How to calculate an electricity bill
Calculate electricity units from watts and hours, estimate the energy charge and understand why an Indian electricity bill may include other charges.
What is one electricity unit?
One electricity unit is one kilowatt-hour, written as 1 kWh. A 1,000-watt appliance running for one hour uses 1 kWh. A 100-watt appliance running for ten hours also uses 1 kWh.
Use the basic formula
Monthly units = watts × quantity × hours used per day × days used ÷ 1,000. For an appliance that switches on and off, also multiply by a realistic operating factor.
Estimate the energy charge
Energy charge = monthly units × your electricity rate per kWh. Use the rate or slab details from your own bill. The example rate in a calculator is not a universal Indian tariff.
Why the final bill may be different
A bill may also include fixed charges, slab pricing, fuel adjustments, duties, taxes or subsidies. Appliance cycling, voltage, weather and condition can change actual use. Compare the estimate with several real bills.
Worked appliance example
If a 100 W appliance runs for 5 hours a day for 30 days, the energy is 100 × 5 × 30 ÷ 1,000 = 15 kWh. Multiply those units by the rate printed on your own bill to estimate only the energy-charge portion.
Read the bill in the right order
First identify the billing dates and units. Next separate the energy charge from fixed and adjustment lines. Then compare several similar months; one month can be distorted by weather, occupancy or an estimated meter reading.
Build a room-by-room appliance list
Write down each appliance, its input watts, quantity and realistic daily use. Use the electrical input on the nameplate or product sheet, not an output rating such as cooling capacity. For an appliance that cycles, keep the full input and operating factor as separate values. Separating them makes the estimate easier to correct when you later compare it with a meter or bill.
Worked monthly appliance table
This example uses entered values only. The table calculates units before any money is added, which means the same energy estimate can be checked against any current tariff. The final row shows why a high-power appliance used briefly can consume less than a moderate load left on for many hours.
| Appliance scenario | Calculation | Monthly energy |
|---|---|---|
| 2 × 9 W lamps, 5 h/day | 18 W × 5 × 30 ÷ 1,000 | 2.70 kWh |
| 75 W fan, 10 h/day | 75 W × 10 × 30 ÷ 1,000 | 22.50 kWh |
| 1,500 W heater, 0.5 h/day | 1,500 W × 0.5 × 30 ÷ 1,000 | 22.50 kWh |
Apply a rate without hiding slab rules
For a simple scenario, 47.70 kWh from the table at an example ₹8 per kWh gives ₹381.60 as the energy-charge estimate. ₹8 is only an example input: replace it with the rate or slab treatment applicable to the connection and billing period. If the bill uses multiple slabs, calculate the applicable blocks or use the bill’s effective energy-charge rate instead of pretending one rate applies universally.
Reconcile the estimate with a real bill
Match the same billing dates, then compare estimated appliance units with the meter units on the bill. Investigate seasonal loads, standby use, pump or compressor cycling, guests, estimated meter readings and changed operating hours before adjusting the formula. Do not force the appliance list to equal a bill that also includes a different date range. Use several similar months to separate a repeated pattern from a one-off event.