Solar Bill Calculation Formula India — Explained Simply (2026)
Introduction
If you're planning rooftop solar in India, the sales pitch usually focuses on “zero bills” and “fast payback.” But the real outcome depends on a few simple formulas: how many units you consume, what tariff you pay, how much your system generates, and how net metering credits are applied.
Understanding the solar bill calculation formula before buying helps you compare quotes apples-to-apples, avoid unrealistic savings claims, and choose the right system size (not too small, not unnecessarily large). It also makes it easier to verify subsidy, final cost, and payback using a calculator.
Basic Solar Savings Formula (Monthly)
Monthly Savings = Units Consumed × Electricity Tariff Rate
This is the simplest “bill reduction” estimate: you multiply how many units you normally buy from the grid by your per-unit tariff.
Worked Example (Ahmedabad)
Assume a family in Ahmedabad:
- Units consumed: 300 units/month
- Tariff rate: ₹5.50/unit
Monthly bill (before solar):
- 300 × 5.50 = ₹1,650
Now assume a 3kW rooftop solar system produces around ~360 units/month (planning estimate; varies by season and roof conditions). If your usage pattern allows you to self-consume and/or use net metering credits effectively, your grid import can drop close to ₹0 in many months.
Monthly savings estimate:
- ₹1,650/month
This “near ₹0 bill” scenario depends on net metering availability, how much you consume during the day, and how your DISCOM settles credits. Use the calculator below to verify with your assumptions.
Net Metering Formula (India)
Net Bill = (Units Consumed − Units Exported) × Tariff Rate
If your solar system produces more than you consume at certain times, the extra energy is exported to the grid. With net metering, those exported units can be credited on your bill (as per DISCOM rules).
Net Metering Example
- Units consumed in a month: 300
- Solar produces: 400
- Units exported: 100
- Tariff rate: ₹5.50/unit
Credit value (simple illustration):
- 100 × 5.50 = ₹550 credit
So your bill reduces because your net import is lower. In many billing cycles, the credit is adjusted against your consumption, and the exact settlement (carry-forward rules, banking, charges) depends on DISCOM policy.
Do not assume every exported unit is paid at the full retail tariff in all cases. Billing rules can include fixed charges, minimum charges, or different settlement for surplus.
Payback Period Formula
Payback Years = Total System Cost After Subsidy / Annual Savings
This is the most common ROI metric in residential solar: how many years of savings it takes to recover your net investment.
Payback Example (3kW)
Assume:
- 3kW system final cost after subsidy: ₹1,12,000
- Monthly savings estimate: ₹1,650
Annual savings:
- ₹1,650 × 12 = ₹19,800
Payback:
- 1,12,000 / 19,800 = 5.6 years
Payback improves when tariffs rise over time, when daytime self-consumption is higher, and when the system is well-designed (low shading, good inverter, correct tilt).
Verify Your Calculation
Use the savings calculator below to verify subsidy + savings estimates using your inputs.
FAQ
What is the formula to calculate solar panel output?
A simple planning formula is: Monthly units ≈ System size (kW) × Peak sun hours/day × 30 × 0.8. The 0.8 factor accounts for real-world losses (temperature, wiring, inverter efficiency).
How is net metering calculated in India?
In basic terms, net billing considers how many units you import from the grid minus how many units you export, and then applies tariff and charges as per DISCOM rules. Settlement rules can differ by state and DISCOM.
How do I calculate my electricity bill savings with solar?
Start with Units offset × Tariff rate, then adjust for how much solar you self-consume vs export, and include fixed charges and net metering settlement rules.
What is a good payback period for solar in India?
Many residential rooftops target 4–7 years payback (very case-dependent). Lower payback is more likely with high tariffs, strong sun hours, and good net metering outcomes.
How many units does a 3kW solar panel produce per day?
A planning estimate is: 3kW × ~5 peak sun hours × 0.8 ≈ ~12 units/day. Actual generation varies by location, season, shading, and system design.
Use our free calculator to get your exact numbers
If you want to verify subsidy, EMI and savings with your real inputs, use the free calculator here: Open Solar Calculator.
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