Online Electricity Calculator

Most people guess their electricity costs rather than calculate them — and the guesses are almost always wrong. A single space heater running 6 hours a day through winter adds roughly $65-80 to your electricity bill over three months, but most households assume it is around $20-30. The gap comes from skipping the math.

The calculation itself is simple: watts divided by 1,000 gives you kilowatts, multiplied by hours gives you kilowatt-hours (kWh), and multiplied by your rate in dollars per kWh gives you the cost. A 1,500 W heater running 4 hours uses 6 kWh. At $0.15/kWh that is $0.90 per day, $27 per month. That is the entire calculation.

What makes this useful is comparing devices. An older CRT monitor at 100 W running 8 hours a day costs about $4.38/month. A 27-inch LCD at 30 W costs $1.31/month — the same task at less than a third of the cost. Running this calculation before buying or replacing an appliance turns a vague sense of 'more efficient' into a payback period you can actually evaluate.

Use this calculator when you are evaluating whether to replace an old appliance, deciding between two models at different efficiency ratings, estimating the cost of a new device before buying, or building a monthly electricity budget by device category.

It is also useful when debugging a high electricity bill. Run the calculation for your largest consumers — HVAC, water heater, electric dryer, electric oven — and see which one accounts for a disproportionate share. Often one appliance explains most of the variation between months.

This calculator is not the right tool for calculating your total household bill. It does not account for tiered pricing (where your rate increases once you exceed a monthly kWh threshold), time-of-use pricing (peak vs. off-peak rates), demand charges on commercial accounts, or transmission and distribution fees that typically add 30-60% to the raw energy cost. For an exact bill forecast, use your utility's own tools or bill history.

The most common mistake is entering kilowatts as if they were watts. A 1.5 kW kettle entered as 1.5 instead of 1,500 produces a result 1,000 times too small — the monthly cost looks like fractions of a cent, and people assume electricity is practically free for that device. Always check the unit on the appliance label before entering.

A second mistake is using peak wattage for devices that cycle on and off. A refrigerator might be rated at 150 W but only runs its compressor about 30-40% of the time. Entering 150 W at 24 hours overstates the cost. For cycling devices, either use the manufacturer-stated annual kWh figure (usually on the EnergyGuide label) or apply an estimated duty cycle.

The third mistake is forgetting standby power. Many electronics draw 2-10 W continuously even when switched off — TVs, game consoles, smart speakers, and set-top boxes are common offenders. A TV drawing 5 W in standby 24 hours a day costs about $6.57/year. Across 10 devices on standby, that is $65+/year invisible on any single appliance calculation.

The formula has three steps. First, convert watts to kilowatts: kW = Watts / 1,000. Second, calculate daily energy: kWh per day = kW x hours per day. Third, calculate cost: daily cost = kWh per day x rate per kWh.

From there, monthly cost = daily cost x days per month, and annual cost = daily cost x 365. Notice that annual cost uses a full 365-day year, while monthly cost uses your actual usage days. This is intentional — an air conditioner used 25 days a month in summer but zero days in winter should not have its monthly cost extrapolated to 12 months.

One number worth understanding is the kWh figure itself. 1 kWh is roughly the energy needed to boil 10 liters of water, run a laptop for 10-20 hours, or power a 100 W bulb for 10 hours. Once you internalize that unit, every line on your electricity bill becomes readable.

The formula assumes constant wattage over the full usage period, which is accurate for resistive loads (heaters, incandescent bulbs, kettles) but less accurate for devices with variable draw. A laptop under heavy load might pull 65 W; at idle, 12 W. Motors at startup briefly draw 5-7x their running wattage. For devices with variable profiles, measure actual draw with a plug-in energy monitor and use the measured average — rated wattage overstates cost for anything with significant idle time.