Electricity & Energy
Electricity Calculator
Turn appliance wattage and running time into kilowatt-hours and running cost, using your own tariff and currency label.
Electricity Use and Cost
Energy and running cost for one or more devices.
From Watts to Kilowatt-Hours
Power is the rate at which energy is used, measured in watts. Energy is power multiplied by time, and the unit on your electricity bill is the kilowatt-hour: one kilowatt drawn for one hour.
The division by a thousand converts watt-hours into kilowatt-hours. A 1000 W appliance running for one hour uses exactly 1 kWh, which is the reference point worth memorising, since every other figure scales from it.
If a device is labelled with voltage and current rather than wattage, Ohms Law bridges the gap: P = V × I. A 230 V appliance drawing 4.3 A is consuming about 989 W.
How to Use the Electricity Calculator
Enter the device's power rating and set the unit, which accepts milliwatts through megawatts. Set the quantity if you are calculating for several identical devices, such as a run of light fittings.
Enter how many hours a day the device actually runs, between 0 and 24, and the number of days you want the period total for. Thirty days gives roughly a monthly figure, and 365 an annual one, though monthly and yearly estimates are also calculated independently using average month and year lengths.
The price per kWh is optional. Enter the rate from your bill and choose a currency label, or select Custom to type your own. No currency is assumed, and the calculator simply attaches the label you choose to the numbers it produces.
Worked Example: A 2 kW Heater Running for Three Hours
A 1000 W device running for one hour:
Now something more realistic. A 1000 W air conditioner running four hours a day for thirty days uses 1000 × 4 ÷ 1000 = 4 kWh per day, and 120 kWh over the month. At a rate of 0.15 per kWh, that is 0.60 per day and 18.00 for the period.
Compare lighting. Ten 9 W LED bulbs running five hours a day use 10 × 9 × 5 ÷ 1000 = 0.45 kWh daily, about 13.7 kWh a month, costing roughly 2.05 at the same rate. The same ten fittings using 60 W incandescent bulbs would use 3 kWh a day and cost around 13.70 a month, which is where the case for replacing lighting comes from.
Reading Your Energy and Cost Results
Daily energy is the consumption in one 24-hour period at the hours you specified. The period figure multiplies that by your chosen number of days.
The monthly and yearly estimates use 30.44 days and 365.25 days respectively, which are the average lengths across a full year including leap years. They are therefore slightly different from simply multiplying the daily figure by 30 or 365, and they are the more accurate basis for annual budgeting.
Cost figures appear only when you supply a tariff. They are direct multiplications of energy by rate and do not include standing charges, tiered rates, time-of-use pricing or taxes, all of which appear on a real bill.
Where Energy Cost Estimates Help
The most common use is identifying which appliances actually matter. Devices that draw a lot but run briefly, such as a kettle, often cost less over a month than something modest that runs constantly, such as a fridge or a set of standby loads. Running both through the calculator makes the comparison concrete.
It also supports upgrade decisions. Comparing the annual cost of an old appliance against a more efficient replacement gives a simple payback period: divide the price difference by the annual saving.
For solar and battery sizing, daily kilowatt-hours is exactly the input those calculations need, so totalling your loads here is a reasonable first step before sizing an array or a battery bank.
Standby consumption deserves its own pass. A device drawing only 3 W continuously uses 3 × 24 ÷ 1000 = 0.072 kWh a day, which is about 26 kWh a year. That looks trivial on its own, and it is until you count the ten or fifteen devices in a typical home that never fully switch off. Running the total through the calculator turns an abstract worry into a number you can act on.
Limits of the Energy Cost Estimate
- Nameplate wattage is the maximum draw, not the average. Appliances that cycle, such as refrigerators, air conditioners and washing machines, consume considerably less over a full day than continuous operation implies.
- Motors and heaters draw a surge at start-up that the nameplate figure does not reflect.
- Bills include standing charges, tiered or time-of-use rates and taxes. This calculator handles a single flat rate only.
- For an accurate figure on a variable load, a plug-in energy monitor measuring actual consumption is far better than any calculation.
Questions
Electricity Cost FAQs
Where do I find my price per kWh?
On your electricity bill, usually shown as a unit rate or energy charge. Use the rate excluding any fixed standing charge, since that is billed regardless of consumption.
Why are the monthly and yearly figures not exactly 30 and 365 times the daily one?
They use 30.44 days and 365.25 days, the average month and year lengths, which gives a more accurate long-term estimate than round numbers.
My appliance is labelled in amps, not watts. What do I do?
Multiply amps by the supply voltage to get watts, using P = V × I. A 230 V appliance drawing 2 A is consuming about 460 W.
Does the calculator assume a particular currency?
No. You choose a symbol or type your own label, and it is simply attached to the numbers. The arithmetic is identical whichever label you pick.
Find the Current With the Ohms Law Wheel
Open the full Ohms Law Wheel calculator to solve voltage, current, resistance and power from any two known values.