How to Calculate Electricity Units: Watts to kWh (1 Unit = 1 kWh)

The short answer

One electricity "unit" on your bill is 1 kilowatt-hour (kWh): the energy used by a 1,000-watt appliance running for one hour. To find the units an appliance uses, multiply its power in watts by the hours it runs and divide by 1,000: kWh = watts × hours ÷ 1000. A 75 W fan running 12 hours uses 75 × 12 ÷ 1000 = 0.9 units. If you have the energy in Wh, joules or BTU, the Energy Converter turns it into kWh.

Watts vs kWh: the difference

• Watts (W) or kilowatts (kW) measure power: how fast an appliance uses energy at a moment. It is printed on the appliance label or in the manual.
• Kilowatt-hours (kWh) measure energy: power multiplied by time. This is what the meter counts and what you pay for.

A 1,500 W heater is not "1,500 units". It uses 1.5 units for every hour it is on.

Step-by-step calculation

Find the power rating in watts on the label, for example "75 W" or "1.5 kW". If it is in kW, multiply by 1,000 to get watts.

Estimate hours of use per day. Be honest; this is where most estimates go wrong.

Multiply watts by hours to get watt-hours (Wh) per day.

Divide by 1,000 to get kWh, which is units per day.

Multiply by days (for example 30) to get units per month.

Multiply by your tariff if you want an approximate cost. Tariffs vary by state, supplier and slab, so take the rate per unit from your own bill.

Worked examples

Appliance (example rating)	Hours per day	Units per day	Units in 30 days
LED bulb, 9 W	6	0.054	1.62
Ceiling fan, 75 W	12	0.9	27
Water heater (geyser), 2,000 W	0.5	1	30
Air conditioner, 1,500 W	8	12	360

The ratings here are examples to show the arithmetic. Check the label on your own appliance, because ratings vary widely between models.

Fan: 75 W × 12 h = 900 Wh = 0.9 kWh a day. Over 30 days: 0.9 × 30 = 27 units.

Geyser: 2,000 W × 0.5 h = 1,000 Wh = 1 kWh a day, or 30 units a month. A short run of a high-power appliance can use as much as a fan running all day.

AC: 1,500 W × 8 h = 12,000 Wh = 12 kWh a day. Treat this as an upper estimate: the label usually shows rated input power, and an AC's compressor does not run at full power for every minute it is switched on, especially inverter models. Your actual use may be lower, but the calculation shows why the AC dominates a summer bill.

Using the Energy Converter

The converter is useful when the figure you have is not already in kWh:

Open the Energy Converter.

Type the value, for example 900.

Set From to Watt-hour (Wh).

Choose Kilowatt-hour (kWh) as the result. 900 Wh shows as 0.9 kWh.

The All Energy conversions list also shows the same energy in joules, kilojoules, calories, kilocalories and BTU.

Some reference values: 1 kWh = 3,600,000 joules (3.6 MJ), about 860 kcal and about 3,412 BTU. If a product sheet gives energy in megajoules or BTU, convert it to kWh first, then compare it with your bill.

The converter handles energy units only. It does not multiply watts by hours for you, so do step 3 above with a calculator, then convert the result.

Checking your estimate against the bill

Add up the monthly units for your main appliances and compare the total with the units on your bill:

• Much lower than the bill? You may have missed always-on appliances such as the refrigerator, Wi-Fi router, set-top box or water pump, or underestimated hours.
• Much higher than the bill? Appliances like ACs, fridges and heaters with thermostats switch on and off, so they do not draw full rated power all the time.
• Still unsure? Read your meter at the same time on two days. The difference is the units used in those 24 hours. Switch off a single large appliance for a day to see its share.

Common mistakes

• Forgetting to divide by 1,000. 75 W × 12 h is 900 Wh, not 900 units.
• Mixing kW and W. "1.5 kW" is 1,500 W. Using 1.5 as watts makes the answer 1,000 times too small.
• Using AC "tonnage" as power. A 1.5-ton AC describes cooling capacity, not electrical input. Use the watt or kW input figure from the label or manual.
• Ignoring standby power. Small loads left on 24 hours a day add up across a month.
• Assuming one rate per unit. Many tariffs charge more per unit as monthly use rises, plus fixed charges and taxes. Use your bill's actual figures for cost.

Quick formulas

• Units (kWh) = watts × hours ÷ 1000
• Hours per unit = 1000 ÷ watts (a 100 W appliance runs 10 hours on 1 unit)
• Watts = kWh × 1000 ÷ hours

Related tools

• Converting food energy rather than electricity: kJ to kcal: How to Read Calories on a Food Label.
• Working out percentage changes between two bills: Percentage Calculator.