Appliance Power Consumption Calculator
How the formula works
Electricity is billed in units, and one unit is one kilowatt-hour (kWh). An appliance that draws 1 kW for 1 hour uses 1 kWh. This calculator finds the energy used from the power rating of the appliance and the time it runs.
1. Energy per day
Energy per day (kWh) = power (W) x quantity x hours per day x load factor / 1000
2. Energy per month and per year
Energy per month = energy per day x days used per month (30 days if skipped)
Energy per year = energy per month x 12
3. Running cost
Cost = energy (kWh) x tariff per unit
4. Usage time from energy
Run time (hours) = energy (kWh) x 1000 / (power (W) x quantity x load factor)
5. Average power from energy
Average power (kW) = energy (kWh) / run time (hours)
Power in watts = kW x 1000. Horsepower = kW / 0.746
Where
- Power (W) = rated power on the appliance nameplate
- Load factor = the share of time the appliance actually draws full power. A refrigerator compressor, for example, switches on and off.
- 1 unit = 1 kWh = 1 kW used for 1 hour = 3.6 MJ
Typical power ratings used
- LED bulb 9 W, LED tube light 20 W, ceiling fan 75 W, LED television (32 in) 50 W
- Desktop computer 150 W, laptop 60 W, Wi-Fi router 10 W
- Refrigerator (200 L) 150 W, air conditioner 1 TR 1000 W and 1.5 TR 1500 W
- Water heater 2000 W, washing machine 500 W, microwave oven 1200 W, electric iron 1000 W
- Mixer grinder 500 W, electric kettle 1500 W, room heater 1500 W, induction cooktop 1800 W
How to use the calculator
- Select the calculation. For energy, choose the appliance, enter the quantity and the hours of use per day.
- For a refrigerator or air conditioner, choose a load factor, because they do not draw full power all the time.
- Optionally enter the days used per month and your tariff per unit to get the running cost.
- Click Calculate, then Download PDF Result if you need a record.
Important notes
- Typical values only. Check the nameplate or the energy label of your own appliance for the real rating.
- Inverter air conditioners and refrigerators draw less power than their rating most of the time. Use a lower load factor for them.
- The cost is based on one flat tariff. Real bills may have slabs, fixed charges, taxes and time-of-day rates.
Worked example
Example 1 – Four ceiling fans with running cost
Fan 75 W, 4 fans, 10 hours per day, 30 days, tariff 8 per unit.
Energy per day = 75 x 4 x 10 / 1000 = 3.0 kWh
Energy per month = 3.0 x 30 = 90 kWh, per year = 1080 kWh
Cost per month = 90 x 8 = 720
Example 2 – Air conditioner with load factor
1.5 TR split AC, 1500 W, 8 hours per day, load factor 50 %, 26 days per month.
Energy per day = 1500 x 1 x 8 x 0.50 / 1000 = 6.0 kWh
Energy per month = 6.0 x 26 = 156 kWh, per year = 1872 kWh
Example 3 – How long did the geyser run?
Water heater 2000 W, 120 kWh consumed in 30 days.
Run time = 120 x 1000 / 2000 = 60 hours
Per day = 60 / 30 = 2 hours
Example 4 – Average power from a meter reading
A machine used 45 kWh in 30 hours.
Average power = 45 / 30 = 1.5 kW = 1500 W
Equivalent horsepower = 1.5 / 0.746 = 2.01 HP
Common mistakes
- Mixing watts and kilowatts. 1000 W is 1 kW, and units are charged on kWh, so divide watts by 1000.
- Assuming every appliance draws full power all day. Refrigerators and air conditioners cycle on and off, so use a load factor.
- Using the rated power of an inverter appliance as the average. Inverter air conditioners often run well below their rating.
- Counting hours per day as 24 for appliances that run only part of the day.
- Forgetting standby power. Televisions, set-top boxes and chargers draw a small power even when they are off.
- Using one flat tariff for the whole bill. Many tariffs have slabs, so the last units cost more than the first units.
- Ignoring the number of appliances. Ten fans use ten times the energy of one fan.
- Reading kW as kVA. The units on your bill are kWh. kVA includes power factor and is used for sizing, not billing, in homes.
- Using nameplate power for old or poorly maintained equipment. Real power drawn can be higher than the rating.
Frequently asked questions
What is one unit of electricity?
One unit is one kilowatt-hour (kWh). It is the energy used by a 1000 W appliance running for one hour.
How do I calculate the units used by an appliance?
Units per day = power in watts x hours per day / 1000. For example, a 75 W fan running 10 hours uses 75 x 10 / 1000 = 0.75 units per day.
How do I calculate the electricity cost of an appliance?
Multiply the units used by the tariff per unit. If the fan uses 22.5 units a month and the tariff is 8 per unit, the cost is 180 per month.
What is the load factor or duty cycle?
It is the share of time that an appliance draws its full power. A refrigerator compressor runs only part of the time, so a load factor of about 30 % gives a better estimate.
How much electricity does an air conditioner use?
A 1.5 TR split AC is rated about 1500 W. If it runs 8 hours a day at a 50 % load factor, it uses about 6 units per day. Inverter models and room conditions change the real figure.
How much electricity does a refrigerator use?
A 200 L refrigerator is rated about 150 W but its compressor works only part of the day. At a 30 % load factor it uses about 1.08 units per day, or about 32 units per month.
What is the difference between kW and kWh?
kW is power, the rate at which energy is used at a moment. kWh is energy, the total used over time. A 2 kW heater running for 3 hours uses 6 kWh.
How can I find the real power of my appliance?
Check the nameplate or the energy label. For a better figure, use a plug-in energy meter, or use the Average Power mode here with the units shown on your meter and the time it ran.
How many units does a house use per month?
It depends on the number of appliances and how long they run. Add up each appliance with this calculator to estimate your monthly total, then compare it with your bill.
Why is my bill higher than the calculated value?
Your bill may include fixed charges, taxes, slab rates and standby power. Appliances may also draw more power than their rating when old or dusty.
How can I reduce the units used by appliances?
Choose appliances with a good energy rating, use LED lighting, switch off standby loads, set the air conditioner to a higher temperature and keep filters and coils clean.
Is this calculator a replacement for an energy audit?
No. It gives a quick estimate from typical ratings. A real energy audit measures actual power use with meters.