If you are weighing rooftop solar for a factory, warehouse or commercial building in Tamil Nadu, one policy question quietly decides your payback: what happens to the units you generate but don't use on the spot? The answer sits inside the metering arrangement your connection falls under. Get it right and the plant pays for itself on schedule. Get it wrong, size for a beautiful, fully-covered roof and export half of it, and the economics soften in a way no amount of good engineering later can fix.
This guide explains the three arrangements in plain language, what "banking" actually means, and why C&I consumers in particular should size to daytime self-consumption rather than to maximum roof coverage.
One important caveat before any numbers. Net-metering, gross-metering and net-billing rules, and the rate at which exported surplus is credited, are set by the state regulator (the Tamil Nadu Electricity Regulatory Commission, TNERC), currently the Generic Tariff Order for Grid-interactive Solar Energy Generating Systems (GISS), Order No. 8 of 2021 dated 22 October 2021, and administered by your DISCOM (TANGEDCO/TNPDCL). These orders are revised from time to time, and the applicable arrangement depends on your consumer category, voltage level and sanctioned capacity: net-metering for domestic LT, net-feed-in for other LT and for HT plants of 1–999 kW, and parallel operation for plants of 1 MW and above. Treat every figure and timeline below as indicative, meant to show how the mechanics work, not as a permanent rule. Always confirm the current TNERC order and DISCOM circular that applies to your connection before you finalise sizing.
Net vs gross vs net-billing: what each does to your bill
All three describe how your solar generation is measured and settled against what you draw from the grid. The difference is entirely in how exported units are valued.
Net metering
A single bidirectional meter records the net of what you import and what you export. If in a billing period you drew 100,000 units and exported 20,000, you are billed for 80,000. In effect, every exported unit is worth the same as a unit you would otherwise have bought, your retail tariff. This is the most generous arrangement for the consumer, which is precisely why it is usually reserved for smaller or LT categories and increasingly restricted for large loads.
Gross metering
Here the logic flips. All your solar generation is exported to the grid and metered separately, and you continue to buy your entire consumption from the DISCOM at the retail tariff. Your export is paid at a fixed feed-in rate set by the regulator. You never "self-consume" behind the meter, the plant becomes a small generating asset selling to the grid. Whether this works for you depends entirely on how the feed-in rate compares with your tariff. In Tamil Nadu, though, it is rarely where a commercial or industrial rooftop actually lands: the TNERC category tables put other-than-domestic LT and HT plants of 1–999 kW on net-feed-in, which leaves gross metering a seldom-used option rather than the norm for C&I.
Net-billing (net feed-in)
This is the middle path, and the one HT and larger C&I consumers most often find themselves on. You self-consume as much of your generation as you can in real time, those units simply never get bought from the grid, so they are worth your full retail tariff. But any surplus you export is credited separately at a feed-in tariff set by TNERC, not at your consumption tariff. Under Order 8/2021 that feed-in tariff is tiered by plant size: ₹3.61/unit up to 10 kW, ₹3.37 for 11–150 kW and ₹3.10 for 151–999 kW, against a consumption tariff of ₹9 or more. On top of that, a DISCOM network charge applies to generation (indicatively ₹1.04/unit for net-feed-in above 150 kW, ₹1.60/unit for LT). The settlement is monetary, not in units: the value of your imports at retail tariff, less the value of your exports at the feed-in tariff, is what you actually pay. Two units, two very different values, depending on whether you use them or export them.
What settlement looks like, and why "banking" rarely applies
"Banking" (exporting units now and drawing them back later as a unit credit) belongs to net-metering. Under the net-feed-in arrangement that C&I plants actually run on, there is no unit bank: each billing cycle is settled in money. The value of what you import is charged at your retail tariff, the value of what you export is credited at the feed-in tariff, and you pay the difference.
Two consequences follow directly from that:
- An exported unit is not stored at retail value. It is converted to rupees at the feed-in tariff (₹3.10–₹3.61) the moment it leaves your meter, so it can never be "drawn back" as a ₹9 unit later.
- Network charges apply to generation. The DISCOM levies a network charge on what you generate (indicatively ₹1.04/unit for net-feed-in above 150 kW, ₹1.60/unit for LT), so a plant that exports heavily earns less than the headline feed-in rate suggests.
The takeaway is the same either way: a unit you consume on site is worth far more than one you export, and no settlement mechanism closes that gap. Because the exact rates and charges are revised periodically, confirm the prevailing TNERC order for your connection before you finalise sizing.
Why C&I consumers should size to daytime self-consumption
Put the two ideas together, net-billing credits exports at the feed-in tariff and levies a network charge on generation, and the sizing logic becomes almost self-evident.
Solar generates during the day. A factory that runs a full daytime shift consumes during the day. When your generation profile and your load profile overlap, those units are self-consumed and worth your full ₹9-plus tariff. When you oversize, covering every last square metre of roof so that midday output exceeds what the plant is actually using, the excess spills over into export, where under net-billing it earns only the ₹3.10 feed-in tariff (less a network charge on generation). You have spent CAPEX on panels whose output is worth a fraction of what you assumed.
So the right question is not "how many kW will fit on my roof?" It is "how many kW keeps my generation under my daytime load through the year?" That is the size that maximises the number of units earning the full tariff, and it is usually smaller than the roof could physically hold.
What this means for sizing
- Start from your daytime load profile, not your roof area, the plant should generate roughly what you consume while the sun is up.
- Under net-billing, a self-consumed unit (retail tariff, ₹9+) is worth far more than an exported one (TNERC feed-in tariff, ₹3.10–₹3.61 by size), so aim for high self-consumption, not maximum coverage.
- Model a realistic self-consumption ratio across seasons and weekends, when the plant may run leaner and exports rise.
- Treat banking as a cushion for day-to-day mismatch, not as a way to bank a year of surplus at full value.
- Re-check the exact metering arrangement, export rate and banking terms for your category and voltage against the current TNERC/DISCOM order before freezing the design.
The DISCOM application flow, end to end
Connecting a C&I plant to the grid follows a defined sequence. Timelines vary with capacity, feeder readiness and how complete your paperwork is, but the shape is consistent:
- Application & registration, you apply to the DISCOM with plant capacity, sanctioned load and connection details.
- Feasibility / technical approval, the utility checks whether your transformer, feeder and sanctioned capacity can accommodate the plant, and confirms the metering arrangement.
- Installation & net-meter testing, after the plant is built, the bidirectional (or gross) meter is installed and tested by the DISCOM.
- Commissioning & agreement, a joint inspection, the connection agreement, and formal energisation.
Realistically, budget an indicative 45 to 90 days from application to commissioning, depending on approvals and meter availability. It is not a formality to rush at the end, the metering arrangement confirmed during feasibility is what governs the numbers for the plant's whole life, so it belongs in the conversation on day one.
Where this leaves you
Net metering, gross metering and net-billing are not interchangeable labels, each one prices your exported units differently, and that price, more than almost anything else, shapes your payback. For most C&I consumers in Tamil Nadu, the honest planning assumption is net-billing: reward self-consumption, respect the gap between the retail tariff and the export rate, and size the plant to the load rather than the roof.
Because the specific rules shift with each regulatory cycle, the safest move is to design around the current order for your exact connection. If you would like that checked properly, you can use our sizing calculator for a first-cut estimate, browse how we approached comparable plants in our case studies, or ask us for a feasibility assessment that models your real load profile against today's rules.