How to Calculate Electricity Cost in 5 Simple Steps

Electricity cost in one line

Electricity cost is counted in three moves: multiply watts by hours and days, divide by 1,000 for kilowatt-hours, and multiply by your rate for the bill. A 100-watt device run 5 hours a day for 30 days uses 15 kWh, and at $0.17 a kWh that is $2.55 a month, or $30.60 across a year of identical months. Everything else in this guide is about getting the watts, the hours, and the rate honest, because the formula is simple and the inputs are where householders fool themselves.

Step 1: Find the watts, honestly

Find the watts the device actually draws: the worked example uses 100 watts, an old-style bulb or a small fan. The label watts are a maximum, not an average — a fridge rated at 150 watts cycles on and off and may average 60, a television rated at 120 watts may draw far less on a dim scene, and a charger rated at 65 watts draws that only while charging a flat battery. The honest watts come from a plug meter over a real week, or from the device’s own energy label where one exists, and a guessed wattage is a guessed bill with decimals attached. Write the watts down before moving on, because a bill built on a label’s peak describes a device that never rests, and most devices rest more than they work.

Step 2: Count the hours it actually runs

Count the hours the device actually runs each day: the example uses 5 hours. Running hours are not plugged-in hours — a television plugged in for 16 hours but watched for 5 costs the 5-hour figure, not the 16-hour one, and a charger left in the socket after the battery is full costs almost nothing, not its full rating. Count a real week and average it, because a single day misleads: a washing machine used twice on Saturday and never on Tuesday averages less per day than Saturday alone suggests, and a heater used only on cold evenings averages less across a mild week than a cold night suggests. Days in the period work the same way: 30 days is a month, and a device used only on weekdays should be counted over its real days, not a borrowed month.

Step 3: Multiply for the kilowatt-hours

Multiply watts by hours by days, then divide by 1,000: 100 × 5 × 30 ÷ 1000 = 15 kWh. This is the energy the device actually used, and it is the number the bill counts — the meter does not know about watts or hours, only about kilowatt-hours, which is why two devices with wildly different labels can cost the same if their hours differ enough. Check the kWh against feel before trusting it: 15 kWh should feel like a small slice of a household bill, and if the kWh for one bulb rivals the whole house, the watts or the hours are wrong — usually the hours, counted as plugged-in time instead of running time. The kWh is also the number to compare devices by, because it strips out the rate entirely: a thriftier device is the one with the smaller kWh, wherever in the country it runs.

Step 4: Multiply by your rate for the bill

Multiply the kWh by the rate on your bill: 15 × $0.17 = $2.55 a month. The rate is the only part of the calculation that is personal in the money sense — the same 15 kWh costs $3.75 where power is $0.25 a kWh, and $1.80 where it is $0.12 — which is why a bill comparison between households proves nothing about their devices, only about their tariffs. Find the rate on the bill itself, not on a memory: some bills state an all-in rate including delivery, others state energy alone with delivery added separately, and the rate that settles the month is the all-in one. Where the tariff is tiered, use the tier the device’s kWh actually fall into, because the first cheap tier and the last dear tier are different rates wearing one bill.

Step 5: Stretch the month to the year, and sanity-check it

Multiply the month by 12 for the planning year: $2.55 × 12 = $30.60. This is a planning year, not a prediction — real months differ in length, real use differs with the seasons, and a heater’s January has nothing in common with its June — but the planning year is the figure that settles whether a more efficient replacement pays for itself, and whether a habit is worth changing. Sanity-check the year against the whole bill: a single bulb at $30.60 a year inside a $1,200 annual bill is plausible, while a single bulb claiming a quarter of the bill is a sign that the watts or hours were counted wrong. Finally, re-run the maths when anything changes — a new tariff, a new device, or a new routine all move the answer, and a bill calculated once and trusted forever drifts wrong by dollars a month. For the interactive version, use the electricity cost calculator, which runs the same three moves on any watts, hours, days and rate.

Worked example, start to finish

Watts 100, hours 5 a day, days 30, rate $0.17 a kWh. kWh: 100 × 5 × 30 ÷ 1000 = 15. Monthly cost: 15 × $0.17 = $2.55. Yearly cost: $2.55 × 12 = $30.60. The same five steps on a 200-watt device give 30 kWh, $5.10 a month, and $61.20 a year; on the same device run 8 hours a day they give 24 kWh, $4.08 a month, and $48.96 a year; and at a rate of $0.25 a kWh they give the same 15 kWh but $3.75 a month and $45.00 a year. Watts, hours, days and rate are the only four levers, and only the first three move the energy — which is why a household that measures its watts and counts its hours stays honest about its bill for years, while a household that guesses either drifts wrong by whole kilowatt-hours.

Three mistakes to avoid

The first mistake is using label watts as average watts: a peak rating describes a moment, not a month, and a bill built on peaks overstates every cycling appliance in the house. The second mistake is counting plugged-in hours as running hours: standby power is real but small, and a device that sits plugged in for 16 hours and runs for 5 costs the 5-hour figure, not the 16-hour one. The third mistake is using a remembered rate instead of the bill’s rate: tariffs change, delivery charges hide, and tiers bite, so a rate from last year is a rate from another bill entirely. Measure the watts, count the running hours, multiply, divide by 1,000, multiply by the bill’s rate, and stretch to the year — in that order, every time the bill is questioned.