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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
A critical point that many consumers miss: TANGEDCO's domestic tariff slabs are defined for bi-monthly consumption, not monthly. When you see "0-100 units: Free," that means 0-100 units over two months (effectively 0-50 units per month). This matters significantly for solar savings calculations.
| Bi-Monthly Slab | Equivalent Monthly | Energy Charge |
|---|---|---|
| 0-100 units | 0-50 units/month | Free |
| 101-200 units | 51-100 units/month | Rs 1.50/unit |
| 201-400 units | 101-200 units/month | Rs 2.00/unit |
| 401-500 units | 201-250 units/month | Rs 3.00/unit |
| 501-600 units | 251-300 units/month | Rs 4.00/unit |
| 601-800 units | 301-400 units/month | Rs 5.50/unit |
| 801-1000 units | 401-500 units/month | Rs 6.60/unit |
| Above 1000 units | Above 500 units/month | Rs 7.60/unit |
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:
| 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) |
| Tariff slab | Rs 5.50/unit (601-800 slab) | Rs 2.00/unit (201-400 slab) |
| Energy charge | Rs 3,850 | Rs 700 |
| Fixed charge | Rs 150 | Rs 70 |
| Total bill | Rs 4,000 | Rs 770 |
| Savings | -- | Rs 3,230 per bi-monthly cycle |
Notice the dramatic slab-drop effect. The household moved from the Rs 5.50 slab to the Rs 2.00 slab, saving not just on the 350 exported units but on the rate applied to the remaining 350 units consumed from the grid.
The Slab-Drop Bonus: Why Bi-Monthly Billing Amplifies Solar Savings
TANGEDCO's whole-slab billing (where the per-unit rate for the highest slab you reach applies to ALL units) creates a powerful savings multiplier:
Without Solar: 700 units bi-monthly
- All 700 units charged at Rs 5.50 = Rs 3,850 + Rs 150 fixed = Rs 4,000
With Solar: Net 350 units bi-monthly
- All 350 units charged at Rs 2.00 = Rs 700 + Rs 70 fixed = Rs 770
The Math Behind the Magic
- Units replaced by solar: 350 units
- Value at old slab rate (Rs 5.50): Rs 1,925
- Additional savings from slab drop on remaining 350 units: Rs 1,225 (350 x (Rs 5.50 - Rs 2.00))
- Fixed charge reduction: Rs 80
- Total savings: Rs 3,230 -- which is Rs 9.23 per unit of solar export
This effective savings rate of Rs 9.23 per unit (far above the Rs 5.50 tariff) is why solar ROI in Tamil Nadu is significantly better than the tariff rate alone 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 | Tariff 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 | Free slab (0-100) |
| 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 | Free slab (0-100) |
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 saves you Rs 4-8 (by displacing grid consumption), but surplus units at year-end earn only Rs 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 minimum fixed charge (Rs 50-70 for domestic). 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.
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