Q.A household uses the following appliances daily: a TV (85 Watts) for 4 hours/day, a toaster (1000 Watts) for 15 minutes/day, a kettle (2400 Watts) for 2 hours/day, and 15 light bulbs (100 Watts each) for 5 hours/day. Given a flat rate tariff of 22 paise (cts) per kWh (unit) and a line charge of ₹1.10 per day, calculate the total monthly electricity bill for a 30-day month.

The Intuition: You Pay for What You Use

Think about your phone's data plan. You get a certain amount of data for a fixed monthly fee. If you go over, you pay extra per gigabyte. If you use less, you might carry it forward or lose it — depends on the plan.

Now imagine a simpler world: no fixed fee, no free allowance. You just pay exactly for what you consume. If you use 10 GB, you pay for 10 GB. If you use 100 GB, you pay for 100 GB. The price per GB is constant.

That's the core idea behind utility bills — electricity, water, gas, etc. You consume a certain quantity (kilowatt-hours for electricity, kilolitres for water, cubic metres for gas), and you pay a certain rate per unit. Multiply quantity by rate, and you get your bill.

But real life is never that simple. Utility companies add layers: fixed charges (just for having the connection), tiered rates (the more you use, the higher the rate per unit), taxes, and sometimes subsidies for low consumption. The bill you receive is the sum of all these components.

---

The Precise Statement

An electricity, water, or other supply bill is calculated as:

Where:

• Fixed Charges — a constant amount per billing period, independent of consumption. Covers meter reading, maintenance, and connection costs.
• Variable Charges — depends on the quantity consumed. Often structured in slabs (tiers) where the rate per unit changes as consumption increases.
• Taxes and Levies — government duties, service taxes, or surcharges applied as a percentage of the bill or as a fixed amount.

The variable charge is the main part. For a single-rate tariff (no slabs):

For a slab-based tariff, you calculate separately for each slab. Example: first 100 units at ₹3/unit, next 200 units at ₹5/unit, above 300 units at ₹7/unit. If you consume 350 units:

• First 100 units: $100 \times 3 = ₹300$
• Next 200 units: $200 \times 5 = ₹1000$
• Remaining 50 units: $50 \times 7 = ₹350$
• Total variable charge: $300 + 1000 + 350 = ₹1650$

Then add fixed charges and taxes.

---

Interpreting the Bill

The bill tells you more than just the amount due. It shows:

• Consumption pattern — compare with previous months. A sudden spike might indicate a leak (water) or a faulty appliance (electricity).
• Slab reached — if you crossed into a higher slab, your average cost per unit increased. This is why conserving just a few units can sometimes save disproportionately more money.
• Fixed vs variable split — a high fixed charge relative to variable charge means even if you consume very little, your bill won't drop much. This is common in areas with high infrastructure costs.

---

A Worked Example (Electricity)

Suppose the tariff is:

Fixed charge: ₹100/month

Tax: 5% of (fixed + variable)

You consumed 420 units.

Step 1: Variable charge

• First 100 units: $100 \times 3.00 = ₹300$
• Next 200 units (101–300): $200 \times 4.50 = ₹900$
• Next 120 units (301–420): $120 \times 6.00 = ₹720$
• Total variable: $300 + 900 + 720 = ₹1920$

Step 2: Fixed charge

₹100

Step 3: Tax

$5\% \text{ of } (1920 + 100) = 0.05 \times 2020 = ₹101$

Step 4: Total bill

$1920 + 100 + 101 = ₹2121$ …