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Electricity cost calculator
Put your watts, hours per day, days and rate in: a 100-watt device run 5 hours a day for 30 days uses 15 kWh a month — $2.55 a month at $0.17 a kWh, $30.60 a year.
★ MONEY · ENERGY · BILLS
Electricity Cost Calculator: $2.55 a Month, 100 Watts
On the defaults — a 100-watt device run 5 hours a day for 30 days at $0.17 a kWh — electricity cost is $2.55 a month, 15 kWh a month, and $30.60 a year. This is the electricity cost tool: multiply watts by hours and days for the energy, divide by 1,000 for kilowatt-hours, and multiply by the rate for the bill — a device quoted in watts alone tells you nothing until the hours and the rate are known.
What does $2.55 a month, 15 kWh a month mean?
It means a 100-watt device running 5 hours a day for a 30-day month draws 15 kilowatt-hours, and at $0.17 a kWh that energy costs $2.55 for the month, or $30.60 across a year of identical months. The watts set the appetite, the hours and days set how long it eats, and the rate sets only the price of what was eaten: the same 15 kWh costs $3.75 where power is $0.25 a kWh, while the energy itself never changes. The only judgement is the watts and the hours together — a label’s watts are a maximum, and a device that cycles on and off draws far less on average than its label claims.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| kWh a month | watts × hours × days ÷ 1000 | 15 kWh — 100 × 5 × 30 ÷ 1000. |
| Cost a month | kWh × rate | $2.55 — 15 × $0.17. |
| Cost a year | monthly cost × 12 | $30.60 — $2.55 × 12. |
Three things to check before you trust the number
First, check the watts honestly: a 100-watt bulb draws 100 watts only while it is on, while a fridge rated at 150 watts cycles and averages far less — the calculator trusts the watts you enter, so enter the average draw, not the peak on the label. Second, check the hours against a real day: 5 hours means the device is actually running for 5 hours, not merely plugged in for 5 — standby watts are small, but counting plugged-in hours as running hours inflates the month. Third, check the rate on the bill, not on a memory: $0.17 a kWh is a planning rate, and the rate on your own bill, including delivery charges if your bill bundles them, is the rate that settles the month.
Worked example
100 watts, 5 hours a day, 30 days, $0.17 a kWh. kWh: 100 × 5 × 30 ÷ 1000 = 15. Monthly cost: 15 × $0.17 = $2.55. Yearly cost: $2.55 × 12 = $30.60. In practice that means a single 100-watt bulb left on five hours a day costs about two and a half dollars a month — the number to quote when someone asks whether switching it off matters, and the yearly figure to use when a more efficient bulb promises to pay for itself.
Every branch, checked
With 200 watts instead of 100, the month rises to 30 kWh — $5.10 a month, $61.20 a year. With 8 hours a day instead of 5, the month rises to 24 kWh — $4.08 a month, $48.96 a year. With 20 days instead of 30, the month falls to 10 kWh — $1.70 a month, $20.40 a year. With a rate of $0.25 a kWh instead of $0.17, the energy stays 15 kWh but the month rises to $3.75, and the year to $45.00.
The reverses, worked backwards
The watts behind $2.55 are 100: at 50 watts the month is 7.5 kWh ($1.28 a month, $15.36 a year); at 150 watts it is 22.5 kWh ($3.83 a month, $45.96 a year). The hours behind $2.55 are 5: at 3 hours the month is 9 kWh ($1.53 a month, $18.36 a year); at 8 hours it is 24 kWh ($4.08 a month, $48.96 a year). The rate behind $2.55 is $0.17: at $0.12 the month is still 15 kWh but costs $1.80 ($21.60 a year); at $0.25 it is still 15 kWh but costs $3.75 ($45.00 a year) — the rate never touches the energy, only the money.
What the defaults assume
The defaults assume a steady 100-watt draw, 5 hours a day, for 30 days, at $0.17 a kWh. They do not assume a cycling appliance, a device on standby, tiered pricing, or taxes and fees beyond the rate entered — a fridge, a charger left plugged in, and a bill with tiered rates all need their own average watts and their own true rate, never the defaults borrowed.
Who uses this
- A homeowner checking a single bulb: the $2.55 month settles whether the bulb is worth switching off.
- A renter comparing two appliances: the kWh figure off the calculator is the line to compare, not the watt label.
- A household budgeting the power bill: the $30.60 year is the budget line for one device, not a guess.
- A shopper choosing an efficient replacement: the yearly saving is the payback figure for the dearer, thriftier device.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the average watts and the true rate. Real devices cycle, standby loads sip power, and bills bundle delivery charges, tiers and taxes that a single rate cannot capture — so treat $2.55 as the planning month for a steady 100-watt device at 5 hours a day and $0.17 a kWh, and check it against a metered month before quoting it to anyone else.
Cite this calculator
Bright Side Kit, “Electricity Cost Calculator: $2.55 a Month, 100 Watts”, https://brightoolbox.com/electricity-cost-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (100 watts, 5 hours a day, 30 days, $0.17 a kWh) so the result can be checked.
Frequently asked questions
How much does a 100-watt bulb cost a month?
$2.55 on the defaults: 100 watts × 5 hours × 30 days ÷ 1000 = 15 kWh, and 15 × $0.17 = $2.55, or $30.60 across a year of identical months.
What is a kilowatt-hour?
It is 1,000 watts drawn for one hour: a 100-watt device running for 10 hours uses exactly 1 kWh. The bill counts kilowatt-hours, not watts, which is why hours matter as much as the label.
Does a higher rate change the kWh?
No: the rate moves only the money. At $0.25 a kWh the same device still uses 15 kWh a month, but the month costs $3.75 instead of $2.55.
Why is the yearly figure $30.60, not a metered year?
Because it repeats the same 30-day month twelve times. A real year has months of different lengths and different use, so treat $30.60 as the planning year for identical months, not a prediction.
Should I use the label watts or the average watts?
The average watts: a fridge rated at 150 watts may average 60, and a heater rated at 1,500 watts may cycle at half that. A plug meter over a real week gives the honest watts.
What if my device runs 24 hours a day?
Then enter 24 hours: a 100-watt device running around the clock uses 72 kWh in a 30-day month, costing $12.24 at $0.17 a kWh. The calculator allows up to 24 hours, never more.