 3 days a week counts as about 0.43 hours per day on average. That keeps your monthly and yearly totals correct.

Why does the tool use 30 days for a month and 365 days for a year?

They are simple, steady numbers. Real months are 28 to 31 days, so a 30-day month keeps every appliance on the same scale.

Your real bill may cover 29 or 32 days. Expect a small difference, usually under 5%.

Why is my real power bill higher than this estimate?

Bills include more than energy. Common extras are:

Delivery, service, and meter fees
Taxes
Tiered rates that rise after you pass a set amount
Appliances you forgot to add
Standby power from devices left plugged in

The calculator shows energy cost only.

What rate should I enter if my bill has tiers or extra fees?

Use your effective rate. Take the total dollar amount on your bill and divide it by the kWh listed on the same bill.

That single number covers fees and tiers, so your cost estimate lands closer to reality.

Should I use running watts or surge watts?

Use running watts. Surge or startup watts last only a second or two when a motor kicks on, so they barely change your kWh.

Surge watts matter for sizing a generator or breaker, not for your bill.

My fridge is listed at 150 watts, but the label says 700. Which is right?

Both. The label shows peak draw when the compressor runs. A fridge cycles on and off, so it only runs part of each hour.

The 150 W midpoint in this tool is an average across the full day, which is what you want for kWh.

What does the Share column mean?

It shows what percent of your total daily energy each appliance uses.

The item with the biggest share is your best target for savings. Cutting 20% off a 40% item helps far more than cutting a 2% item.

Can I save my appliance list?

No. The list clears when you reload or reset the page.

Write down or screenshot your totals before you leave. You can also print the page.

How accurate is the Whole-Home Monthly Estimator?

It is a ballpark, not a bill. It uses floor area, people, habits, and climate to build a typical number.

It cannot know your insulation, window count, appliance age, or heating fuel. For sharper numbers, list your real appliances in the first tab.

Does the home estimator assume electric heat?

It assumes a mixed home with electric cooling and some electric heat.

If you heat with gas, oil, or propane, your electric use will be lower than shown. Pick a conservative habit setting like −10% or −20% to adjust.

Why is home size limited to 425 to 3,500 sq ft?

That range covers most single homes and apartments, and the built-in formula is tuned for it.

Outside that range the estimate drifts too far. For very large or very small spaces, add your appliances one by one instead.

What is the difference between the two tabs?

The Appliance kWh Calculator adds up devices you pick, so it is exact for the items you list.

The Whole-Home Monthly Estimator guesses your total bill from home facts, with no device list needed. Use it when you want a quick check.

My appliance is rated in BTU/h or horsepower. Can I still use it?

Yes. Pick BTU/h or hp in the box next to the power field and type the number.

The tool converts it to watts for you. Switching units also converts any value already typed in.

For an air conditioner, is the BTU rating the same as power used?

No. The BTU rating on an AC is cooling output, not the electricity it pulls.

A 10,000 BTU window unit may only draw about 1,000 watts. Use the watts or amps on the nameplate, not the cooling BTU.

Does this count devices that are off but still plugged in?

Only if you add them. Standby power, sometimes called vampire load, is often 1 to 10 watts per device.

To include it, add the device with its standby watts, 24 hours a day, 7 days a week.

Can I use another currency?

Yes. Type your local price per kWh in the rate box. The math does not care about the currency.

Results show a dollar sign, so read those numbers as your own money units.

Why did my numbers change when I switched the energy display unit?

The energy did not change, only the label. 1 kWh is the same energy as 3.6 MJ or 3,412 BTU.

Each result card also shows the kWh value in small text so you can check it.

How do I compare two appliances before I buy?

Add both to the list with the same hours and days. Then look at the Energy per Month column and the yearly cost.

The gap between them is your yearly savings from the better model.

Can I use this for a shop, office, or RV?

Yes, for the appliance tab. Any device with a wattage works, no matter where it is plugged in.

The whole-home tab is built for houses and apartments, so skip it for other spaces.

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