How to use this calculator
- Enter the tariff per kWh from your electricity bill and, optionally, a fixed monthly charge and a currency label such as IQD or USD.
- For each appliance enter a name, the power in watts, the quantity, the hours of use per day and the days of use per month.
- For appliances that switch themselves on and off, such as refrigerators, freezers and air conditioners, enter a duty cycle below 100 %, or keep 100 % and enter a measured average power.
- Add or remove rows with the buttons; up to 30 rows are kept in the page link, so the whole list can be shared.
- Read the monthly and annual energy and cost and the share of each appliance.
How it works: the formula
Energy is power multiplied by time. A 1000 W appliance running for one hour uses 1 kWh, the unit on the bill. Multiplying the watts by the number of units, the hours per day and the days per month gives watt-hours per month, and dividing by 1000 gives kWh. The duty cycle scales the result for appliances that do not draw their full power all the time they are switched on. The monthly cost is the energy times the tariff plus any fixed charge, and a year is taken as twelve such months.
Variables and units
| Symbol | Meaning | Unit |
|---|---|---|
| P | Power of one unit (rating plate or measured average) | W |
| n | Number of identical units | — |
| h | Hours of use per day | h |
| d | Days of use per month | d |
| duty | Share of the running time at which the entered power is drawn | % |
| tariff | Price of one kilowatt-hour in your currency | per kWh |
Worked example
The default rows are editable examples, not typical values: a 150 W refrigerator running 24 h at an assumed 50 % duty cycle, eight 10 W LED lamps for 6 h a day and a 750 W water pump for 2 h a day, all for 30 days, at a tariff of 0.10 per kWh:
With a fixed charge of 5 per month the bill becomes 16.34 per month and 196.08 per year.
Assumptions
- A flat tariff: every kWh costs the same.
- The entered watts are drawn for the whole running time, scaled by the duty cycle.
- One year is twelve months of the entered pattern; seasonal changes are not modelled unless you enter an average.
- Standby consumption is included only when you enter it as its own row.
Limitations
- Tiered (block) tariffs, time-of-use rates, taxes, meter rent and demand charges are not modelled; enter an average rate or work out the bill from the utility's tariff sheet.
- Rating-plate power is often a maximum. Appliances with motors, compressors, heaters and thermostats draw less on average; a plug-in energy meter gives the real figure.
- The duty cycle of a refrigerator or air conditioner depends on the room temperature, the thermostat setting and how often the door is opened, so any fixed value is an estimate.
- Losses in the wiring and in a generator, inverter or UPS supplying the loads are not included.
References
Standards and texts that cover this topic. Use the edition adopted in your jurisdiction.
- IEC 80000-6, Quantities and units — Electromagnetism (active power and active energy).
- IEC 62301, Household electrical appliances — Measurement of standby power.
- Your electricity supplier's published tariff, which defines the energy rate, any blocks or time bands and the fixed charges.
Frequently asked questions
Where do I find the power of an appliance?
On the rating plate or label, usually on the back or underneath, in watts (W) or as volts and amperes. If only V and A are given, V × A gives an upper limit in VA. For appliances whose consumption varies, a plug-in energy meter measured over a day or a week is far more accurate than the label.
What duty cycle should I use for a refrigerator or air conditioner?
There is no single correct value: the compressor runs more in hot weather, with frequent door opening or a low thermostat setting. The 50 % in the example is only an assumption. The most reliable way is to measure the energy over a few days with a plug-in meter and enter the average watts with 100 % duty.
Why is my bill different from the result?
Bills often include tiered prices, taxes, fixed charges and the energy of appliances you have not listed, such as water heaters or standby loads. The meter reading period may also not be exactly 30 days. Compare the total kWh first, then the cost.
Does the calculator work with any currency?
Yes. It only multiplies the kWh by the tariff you type, so the costs come out in the same currency. The currency label is just text shown next to the results.
Can I share the list of appliances?
Yes. The rows are stored in the page address in a compact form, so the copied link opens the same list. Up to 30 rows are kept.