Methodology
How BillUnits.in researches, calculates, verifies, and maintains its electricity bill calculators.
This page explains how our calculators turn a unit reading into a bill estimate. For where our tariff figures come from, see our Data Sources page; for how our content is written and reviewed, see our Editorial Policy.
What goes into an electricity bill
An Indian electricity bill is rarely a single per-unit rate. Depending on your state, DISCOM, and consumer category, a domestic bill is built from some combination of these components:
- Energy charges — the cost of the units (kWh) you consumed, applied through progressive slabs.
- Slab rates — consumption tiers, where only the units falling within each tier are charged at that tier’s rate.
- Fixed charges — a monthly charge usually tied to your sanctioned load (kW), payable regardless of usage.
- Fuel & Power Purchase Adjustment (FPPCA / FAC) — a variable charge linked to fuel costs, set periodically by the regulator.
- Electricity duty — a state-level tax, where applicable.
- Meter charges — metering / meter rent, where applicable.
- Subsidies & rebates — free-unit schemes and category rebates announced by individual states.
How our calculators apply them
India uses a progressive (telescopic) slab system: when your consumption crosses into a higher slab, only the units inside that higher slab are charged at the higher rate — not your entire consumption. Our calculators follow the same order an official bill does:
- Split total units across the applicable slabs and charge each slab at its own rate to get energy charges.
- Add fixed charges for the sanctioned load.
- Add FPPCA / FAC and electricity duty where the tariff order applies them.
- Apply any subsidy or free-unit benefit for the consumer category.
Worked example — TNEB domestic, 300 units
The figures below are illustrative — they exist only to show how the calculation works, not to state TANGEDCO’s current published rates. For the actual figures behind the live calculator, see the linked order on our Data Sources page.
Step 1 — Energy charges, slab by slab
| Slab | Units in slab | Illustrative rate | Charge |
|---|---|---|---|
| 0–100 units | 100 | ₹0.00 (free) | ₹0 |
| 101–200 units | 100 | ₹2.35 | ₹235 |
| 201–300 units | 100 | ₹4.70 | ₹470 |
| Energy charges subtotal | ₹705 | ||
Step 2 — Add the remaining components
| Energy charges (from Step 1) | ₹705 |
| Fixed charge (illustrative) | + ₹30 |
| FPPCA / FAC (illustrative — nil in this example) | + ₹0 |
| Electricity duty (illustrative — nil for this slab) | + ₹0 |
| Estimated bill | ₹735 |
The key idea: the first 300 units are not all charged at ₹4.70. The free first 100, the ₹2.35 middle slab, and the ₹4.70 top slab each apply only to their own band — which is exactly why a flat “units × rate” estimate is usually wrong.
How we build and maintain each calculator
Every calculator on BillUnits.in follows the same process:
- Research — we read the latest tariff order and supporting notifications for that state and category.
- Formula preparation — slabs, fixed charges, and adjustment components are translated into the calculation logic above.
- Development — the logic is built into a calculator that runs entirely in your browser.
- Testing — outputs are checked against sample real bills for that state before the calculator goes live.
- Publication — the calculator is published with supporting explanation.
- Periodic review — pages are revisited and updated when a tariff revision or regulatory change is issued.
Where our tariff data comes from
All tariff figures are taken from official, publicly available sources — State Electricity Regulatory Commission (SERC) tariff orders and DISCOM notifications. The specific orders behind each calculator are listed, with links, on our Data Sources page.
Accuracy and limitations
BillUnits.in produces estimates for informational purposes only. Your actual bill can differ because of billing-cycle timing, mid-period tariff revisions, FAC changes, category-specific charges, or rounding. Always treat your official DISCOM bill as the authoritative figure. Full terms are on our Disclaimer page.