TANGEDCO bills domestic consumers every two months (bi-monthly), not monthly. This billing cycle has a direct and sometimes surprising impact on how your solar net metering savings are calculated. Understanding the mechanics -- how credits accumulate, when they are applied, and what happens to surplus generation -- helps you accurately predict your post-solar electricity bill and avoid unpleasant surprises.

How TANGEDCO Bi-Monthly Billing Works

The Basic Cycle

TANGEDCO reads your meter once every two months. Your bill reflects the total units consumed (or, with solar, the net units consumed) over that 60-day period. The billing months vary by section:

Some sections bill January-February, March-April, May-June, etc.
Others offset by one month: February-March, April-May, June-July, etc.
Your specific billing cycle is printed on your TANGEDCO bill

Domestic Tariff Slabs Are Bi-Monthly

Critical point most consumers miss: TANGEDCO's LT-IA domestic slabs are defined for bi-monthly consumption, not monthly. Per TNERC Tariff Order No. 6 of 2024 (effective 1 July 2024), the schedule is telescopic (each slab rate applies only to units inside that slab) and uses a two-tier free-supply subsidy — the 500-unit bi-monthly boundary changes the free-unit quota.

Tier 1 — Consumption ≤ 500 units bi-monthly (200 free units)

Bi-Monthly Slab	Equivalent Monthly	Energy Charge
1–200 units	0–100 units/month	₹0 (free)
201–400 units	101–200 units/month	₹4.70/unit
401–500 units	201–250 units/month	₹6.30/unit

Tier 2 — Consumption > 500 units bi-monthly (only 100 free units)

Bi-Monthly Slab	Equivalent Monthly	Energy Charge
1–100 units	0–50 units/month	₹0 (free)
101–400 units	51–200 units/month	₹4.70/unit
401–500 units	201–250 units/month	₹6.30/unit
501–600 units	251–300 units/month	₹8.40/unit
601–800 units	301–400 units/month	₹9.45/unit
801–1000 units	401–500 units/month	₹10.50/unit
Above 1000 units	Above 500 units/month	₹11.55/unit

The bi-monthly definition matters because a household drawing 251 units/month (502 bi-monthly) gets pushed into Tier 2 — losing 100 free units and unlocking the ₹8.40 slab — even though monthly consumption looks modest.

How Net Metering Works with Bi-Monthly Billing

The Bi-Directional Meter

When you install solar with net metering, TANGEDCO replaces your standard meter with a bi-directional (import/export) meter. This meter records:

Import units: Electricity drawn from the grid (when your consumption exceeds solar generation -- typically evenings and nights)
Export units: Surplus solar electricity sent to the grid (when solar generation exceeds your consumption -- typically midday)

Net Calculation at Billing Time

At the end of each bi-monthly billing cycle, TANGEDCO calculates:

Net consumption = Import units - Export units

If the result is positive (you drew more from the grid than you exported), you are billed for the net amount at the applicable tariff slab.

If the result is negative (you exported more to the grid than you drew), the surplus is carried forward as a credit to the next billing cycle.

Worked Example

A 3BHK home in Chennai with a 3 kW solar system (2 kW connected load):

Parameter	Without Solar	With Solar
Bi-monthly consumption (import)	700 units	700 units
Solar generation (bi-monthly)	0 units	780 units
Export to grid	0 units	350 units
Self-consumption of solar	0 units	430 units
Net import (billed)	700 units	350 units (700 − 350)
Tier	Tier 2 (>500, 100 free)	Tier 1 (≤500, 200 free)
Energy charge (telescopic)	₹3,825 ¹	₹705 ²
Fixed charge (2 kW)	₹75	₹75
Total bill (excl. FCA + duty)	₹3,900	₹780
Savings	—	₹3,120 per bi-monthly cycle

¹ 700 units Tier 2 = 100 free + 300×₹4.70 + 100×₹6.30 + 100×₹8.40 + 100×₹9.45 = ₹3,825 ² 350 units Tier 1 = 200 free + 150×₹4.70 = ₹705

The household moved from Tier 2 (losing 100 free units + paying the ₹8.40 / ₹9.45 top slabs) into Tier 1 (200 free units + only the ₹4.70 slab applies). Solar's value comes from both the displaced units and the tier reset.

The Tier-Reset Bonus: Why Bi-Monthly Billing Amplifies Solar Savings

TANGEDCO's two-tier subsidy creates a powerful savings multiplier when solar pushes a household from Tier 2 back into Tier 1:

Without Solar: 700 units bi-monthly (Tier 2)

Energy: ₹3,825 (telescopic ladder up to ₹9.45)
Fixed: ₹75 → Total ≈ ₹3,900

With Solar: Net 350 units bi-monthly (Tier 1)

Energy: ₹705 (only the ₹4.70 slab applies, 200 units free)
Fixed: ₹75 → Total ≈ ₹780

The Math Behind the Effect

Units displaced by solar: 350 units
Saved at top-slab rates (avoided ₹8.40 / ₹9.45): roughly ₹1,785 on the units that would have sat in those slabs
Extra free units recovered by returning to Tier 1: 100 units × ₹4.70 ≈ ₹470 implied subsidy
Remaining displacement savings on ₹4.70 / ₹6.30 units: roughly ₹865
Total savings: ≈ ₹3,120 — an effective ₹8.90 per displaced unit, well above the average grid tariff

That implicit ₹8.90/unit return is why solar ROI in Tamil Nadu beats what the headline ₹4.70 slab rate would suggest.

Seasonal Variation and Bi-Monthly Impact

Solar generation is not uniform throughout the year. Here is how it interacts with bi-monthly billing across seasons:

Billing Period	Solar Generation (3 kW)	Typical Consumption	Net Import	Tier / Slab
Jan–Feb	680 units	600 units	−80 (surplus)	Credit carried forward
Mar–Apr	840 units	800 units	−40 (surplus)	Credit carried forward
May–Jun	820 units	900 units	80 units	Tier 1, fully inside 200 free units
Jul–Aug	760 units	750 units	−10 (surplus)	Credit carried forward
Sep–Oct	720 units	700 units	−20 (surplus)	Credit carried forward
Nov–Dec	580 units	650 units	70 units	Tier 1, fully inside 200 free units

In this example, the consumer pays zero or minimal electricity bills throughout the year. The surplus generated in high-production months offsets the deficit in high-consumption months (summer) and low-production months (northeast monsoon).

What Happens to Surplus Credits?

Within the Financial Year

Surplus credits (export exceeding import in a billing cycle) are carried forward to the next bi-monthly billing cycle indefinitely within the settlement period.

At Settlement (Annual or as Per TANGEDCO Policy)

At the end of the settlement period (currently annual, April-March), any remaining surplus credits are:

Compensated at the APPC rate (Average Power Purchase Cost), currently approximately Rs 2.00-2.50/unit
This rate is significantly lower than the tariff you would have paid, so you do not want excess surplus at year-end

Practical Implication

Size your system to generate approximately equal to your annual consumption, not significantly more. The first unit of solar can save ₹4.70–₹11.55 (by displacing top-slab grid consumption), but surplus units at year-end earn only ₹2.00–₹2.50.

Common Misunderstandings

"My solar system generates 400 units/month but my bill hasn't decreased proportionally"

Check the bi-monthly cycle dates. Your first post-solar bill may cover a period that includes pre-solar days. Also, TANGEDCO may take 1-2 billing cycles after net meter installation to correctly reflect solar credits.

"I generated more than I consumed but still got a bill"

Check the fixed charges. Even with zero net consumption, you still pay the connected-load fixed charge (₹30 for ≤500 W, ₹45 for 501 W–1 kW, ₹45 + ₹30 per additional kW). Also, verify that the export reading on your bill matches your inverter's generation data.

"My bill shows both import and export, but the export credits seem low"

TANGEDCO nets import and export at the meter level. If your export was 300 units and import was 500 units, you are billed for 200 units. The "export" line on the bill shows gross export, not a credit value. The netting is done automatically.

"I want monthly billing instead of bi-monthly"

TANGEDCO does not offer monthly billing for domestic consumers. However, you can track your daily generation and consumption through your inverter's monitoring app to understand your real-time solar performance.

Optimising Your Bill with Bi-Monthly Cycles

Shift Consumption to Daytime

Every unit consumed directly from your solar panels (self-consumption) is a unit you do not need to export and re-import. Self-consumption is always more valuable than export because:

No grid losses
No dependency on net metering settlement
Immediate slab reduction benefit

Practical tips:

Run washing machines, water heaters, and dishwashers during 9 AM - 3 PM
Charge devices and battery backups during peak solar hours
Set AC pre-cooling during afternoon hours so evening cooling load is reduced
If you have an EV, charge it during daytime solar hours

Monitor Each Billing Cycle

Track your import/export readings at least monthly (even though billing is bi-monthly) to catch issues early. Your inverter monitoring app provides daily generation data; compare this with your meter readings to ensure the system is performing as expected.

Next Steps

Understanding bi-monthly billing mechanics helps you:

Size your system correctly (not too large, not too small)
Maximise slab-drop savings
Shift consumption patterns for optimal self-consumption
Set realistic expectations for post-solar electricity bills

Use our solar savings calculator to model your specific bi-monthly consumption pattern and see projected savings, or contact Tristar for a detailed analysis based on your last 6 TANGEDCO bills.

TANGEDCO Bi-Monthly Billing: How It Affects Your Solar Savings