Step 1: Turn Your Bill into Monthly Units
Your bill lists units (kWh) consumed; use a 12-month average if AC use swings your seasons. If you only know the amount, divide by your effective tariff: a ₹3,000 bill at roughly ₹8.5/unit means about 350 units. Worked example: the Sharma family in Nagpur averages 350 units a month.
Step 2: Convert Units into System Size
Nagpur gets 4.8 peak sun hours: 350 ÷ (4.8 × 30 × 0.8) = 3.04 kW, rounded up to 3.5 kW. The solar system size calculator applies your own city's figure. If you are also adding appliances soon (AC, EV), size for the future load using the solar load calculator.
Step 3: Panels and Roof Check
3,500 W ÷ 550 W = 7 panels, needing 280 to 350 sq. ft. of shadow-free roof. The Sharmas' 400 sq. ft. terrace passes easily. Verify yours with the roof area calculator and set the mounting angle with the tilt angle calculator (21° in Nagpur).
Step 4: Cost, Subsidy and Net Investment
3.5 kW × ₹58,000 ≈ ₹2.03 lakh gross for on-grid. The central subsidy contributes the capped ₹78,000 (Maharashtra adds no state top-up), leaving a net of about ₹1.25 lakh. Confirm your own numbers with the cost calculator and subsidy calculator.
Step 5: Savings and Payback
The system generates about 403 units a month, wiping out nearly the whole bill: roughly ₹2,900 to ₹3,100 a month saved, or ₹35,000+ a year. Payback lands around 3.5 years, after which the family enjoys two decades of near-free power. The savings and payback calculator adds tariff escalation for the 25-year picture, and the EMI calculator shows the loan route: financing the net cost at 8.5% over 5 years costs about ₹2,560 a month, still below the saving.
Sample Sizing Worksheet You Can Copy
| Step | Your Entry | Sharma Family Example |
|---|---|---|
| 12-month average units | ________ | 350 |
| City peak sun hours | ________ | 4.8 (Nagpur) |
| System kW = units ÷ (PSH × 24) | ________ | 350 ÷ 115.2 → 3.5 kW |
| Panels = kW × 1000 ÷ 550 | ________ | 7 |
| Roof needed = kW × 80–100 | ________ | 280 – 350 sq. ft. |
| Gross cost = kW × ₹58,000 | ________ | ₹2.03 lakh |
| Subsidy (₹30k/₹60k/₹78k cap) | ________ | ₹78,000 |
| Net cost | ________ | ₹1.25 lakh |
| Monthly saving ≈ units × tariff | ________ | ~₹2,900 |
| Payback = net ÷ (saving × 12) | ________ | ~3.6 years |
Fill the blank column with your own numbers as you work through the steps above; the shorthand "PSH × 24" equals PSH × 30 days × 0.8 performance ratio.
The Decisions That Change Your Answer
- On-grid vs hybrid: reliable grid → on-grid (cheapest, fastest payback). Frequent cuts → hybrid with a battery sized in the battery calculator.
- Summer-heavy usage: if summer bills run double, size closer to the summer average or accept grid support in peak months.
- 3 kW sweet spot: the subsidy caps at 3 kW, so homes near that consumption get the best subsidy-per-rupee at exactly 3 kW (see the 3kW system page).
- Future loads: an EV adds roughly 1 kW of needed capacity per 100 km of weekly driving.
Want it all done in one go? The solar calculator for India runs this exact five-step chain from a single bill amount.