Recompute your electricity bill from the meter readings and the tariff on it. See the units the energy line implies, and whether an estimated reading explains the rest.
Why do I have to type the tariff table from my own bill?
Because there is no national household tariff. Rates are set by state regulators and boards, and boards revise them by order. This page checks the arithmetic against the tariff you type. It does not guess one for you, and it does not say whether the tariff itself is right.
What if the bill says the reading was estimated?
Then the meter difference, the units printed on the bill, and the units implied by the energy charge may stop agreeing. That is exactly why this page asks for the bill's own reading mark. An estimated bill can carry assessed units instead of the meter difference, so a mismatch is a clue rather than automatically an arithmetic error.
Why does a longer billing period matter so much?
Because slab billing prices the whole period's units, not your average day. TNPDCL's own bill calculator says to choose Monthly only when the billing cycle is less than 35 days. Stretch the same daily use over 40 days instead of 30 and more units fall into the upper slabs, even if nothing changed in the house except the number of days billed.
Why separate fixed charge, duty and fuel surcharge from the energy charge?
Because a reader usually blames the units first. The energy line is the only part the meter and slab table rebuild. Fixed charge belongs to the connection, duty is a tax line, and fuel surcharge is often a separate adjustment. A bill can feel high while the energy line is perfectly right.
Does this page tell me whether the board charged the right tariff?
No. It checks the bill against the tariff and line items you entered. If those inputs are wrong for your category, the page will still add them correctly. That is deliberate: a confident guess at your board's tariff would be worse than making you type the numbers off the bill.