Electricity bill calculator
How the formula works
An electricity bill is made of the energy charge for the units used, plus a fixed charge, any surcharge, and tax. The energy charge depends on the tariff: a flat tariff has one price for every unit, while a slab (tiered) tariff charges different prices for different blocks of units.
1. Energy charge, flat tariff
Energy charge = units x rate per unit
2. Energy charge, slab tariff
Units are filled into the slabs in order. For each slab: charge = units in that slab x slab rate. The energy charge is the sum of all slab charges.
3. Other charges
Surcharge = surcharge per unit x units
Fixed charge = added once per bill
4. Tax and total
Subtotal = energy charge + surcharge + fixed charge
Tax = subtotal x tax %
Total bill = subtotal + tax
Average cost per unit = total bill / units
5. Units from a bill amount
Pre-tax amount = bill amount / (1 + tax %)
Energy part = pre-tax amount – fixed charge
The energy part is filled into the slabs in order, using the rate of each slab plus the surcharge per unit, until it is used up. The units in each slab are added to get the total units.
Where
- 1 unit = 1 kWh (kilowatt-hour)
- In a slab tariff, “first 100 units, next 200 units, remaining units” means slab 1 covers units 1 to 100, slab 2 covers units 101 to 300, and slab 3 covers every unit above 300
How to use the calculator
- Select the calculation and, if you want, a currency.
- Select the tariff type. For a slab tariff, enter the units and the rate for each slab from your electricity tariff order or your bill.
- Enter the units consumed, or the total bill amount for the reverse calculation.
- Optionally add the fixed charge, the surcharge per unit and the tax or duty.
- Click Calculate, then Download PDF Result if you need a record.
Important notes
- Tariffs differ by country, utility and consumer category (home, shop, industry). Always take the rates from your own tariff order or bill.
- This calculator assumes one monthly billing period. For a two-month bill, add the slab limits and the fixed charge for both months.
- Real bills can include other items such as demand charges, time-of-day rates, power factor adjustments, subsidies and arrears that are not covered here.
Worked example
Example 1 – Flat tariff with fixed charge and tax
250 units, rate 8 per unit, fixed charge 120, tax 5 %.
Energy charge = 250 x 8 = 2000
Subtotal = 2000 + 120 = 2120
Tax = 2120 x 0.05 = 106
Total bill = 2226. Average cost = 8.90 per unit.
Example 2 – Three-slab tariff with surcharge and tax
First 100 units at 3.5, next 200 units at 5, remaining units at 7. Consumption 350 units, fixed charge 50, surcharge 0.4 per unit, tax 10 %.
Slab 1 = 100 x 3.5 = 350
Slab 2 = 200 x 5 = 1000
Slab 3 = 50 x 7 = 350
Energy charge = 1700, surcharge = 350 x 0.4 = 140
Subtotal = 1700 + 140 + 50 = 1890, tax = 189
Total bill = 2079. Average cost = 5.94 per unit.
Example 3 – Four-slab tariff with free units
First 50 units at 0, next 100 units at 4, next 150 units at 6, remaining units at 8. Consumption 400 units.
Slab 1 = 0, slab 2 = 400, slab 3 = 900, slab 4 = 100 x 8 = 800
Total bill = 2100. Average cost = 5.25 per unit.
Example 4 – Units from a bill amount
Use the tariff of Example 2 with a bill of 2079.
Pre-tax amount = 2079 / 1.10 = 1890. Energy part = 1890 – 50 = 1840.
Slab 1 uses 100 x (3.5 + 0.4) = 390. Slab 2 uses 200 x (5 + 0.4) = 1080. Left = 1840 – 390 – 1080 = 370.
Units in slab 3 = 370 / (7 + 0.4) = 50
Units consumed = 100 + 200 + 50 = 350 units, which matches Example 2.
Common mistakes
- Charging all units at the last slab rate. In a slab tariff, only the units above a limit are charged at the higher rate.
- Forgetting the fixed or meter charge, which is added to every bill even if the units used are very low.
- Applying tax only on the energy charge. In many tariffs, tax or duty is also charged on the fixed charge and the surcharge.
- Entering the slab limits as cumulative values. This calculator asks for the units in each slab (first 100, next 200), not the running total (100, 300).
- Using the tariff of a different consumer category. Homes, shops and industries have different rates and slabs.
- Using a two-month reading with a monthly slab table. Slabs are normally set for one month.
- Ignoring the fuel or power purchase surcharge, which can be a large part of the bill.
- Mixing the units and the bill amount. Units are kWh, and the bill amount is money.
- Treating the average cost per unit as the tariff rate. The average includes the fixed charge, the surcharge and the tax.
Frequently asked questions
How is an electricity bill calculated?
Multiply the units used by the tariff rate to get the energy charge, then add the fixed charge, any surcharge and the tax. The total is your bill.
What is a unit of electricity?
One unit is one kilowatt-hour (kWh). It is the energy used by a 1000 W appliance running for one hour.
What is a slab tariff?
In a slab (tiered) tariff, the price per unit rises as you use more. For example, the first 100 units may cost 3.5 each, the next 200 units 5 each, and the rest 7 each.
Why is my average cost per unit higher than the tariff rate?
The average includes the fixed charge, the surcharge and the tax, divided by the units used. When you use very few units, the fixed charge makes the average cost per unit high.
How do I find the units from my electricity bill?
Use the Units Consumed mode. Enter the total bill, your slab rates, the fixed charge, the surcharge and the tax. The calculator works backwards through the slabs to find the units.
What is the fixed charge in an electricity bill?
It is a monthly charge for keeping your connection, often based on the sanctioned load or the meter type. It is charged even if you use no electricity.
What is a fuel or power purchase surcharge?
It is an extra charge per unit that covers changes in the cost of fuel or power bought by the utility. It is usually shown as a separate line on the bill.
Does tax apply to the whole bill?
It depends on the local rules. Some places charge electricity duty on the energy charge, and some also on the fixed charge. Use the tax option for the total subtotal and check your bill.
Can I use this calculator for any country?
Yes. You type your own rates, slabs and currency, so it works for any tariff, as long as it is a flat or slab tariff. Time-of-day and demand-charge tariffs are not covered.
Can I use it for a business or factory bill?
Only for the simple part of it. Industrial and commercial bills often have a demand charge in kVA, time-of-day rates and power factor adjustments, which this calculator does not include.
How can I reduce my electricity bill?
Use efficient appliances, switch off standby loads, use LED lighting and keep your use below the next slab limit where possible. The Appliance Power Consumption Calculator shows which appliance uses the most units.
Is this calculator a replacement for my actual bill?
No. It gives an estimate from the rates you enter. Your utility bill is the final amount and may include other charges, arrears and adjustments.