The best payback model starts from the actual electricity bills, not a generic units-per-kW thumb rule.
By the numbers
BillTrends' current default rooftop-solar yield table totals 1,485 kWh per kWp per year before site-specific edits.
Source: BillTrends default model
BillTrends' default monthly rooftop-solar yield table ranges from 95 to 152 kWh/kWp/month.
Source: BillTrends default model
PM Surya Ghar is a residential rooftop scheme with subsidy capped at 3 kW of system capacity.
Source: PM Surya Ghar (MNRE)
PM Surya Ghar's public benefit promise is up to 300 units of free electricity per month for eligible households.
Source: PM Surya Ghar (MNRE)
PVWatts estimates grid-connected PV energy output from system size, location, tilt, azimuth, losses and weather data rather than a single national yield number.
Source: NREL PVWatts
Start with avoided rupees, not generated units
A 1 kWh solar unit is valuable only to the extent it avoids a grid rupee. If the consumer's effective tariff is high, self-consumption is worth more. If export is credited at a lower rate, exported units may have a weaker payback.
For commercial and industrial users, demand charges, PF behavior, ToD pricing, open access, and banking treatment can change the real saving.
- Use the landed cost per unit from the bill, not only the energy-charge row.
- Separate self-consumed solar units from exported units.
- Check whether fixed and demand charges remain after solar.
Use state yield and roof reality
Maharashtra, Gujarat, Tamil Nadu, Karnataka, Delhi, Punjab, Uttar Pradesh, and other states have different irradiation, weather patterns, soiling, and policy context. BillTrends' current default yield table totals 1,485 kWh per kWp per year, but a payback model should use state or site yield assumptions and then show sensitivity.
Roof area, shading, structure, transformer limits, and sanctioned load can cap the system size even when annual consumption looks large.
- Estimate annual generation from kWp, state yield, degradation, and availability.
- Cross-check monthly generation against the consumption profile; the default monthly yield table ranges from 95 to 152 kWh/kWp/month.
- Flag when daytime load is too low for the proposed system size.
Model NPV, IRR, and payback together
Simple payback answers how many years it takes to recover capex. NPV and IRR answer whether the project is attractive after discount rate, tariff escalation, degradation, O&M, inverter replacement, and financing assumptions.
A consultant-ready model should show all three because different buyers care about different decision metrics.
- Payback years: capex divided by annual net saving.
- NPV: discounted project cash flows minus capex.
- IRR: the discount rate where NPV becomes zero.
Add BESS only where the bill supports it
Battery energy storage can help when ToD arbitrage, demand shaving, backup value, or export limits make storage useful. It can also weaken the economics if the spread between charge and discharge value is small.
BillTrends treats BESS as a sensitivity inside the same report so the solar story and bill-audit story stay connected. For residential cases, PM Surya Ghar subsidy logic is capped at 3 kW and should not be applied to C&I rooftop projects.
- Check peak-hour grid tariff and off-peak or solar charging value.
- Check maximum demand spikes that a battery might reduce.
- Show battery capex, round-trip efficiency, usable cycles, and replacement assumptions.
Related BillTrends pages
FAQ
What is a good rooftop solar payback in India?
It depends on tariff, state yield, capex, financing, net metering, and self-consumption. Many projects look attractive only after the actual bill is modeled.
Does net metering guarantee strong payback?
No. Net metering helps, but export credit, consumption timing, fixed charges, and policy limits can change the result.
Should BESS be included in every solar proposal?
No. Add BESS when ToD savings, demand reduction, export limits, or backup value justify it. Otherwise it may extend payback.