The idea in one line
Tamil Nadu bills are telescopic: each slab has its own rate, and the units at the top of your bill are the dearest. A rooftop system does not shave an average unit off your bill. It takes away the last units, the ones at the highest rate.
Homes: the 500-unit cliff
For a two-month domestic bill in 2026, the first 200 units are free if you use 500 or less. Then 201–400 units cost ₹4.70 and 401–500 cost ₹6.30. Cross 500 and two things happen at once: only 100 units stay free, and the slabs rise to ₹8.40, ₹9.45, ₹10.50 and ₹11.55.
cliff
Take a home using 600 units. The bill is ₹2,880, an average of ₹4.80 a unit. Now let a 2.5 kW system supply 360 of those units during the day. The grid share falls to 240 units, back under the cliff with 200 free units, and that part of the bill drops to ₹188.
- 300 units × ₹4.70₹1,410
- 100 units × ₹6.30₹630
- 100 units × ₹8.40₹840
- 360 units from the roof₹0
- 240 from grid₹188
Check your own number: put your units or rupee amount into the TNEB bill calculator. It shows each slab, the old-versus-new rule and what solar saves.
Businesses: a unit used is worth three exported
Commercial and industrial (C&I) consumers pay a flat energy rate, so every daytime unit solar replaces is worth that full rate: about ₹10.45 for LT commercial, ₹8.25 for LT industrial and ₹7.50 for HT, which also pays ₹608 per kVA of demand and 25% extra in the 6–10 AM and 6–10 PM peaks.
An exported unit is different. Under net feed-in it earns ₹3.10–3.61, and a network charge of ₹1.04–1.59 on generation (currently under litigation) can cut that further.
So size a C&I plant to daytime load, not to the roof. A 100 kWp plant on a shop makes about 12,000 units a month. If 90% is used on site it is worth about ₹1.17 lakh a month; if only half is, about ₹83,000. Same panels, about 29% less value.
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What to do
- Find your marginal rate. Homes: note whether you sit near 500 units. Businesses: your tariff category and how much you use in daylight.
- Size to self-consumption. Every unit used on site is worth two to three times a unit exported.
- Get it checked. Send one recent bill; we will model it against your roof and load.
FAQ
What is the TNEB bill for 600 units?
Per Goggles Energy's TNEB bill calculator, about ₹2,880 in energy charges for a two-month bill: 100 units free, 300 at ₹4.70, 100 at ₹6.30 and 100 at ₹8.40.
Why does crossing 500 units cost so much?
Above 500 units only 100 units are free instead of 200, and higher slabs apply, so the 501st unit adds about ₹478.
Should a factory size solar to its roof?
No. Size it to daytime load. A self-consumed unit saves ₹7.50–10.45; an exported unit earns about ₹3.10–3.61, less any network charge.
Sources: TNERC Tariff Order No. 6 of 2025 (effective 1 July 2025); G.O. (Ms) No. 50, May 2026 (200 free units); TNERC generic tariff order, 2021 (feed-in); Madras High Court proceedings on rooftop network charges, 2024–25. Bill figures computed with Goggles Energy's TNEB calculator; C&I figures indicative.