Electricity Usage Calculator
How much energy a device uses — watts, hours, and days to kWh, no dollars.
Updated
You need
270 kWhused
9 kWh a day over 30 days
- Energy per day
- 9 kWh
- Over the period
- 270 kWh
- Per year at this rate
- 3,285 kWh
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In short
How many kWh does a device use?
Energy in kilowatt-hours is watts divided by 1,000, times hours per day, times days. A 1,500 W device running 6 hours a day uses 9 kWh a day, 270 kWh across a 30-day month, and 3,285 kWh over a full year. Watts on their own tell you nothing about consumption; the runtime multiplier does most of the work.
This tool deliberately stops at energy, because price depends on your state, plan, and even the hour of day, so the kilowatt-hours here are the part that is universally true.
How to use the electricity usage calculator
Enter the power of the device in watts, how many hours a day it runs, and over how many days, and the tool returns the energy it consumes in kilowatt-hours for the whole window, for a single day, and across a year at that same usage. It answers one question and refuses the other.
How much energy something uses is a matter of physics that is identical in every country. What that energy costs is a matter of tariff, state, supplier, and sometimes the hour of the day, so mixing the two into one number hides which half of the answer is solid.
When you want the answer in dollars
When you want dollars, take the kilowatt-hours produced here into the electricity cost calculator and apply the rate from your own bill.
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9 kWh
Per day
1,500 W running 6 hours
270 kWh
Per 30-day month
the tool default window
3,285 kWh
Across a year
at that same daily pace
Watts can be the nameplate figure or the real draw. The label states the maximum the device may pull, and a great deal of equipment never gets near it in ordinary use. Anything thermostat-controlled is the classic offender: a refrigerator, a chest freezer, a window air conditioner, or an electric water heater energises its compressor or element for part of each hour and idles for the rest, so its average draw sits well below the plate.
If your label prints volts and amps but not watts, multiply them, so a 12 A appliance on a 120 V circuit is about 1,440 W. For a figure you can defend rather than assume, a plug-in energy monitor left at the outlet reads the true draw and accumulates the true kilowatt-hours.
Do
- Use genuine runtime rather than the hours the device sits plugged in.
- Enter 24 hours a day for a standby draw, which really never stops.
- Multiply volts by amps when the label prints current but not watts.
- Note your meter reading twice and subtract to check the estimate.
Don't
- Enter a nameplate wattage for a thermostat-controlled appliance without allowing for cycling.
- Expect a fixed figure from a variable load such as a computer or printer.
- Look for dollars here, because this tool stops deliberately at energy.
- Assume one meter reading tells you anything on its own.
What a continuous draw adds up to. Standby lights, network boxes, always-on speakers, and anything left idling run all 8,760 hours of the year, so a small wattage becomes a real block of energy. Every figure is computed as watts times 24 hours, with the month at 30 days and the year at 365.
| Continuous draw, watts | kWh per day | kWh per 30-day month | kWh per year | Share of a 899 kWh household month |
|---|---|---|---|---|
| 0.5 W (a well-designed standby light) | 0.012 | 0.36 | 4.4 | 0.04 percent |
| 1 W | 0.024 | 0.72 | 8.8 | 0.08 percent |
| 2 W | 0.048 | 1.44 | 17.5 | 0.16 percent |
| 3 W (a typical set-top or console standby) | 0.072 | 2.16 | 26.3 | 0.24 percent |
| 5 W | 0.12 | 3.6 | 43.8 | 0.40 percent |
| 8 W | 0.192 | 5.76 | 70.1 | 0.64 percent |
| 10 W (a smart speaker or small router) | 0.24 | 7.2 | 87.6 | 0.80 percent |
| 15 W | 0.36 | 10.8 | 131.4 | 1.2 percent |
| 20 W | 0.48 | 14.4 | 175.2 | 1.6 percent |
| 30 W (ten devices idling at 3 W each) | 0.72 | 21.6 | 262.8 | 2.4 percent |
| 50 W | 1.2 | 36 | 438 | 4.0 percent |
| 75 W | 1.8 | 54 | 657 | 6.0 percent |
| 100 W | 2.4 | 72 | 876 | 8.0 percent |
| 150 W (a fridge, if it never cycled off) | 3.6 | 108 | 1,314 | 12.0 percent |
How honest do the hours have to be?
Be equally careful with hours and days, because runtime usually decides more of the answer than wattage does. A space heater described as on all evening runs five or six hours. A television left on as background noise runs far longer than anyone claims. A fridge sits in the kitchen twenty-four hours a day but its compressor works a fraction of that, which is exactly why the nameplate misleads.
Set the days to the window you care about, so 30 for a typical month, 7 for a week, or 365 for a year, and set the hours to the time power is genuinely flowing. One deliberate exception is standby draw: a device idling at a few watts really does run all 8,760 hours of the year, so 24 hours a day is the correct entry there.
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The formula, worked line by line
Energy is power multiplied by time. Watts are divided by 1,000 to become kilowatts so the units match the kilowatt-hours your meter counts, and one kilowatt sustained for one hour is one kilowatt-hour. There is no rate anywhere in this calculation and no currency in the answer, which is the point: the energy figure is the same whether you live in Idaho, Hawaii, or Hampshire.
Because the expression is a plain multiplication, every input scales the result proportionally. Double the wattage and the energy doubles. Double the hours and it doubles again. That linearity is what makes the shortcut useful: once you know a device uses 1.5 kWh per hour, any window at all is one more multiplication away.
kWh = (watts / 1000) x hours per day x days
daily kWh = (watts / 1000) x hours per day
annual kWh = daily kWh x 365
continuous draw: annual kWh = watts x 8.76- Power
- 1,500 W = 1.5 kW
- Hours per day
- 1.5 × 6 = 9 kWh a day
- Days
- × 30
- Energy for the month
- 270 kWh
Held at that same six hours a day all year it would be 3,285 kWh. Change the runtime to 1 hour a day and the same month is only 45 kWh, which is the clearest demonstration that wattage alone predicts nothing.
The 8.76 shortcut
The last line in the block is the one worth memorising. Anything that draws power continuously uses its wattage times 8.76 kilowatt-hours a year, because 24 hours times 365 days divided by 1,000 is 8.76.
- 1 W left on permanently
- 8.8 kWh a year
- 3 W, a typical standby
- 26.3 kWh a year
- 10 W, a small router
- 87.6 kWh a year
Computed as watts × 8.76, the continuous-draw line in the formula block.
That single factor converts any standby figure into an annual block of energy in your head, and it is why the always-on devices in a house matter more than their tiny wattages suggest.
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Questions people ask
How many kWh does a 1,500 W device use?
It depends entirely on runtime, because watts describe a rate rather than a quantity. A 1,500 W device uses 1.5 kWh for every hour it runs. At 6 hours a day for 30 days that is 9 kWh a day and 270 kWh for the month, and held at that pace all year it would be 3,285 kWh. Cut the runtime to 1 hour a day and the same month is 45 kWh. Multiply 1.5 by your hours per day and then by your number of days to size any window you like.
What is a kWh, exactly?
A kilowatt-hour is one kilowatt of power sustained for one hour: ten 100 W bulbs burning for an hour, or a single 1,000 W microwave running for an hour. It is a unit of energy rather than power, which is the distinction that trips people up. Watts say how fast you are drawing energy at this instant; a kilowatt-hour says how much you drew over a stretch of time. Your meter counts kilowatt-hours and your statement charges for kilowatt-hours, which is why this tool outputs them and nothing else.
How do I read my electricity meter to find usage?
Your meter shows a cumulative kilowatt-hour total that only ever climbs, so a single reading tells you nothing on its own. Note the number at the start of a period and again at the end, then subtract the first from the second, and the difference is exactly the energy the property used in between. Two readings a week apart give you a week of usage; a month apart give you a month. That subtraction is the ground truth this calculator estimates toward, and it is the fastest way to check whether a device tally adds up.
Why does this tool show kWh but not dollars?
Because energy and price are different questions and only one of them has a universal answer. How much energy a device uses follows from watts and time and is identical everywhere. What that energy costs depends on your state, your supplier, your tariff structure, and in many cases the hour of the day you used it, so folding a guessed rate into the result would hide which half of the number is solid. This tool nails the energy cleanly, then hands the kilowatt-hours to the electricity cost calculator where you enter your own rate.
How does the kWh here compare to the line on my bill?
They are the same unit, so they are directly comparable. Your bill totals the kilowatt-hours the whole property drew across the billing period, while this calculator estimates the kilowatt-hours one device draws across a window you choose, so dividing one by the other gives that device its share. For a sense of scale, the Energy Information Administration reported that the average US residential utility customer bought about 899 kWh a month in 2022, on a page last updated 8 January 2024. Households vary hugely around that, so use your own statement as the reference.
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
The 10,791 kWh a year, about 899 kWh a month, average US residential utility customer figure for 2022.
How much electricity does an American home use? (FAQ) — US Energy Information Administration, 2022 data; page last updated 8 January 2024
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