About this tool
What this tool does
Electricity bills are billed in units, and one unit is simply one kilowatt-hour: a one thousand watt appliance running for one hour. That single relationship is enough to cost any appliance exactly, because the wattage on its label is the power it draws and the hours are the time it spends drawing it. This calculator does that multiplication for one appliance, or adds up a whole home so you can see which device is actually responsible for the number on the bill.
How it works
Energy is watts multiplied by hours, divided by one thousand to convert to kilowatt-hours. A one hundred and fifty watt bulb left on for five hours a day consumes 0.15 × 5, which is 0.75 kilowatt-hours a day, or roughly 22.5 units across a thirty day month. Cost is then units multiplied by the per-unit tariff your provider charges, so a figure that surprises you is nearly always a wrong hours figure rather than a wrong rate.
The important limitation is that a label wattage is a peak rating, not an average. A refrigerator that never draws more than 200 watts still runs its compressor for perhaps a third of the time, so treating 200 watts as continuous overstates it considerably. The same applies to ceiling fans and to inverter air conditioners, which modulate their output instead of switching on and off. This calculator deliberately does not model duty cycles, so read the absolute totals as upper bounds and use the ranking between devices as the reliable part.
The whole-home mode exists because the arithmetic, while trivial, is not something people do in their head. Costing twelve appliances is where mistakes happen, and a total that happens to be wrong by a factor of two tells you nothing about which habit to change. Listing each device separately and showing its share of the total turns an opaque monthly figure into a ranked list, and the ranking is the actionable part: on most households one air conditioner or one water heater accounts for the majority of consumption, which is where any real saving has to come from.
Tiered billing is where a per-unit estimate stops matching the bill. Under a slab structure the first units are charged at one rate and later units at progressively higher rates, so the average cost of a unit rises with total consumption. This calculator models that shape with illustrative multipliers rather than any provider's published tariff, because those differ by country and change often. If your bill shows bands, use the first-slab rate field and treat the multipliers as a shape to sanity-check against your own statement rather than as a quote.
Worked example
A household with a 1.5 ton inverter AC at 1400W for 8 hours, two ceiling fans at 75W for 10 hours, four LED bulbs at 15W for 6 hours, and a refrigerator at 200W for 24 hours, over 30 days at 7.5 per unit.
- AC: (1400 × 8 × 30) / 1000 = 336 kWh
- Fans: (75 × 10 × 2 × 30) / 1000 = 45 kWh
- Bulbs: (15 × 6 × 4 × 30) / 1000 = 10.8 kWh
- Refrigerator: (200 × 24 × 30) / 1000 = 144 kWh
Total ≈ 535.8 units, costing ≈ 4,018.50 at 7.5 per unit. The air conditioner is about 63% of consumption and the refrigerator about 27%, so together those two account for nine tenths of the bill. Note the refrigerator figure is the upper bound described above, since a compressor does not run continuously.
Accuracy and limitations
- Wattages are nameplate ratings used continuously, so totals overstate any appliance that cycles. Real consumption is lower for fridges, freezers, fans and inverter ACs.
- Tariff slabs use illustrative 1.35x and 1.8x multipliers, not any provider's actual bands. Check your own bill for the real rates and adjust the first-slab field.
- This estimates energy cost only. It excludes fixed monthly charges, connection fees, fuel adjustment surcharges, taxes and any standing charge your provider adds.
- Duty cycles, inverter modulation, power factor and seasonal temperature variation are not modelled, so a seasonal estimate from a single month's figures will be rough.
Frequently asked questions
- What is one unit of electricity?
- One kilowatt-hour, written kWh: a one thousand watt appliance running for one hour. It is also called a unit in some countries. A one hundred watt bulb for ten hours is one unit, and so is a ten thousand watt heater for six minutes. Because it is a measure of energy rather than power, the same appliance costs very different amounts depending only on how long it runs.
- How do I read my meter to check this?
- Read the number at the same time on two days a month, take the difference, and that is your consumption in units for the period. Many meters also show a separate reading for what has been exported back to the grid, if you have solar panels, and that should be subtracted. Compare the difference against this calculator's total to see whether the gap comes from an appliance you forgot to list or from duty cycles the model does not know about.
- How much does an air conditioner cost per hour?
- Divide its wattage by one thousand and multiply by your per-unit rate. A 1400W AC at 7.5 per unit is 10.5 units an hour, or about 78.75, for every hour it runs. Note that a 1.5 ton unit does not use its full rating constantly: inverter models modulate below it, so the real figure during a mild spell is lower, and a window unit of similar capacity usually sits closer to its rating.
- Why is my bill higher than this calculator predicts?
- Most often because an appliance runs longer than assumed, or because your tariff has standing charges and slabs that a flat per-unit rate does not capture. Other common causes are winter heating, a fridge or freezer working harder in hot weather, and appliances left on standby. Add a realistic standby figure for anything with a remote or a clock, and compare across a full month rather than a single week, because short windows are dominated by the fridge cycling.
- Is my electricity usage sent anywhere?
- No. Every calculation runs in your browser from the numbers you type. Nothing is transmitted, stored or logged, and there is no server that could receive it.