Begin with the business load profile
A commercial solar proposal should explain when the business uses electricity, not just how much it pays each month. Offices, shops and factories with similar annual consumption can have different solar economics because operating hours, holidays and production schedules differ.
This is an educational worked example, not investment advice, a quoted project price or a forecast for your premises. Every number below is an assumption chosen to demonstrate the calculation. A real Kolkata or West Bengal project needs its actual tariff, connection conditions, roof design and consumption data.
Separate self-consumed energy from exports
Solar electricity used while it is generated can avoid eligible consumption-linked electricity charges. Exported energy depends on the applicable approved arrangement and should not automatically be valued at the same rate. Fixed charges, demand charges and other components need separate review.
The screening formula is: annual gross benefit = self-consumed kWh × avoided variable tariff + exported kWh × applicable export value. Then deduct recurring operating costs to estimate annual net operating benefit. Do not assume a new export entitlement from this formula.
A transparent worked example
| Assumption | Example |
|---|---|
| Initial project investment | ₹7,00,000 |
| Annual generation | 14,000 kWh |
| Self-consumption share | 80% = 11,200 kWh |
| Assumed avoided variable tariff | ₹8/kWh |
| Remaining export | 2,800 kWh |
| Export value assumed for this screening example | ₹0/kWh |
| Annual operating allowance | ₹7,000 |
Gross annual benefit = 11,200 × ₹8 = ₹89,600. After the ₹7,000 operating allowance, the net operating benefit is ₹82,600. Simple payback = ₹7,00,000 ÷ ₹82,600 ≈ 8.5 years.
Zero export value is deliberately assumed for this example; it is not a statement of the current CESC or WBSEDCL settlement rate. The assumed investment and output are not benchmarks or commitments for any system size.
Test a lower self-consumption scenario
Keep the example’s other assumptions unchanged but reduce self-consumption to 60%. Directly used energy becomes 8,400 kWh. Gross benefit becomes ₹67,200, net operating benefit ₹60,200, and simple payback approximately 11.6 years.
This comparison shows why daytime demand matters. Before approving a project, test lower generation, different self-consumption and higher operating costs. A single optimistic payback figure conceals these sensitivities.
What simple payback leaves out
Simple payback does not account for the time value of money. This example also excludes financing, tax effects, generation degradation, tariff changes, downtime, major replacements and residual asset value. A full investment appraisal should model these explicitly rather than call the payback result a guaranteed return.
Ask a qualified adviser about the tax and financing assumptions applicable to your business. Confirm policy and metering terms with the relevant authority. Do not use residential subsidy assumptions in a commercial model without establishing eligibility.
Request a decision-ready proposal
Provide bills, operating hours, available interval data, shutdown patterns and planned changes to machinery or production. Ask for the proposed roof layout, generation methodology, self-consumption assumptions, equipment schedule, operating-cost allowance and written exclusions.
Explore our commercial and industrial solar EPC service and daytime-energy guide. When you request a commercial assessment, tell us your business objectives so the design and economics can be reviewed together.