# Electricity cost calculator

> Electricity cost of running an appliance per hour, day, month and year, from its wattage, hours of use and your price per kWh (unit).

Interactive version: https://www.calcopenly.com/science/electricity-cost-calculator
Subject: Science calculators

The calculator turns an appliance's power rating into energy and money. Energy per day is the power in kilowatts times the hours of use, in kilowatt-hours (kWh); the monthly figure multiplies that by the days used, the yearly one by 12 months, and each is priced at your tariff per kWh. A kWh is the "unit" counted on electricity bills.

People use it to compare appliances or to see what a heater or air conditioner adds to a bill. The default, a 1.5 kW heater running 4 hours a day for 30 days at 0.15 per kWh, uses 6 kWh a day and 180 kWh a month: 27.00 a month and 324.00 a year.

The rated wattage is a maximum. Fridges, air conditioners and heaters with thermostats cycle on and off, so their real use is lower, and tiered tariffs, standing charges and time-of-day rates change the final bill.

## Inputs

- **Appliance power**
- **Hours used per day**
- **Days used per month**
- **Price per kWh (unit)**
- **Number of appliances**

## Results

- Cost per month — main result
- Cost per year
- Cost per day of use
- Cost per hour
- Energy per day of use (kWh)
- Energy per month (kWh)
- Energy per year (kWh)

## Formula

$$
E_{\text{day}} = \frac{P \times h \times n}{1000}\ \text{kWh},\quad \text{cost}_{\text{month}} = E_{\text{day}} \times d \times \text{tariff},\quad \text{cost}_{\text{year}} = 12 \times \text{cost}_{\text{month}}
$$

## Worked examples

### 1.5 kW heater, 4 h a day, 30 days at 0.15

- Appliance power: 1500 W
- Hours used per day: 4 h
- Days used per month: 30 days
- Price per kWh (unit): 0.15
- **Energy per day of use: 6 kWh**
- **Energy per month: 180 kWh**
- **Cost per month: 27.00**
- **Cost per year: 324.00**
- Checked against: DOE method: 1500 W × 4 h ÷ 1000 = 6 kWh/day; × 30 × 0.15 = 27.00; × 12 = 324.00

### 100 W bulb, 5 h a day at 0.12

- Appliance power: 100 W
- Hours used per day: 5 h
- Days used per month: 30 days
- Price per kWh (unit): 0.12
- **Energy per month: 15 kWh**
- **Cost per month: 1.80**
- Checked against: Python 3.8 decimal: 0.1 kW × 5 × 30 × 0.12

### Two 1 hp motors, 8 h on 22 working days at 0.20

- Appliance power: 1 hp
- Hours used per day: 8 h
- Days used per month: 22 days
- Price per kWh (unit): 0.20
- Number of appliances: 2
- **Energy per day of use: 11.9312 kWh**
- **Cost per month: 52.50**
- **Cost per year: 629.97**
- Checked against: Python 3.8 decimal: 2 × 745.69987 W × 8 h = 11.9312 kWh/day; × 22 × 0.20 = 52.4973

### Ten 9 W LED bulbs, 6 h a day at ₹8

- Appliance power: 9 W
- Hours used per day: 6 h
- Days used per month: 30 days
- Price per kWh (unit): 8
- Number of appliances: 10
- **Energy per month: 16.2 kWh**
- **Cost per month: 129.60**
- Checked against: Python 3.8 decimal: 10 × 9 W × 6 h × 30 = 16.2 kWh; × 8 = 129.60

### Unused appliance costs nothing

- Appliance power: 2000 W
- Hours used per day: 0 h
- Days used per month: 30 days
- Price per kWh (unit): 0.15
- **Cost per month: 0.00**
- **Cost per year: 0.00**
- Checked against: 0 hours of use ⇒ 0 kWh (definition)

## Questions

### How do I calculate the cost of running an appliance?

Multiply the power in kilowatts by the hours of use to get kilowatt-hours, then multiply by the price per kWh. A 2,000 W (2 kW) heater used for 3 hours uses 6 kWh; at 0.15 per kWh that costs 0.90. The US Department of Energy gives the same method: watts × hours used per day ÷ 1,000 = daily kWh.

### What is a unit of electricity?

One unit is one kilowatt-hour (kWh): 1,000 watts used for one hour, which is 3.6 megajoules of energy. A 100 W bulb takes 10 hours to use one unit and a 2 kW kettle takes 30 minutes. Bills in the UK, India and many other countries count units, and the price per unit is the tariff to enter here.

### How do I find an appliance's wattage?

Look for the rating label on the back or base of the appliance, or in its manual; it lists watts or kilowatts. If it gives only amps, multiply by the supply voltage, as the US Department of Energy suggests: 5 A at 230 V is 1,150 W. A plug-in energy meter measures real consumption, including standby power and thermostat cycling.

### How much electricity does a fridge use?

Less than its rating suggests, because the compressor switches on and off. The US Department of Energy suggests dividing the hours a fridge is plugged in by three to estimate its full-power running time, so a 150 W fridge runs about 8 hours a day: 1.2 kWh a day, or 36 kWh in a 30-day month. The kWh-per-year figure on its energy label is a better guide when you have it.

### How accurate is the electricity cost calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “1.5 kW heater, 4 h a day, 30 days at 0.15” is checked against DOE method: 1500 W × 4 h ÷ 1000 = 6 kWh/day; × 30 × 0.15 = 27.00; × 12 = 324.00.

### Where does the method come from?

U.S. Department of Energy — Estimating appliance and home electronic energy use; BIPM SI Brochure (9th ed.), Table 8 — the kilowatt-hour is 3.6 MJ.

## Sources

- [U.S. Department of Energy — Estimating appliance and home electronic energy use](https://www.energy.gov/energysaver/estimating-appliance-and-home-electronic-energy-use)
- BIPM SI Brochure (9th ed.), Table 8 — the kilowatt-hour is 3.6 MJ