Solar Plant Cost Calculator — Commercial Solar Financial Model
Put your own figures through the money side of a commercial or industrial rooftop plant: what the solar you use on site is worth, what your export settlement really pays, what O&M takes back — then simple payback, NPV and IRR.
- Expansion
- Operating cost
What this tool estimates
The year-one value of a commercial or industrial solar plant from the generation you declare — self-used kWh at your avoidable energy rate plus exported kWh at your verified settlement rate, less O&M — its simple payback and, with the lifetime inputs, yearly cash flows, NPV, IRR and discounted payback.
What it cannot decide
- How much the plant will generate — that comes from a yield study for your roof, which this tool takes as an input.
- How much of the generation your site will use at the moment it is produced — that needs interval meter data.
- Your export settlement, or your eligibility for net metering, net billing or gross metering — your DISCOM and state regulator decide those.
- Whether the roof can carry the plant or the sanctioned load allows it — a structural engineer and the DISCOM decide those.
- Any subsidy, tax, depreciation or financing effect — none is assumed, and no residential subsidy applies to commercial or industrial consumers.
Result
Your export rate is at or above your avoidable energy rate. That holds only if your settlement credits every exported unit against later imports at that rate. Under net metering a surplus left at the end of the settlement period can be paid at a lower rate — in Punjab, 75% of the PSERC feed-in tariff (PSERC rooftop regulations 2021, reg 12.3(d), as amended in 2026). Check the regulation and your agreement.
The year-one net benefit is zero or negative: O&M takes all of the value, so the plant does not pay back on these inputs.
| Energy | kWh | Share | Rate | Value |
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| Step | Value |
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Cash flow by year
| Year | Generation, kWh | Value | O&M | Replacement | Net | Cumulative |
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| Scenario | Year-one net benefit | Simple payback | NPV |
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Method version 6 October 2026. A screening estimate, not a design: see the limits below.
Where each input should come from
The arithmetic is simple; the inputs are where commercial solar cases go wrong. This is the evidence an engineer or a lender would ask for behind each one.
| Input | Evidence | What goes wrong when it is guessed |
|---|---|---|
| Year-one generation | A yield study for your roof: orientation, tilt, shading, soiling and system losses. | A rule-of-thumb yield per kW ignores the parapet, the water tanks and the dust. |
| Used on site | 15- or 30-minute interval data from the main meter or a logger, matched against the generation profile. | Annual consumption larger than generation does not mean all solar is used: Sundays, lunch breaks and shutdowns export or curtail it. |
| Export settlement rate | Your state regulator's rooftop regulation and your DISCOM agreement — net metering, net billing or gross metering. | Valuing exports at the retail tariff overstates the case wherever exports are settled at a feed-in tariff or a year-end payout. |
| Avoidable energy rate | The energy charge and energy-linked levies in the current tariff order for your category, kWh or kVAh, and time-of-day blocks if they apply. | Bill total ÷ units includes fixed and demand charges that solar does not remove. |
| Project cost | A quotation for your roof with its scope — structure, cabling route, protection, metering and approvals. | A ₹ per kW figure without its scope hides roof strengthening, HT works and statutory fees. |
| O&M and replacement | An O&M quotation and the inverter warranty terms. | Leaving O&M out flatters every year of the cash flow. |
How this calculator works — and where it stops
What it calculates
The money side of a commercial or industrial rooftop solar plant from your own figures: where the year-one generation goes (used on site, exported or curtailed — always adding up to the generation), what it is worth (self-used kWh at your avoidable energy rate plus exported kWh at your verified export settlement rate, less O&M), the simple payback and, with the lifetime inputs, the yearly cash flows, NPV, IRR (only when it is unique) and discounted payback.
Inputs
- Year-one generation at the meter, kWh — AC energy after losses, from a yield study for your roof. The tool does not estimate it.
- Plant capacity in kWp DC (optional) — used only for the specific yield and for a project cost entered per kWp.
- Solar used on site as it is generated (matched self-consumption) and solar exported, each as a % of generation or in kWh a year. Curtailment is the remainder.
- Avoidable energy rate, ₹/kWh — the energy charge plus energy-linked levies that a self-used unit avoids; not the bill total divided by units.
- Verified export settlement rate, ₹/kWh — what your settlement pays per exported unit. Required whenever anything is exported; 0 when exports are unpaid.
- Project cost (₹, ₹ lakh, ₹ crore or ₹ per kWp) and O&M in year one.
- For NPV and IRR: the analysis period, generation decline, avoidable-rate change, export-rate change, O&M escalation, discount rate, and an inverter replacement cost (0 if none) with its year. Leave them blank for the year-one result only — a blank is never taken as zero.
Method
- The used and exported shares are converted to kWh (a % is applied to the generation). Curtailed = generation − used − exported. If used and exported add up to more than the generation, the inputs are refused: curtailment cannot be negative.
- Year-one value = used kWh × avoidable rate + exported kWh × export settlement rate. Curtailed kWh earn nothing.
- Year-one net benefit = value − O&M. A zero or negative net benefit is shown as it is.
- Simple payback = project cost ÷ year-one net benefit, or no payback when the net benefit is not positive.
- Yearly cash flows for years 1 to N: generation × (1 − decline)^(year − 1), split in the year-one shares; the avoidable rate, the export rate and O&M each change at their own rate from year one; the replacement cost is added in its year. Year 0 is minus the project cost.
- NPV discounts the yearly cash flows at your rate, end-of-year. IRR is reported only when the cash flows change sign once; otherwise the result says there is no unique IRR and NPV decides. Payback on the yearly cash flows and discounted payback are interpolated within the year, or reported as not within the period.
- Sensitivity: the avoidable rate 10% lower; 10% of generation moved from on-site use to export (to curtailment when no export rate is entered); exports earning nothing; generation 10% lower; project cost 10% higher — one at a time.
Assumptions
- The split between used, exported and curtailed stays at its year-one shares as output declines. With the load unchanged, the share used on site can only stay the same or rise as output falls, so holding the shares understates later years wherever exports earn less than self-use.
- Fixed and demand charges do not change: a self-used unit avoids the energy charge only.
- Amounts are nominal rupees of their year; the replacement cost is what you expect to pay in that year.
- The discount rate, rate changes and escalation are yours; the tool recommends none.
- No subsidy, tax, depreciation, GST or financing effect. MNRE's central financial assistance under the rooftop programme is for residential consumers, so none applies to a commercial or industrial plant.
Limitations
- Generation and matched self-consumption are inputs the tool cannot check; they need a yield study and interval meter data.
- One export rate per year. A net-metering settlement that credits exports against later imports and pays a year-end surplus at a lower rate (in Punjab, 75% of the PSERC feed-in tariff) has to be blended into the rate you enter, from your own settlement history.
- No time-of-day or kVAh billing detail: enter the avoidable rate for the hours solar displaces.
- Does not check net-metering eligibility, the sanctioned-load or contract-demand cap, the roof structure or approvals.
- No residual value at the end of the period, and one annual decline rate for the whole period.
- A grid-tied plant stops when the grid fails (anti-islanding), so generation lost to outages must already be out of the generation figure.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Year-one generation 1,50,000 kWh
- Used on site 80% of generation; exported 20%
- Avoidable energy rate ₹8/kWh; verified export settlement rate ₹3/kWh
- Project cost ₹40 lakh; O&M ₹50,000 in year one
- Example lifetime inputs, not recommendations: 25 years; generation decline 0.5% a year; avoidable and export rates flat; O&M escalation 5% a year; discount rate 10%; inverter replacement ₹2.8 lakh in year 12
Working
- Used on site = 1,50,000 × 80% = 1,20,000 kWh; exported = 1,50,000 × 20% = 30,000 kWh; curtailed = 1,50,000 − 1,20,000 − 30,000 = 0 kWh.
- Value of solar used on site = 1,20,000 × ₹8 = ₹9,60,000; value of exports = 30,000 × ₹3 = ₹90,000; gross value ₹10,50,000.
- Year-one net benefit = ₹10,50,000 − ₹50,000 = ₹10,00,000.
- Simple payback = ₹40,00,000 ÷ ₹10,00,000 = 4 years.
- Yearly cash flows: year 12 carries the ₹2,80,000 replacement and nets ₹6,28,155; by year 25 the net is ₹7,69,731 as O&M rises and output declines.
- NPV at 10% over 25 years = ₹44,09,076. IRR = 23.92% — unique, because the cash flows change sign once. Payback on the yearly cash flows 4.05 years; discounted payback 5.49 years.
Result. ₹10,00,000 year-one net benefit and a 4-year simple payback. With the example lifetime inputs: NPV ₹44.09 lakh at 10%, IRR 23.92%, discounted payback 5.49 years.
Sensitivity — what moves the answer
- Avoidable energy rate 10% lower (₹7.20/kWh): year-one net benefit ₹9,04,000, simple payback 4.42 years, NPV ₹35.69 lakh.
- 10% of generation moved from on-site use to export at ₹3/kWh: ₹9,25,000, 4.32 years, NPV ₹37.53 lakh — matching solar to the site's load is worth more than the export rate.
- Exports earning nothing: ₹9,10,000, 4.40 years, NPV ₹36.22 lakh.
- Generation 10% lower, same split: ₹8,95,000, 4.47 years, NPV ₹34.91 lakh.
- Project cost 10% higher (₹44 lakh): payback 4.4 years, NPV ₹40.09 lakh.
How engineers use the result
As the first commercial screen once a yield study and interval data exist: it shows how much of the case rests on self-use, how exposed it is to the export settlement, and whether the project clears the owner's discount rate — before the design is fixed and before a proposal is priced.
When a professional design must replace it
Before an investment decision: the generation should come from a yield study for the actual layout and shading, the self-use share from interval data matched to that generation, the export rate from the current regulation and your DISCOM agreement, the avoidable rate from the current tariff order, and the cost from a quotation with its scope. An engineer should also confirm net-metering eligibility, the sanctioned-load cap and the roof's structural capacity, which this tool does not check.
Sources
- MNRE — Grid Connected Rooftop Solar Programme — checked 6 Oct 2026: central financial assistance is provided to residential consumers; the tool applies no subsidy to a commercial or industrial plant
- PSERC (Grid Interactive Rooftop Solar Photo Voltaic Systems) Regulations, 2021, consolidated up to the 3rd Amendment (Regu. 203, Punjab Govt Gazette 11 Apr 2026), PSPCL memo of 8 Jul 2026 — Punjab, checked 6 Oct 2026 — reg 12.3: net-metered exports offset consumption and carry forward within the settlement period, time-of-day consumers net within each time block, credits left at the end are paid at 75% of the feed-in tariff, and fixed and demand charges remain payable; reg 13.2: under net billing exports are valued at the feed-in tariff; reg 4.2: net metering up to the sanctioned load or contract demand (kVA × 0.9), at most 500 kWp on the AC inverter rating. Other states: confirm with your regulator and DISCOM
- NPV, IRR and payback — standard discounted-cash-flow definitions; IRR is reported only when the cash flows change sign once
Questions people ask before using it
What information do I need before using the commercial solar financial calculator?
Five things, ideally on paper. A year-one generation estimate in kWh from a yield study for your roof (AC energy at the meter, after losses). How much of that generation your site will use at the moment it is produced — worked out from 15- or 30-minute interval data from your main meter or a load logger, not from monthly units. The export settlement that applies to your connection and the rate it pays, from your state regulator's rooftop regulation and your DISCOM agreement. Your avoidable energy rate — the energy charge and energy-linked levies in your tariff, not the bill total divided by units. And the EPC and O&M quotations. For NPV and IRR add the analysis period, the module degradation from the warranty, your assumptions for rate changes and O&M escalation, any inverter replacement and your discount rate.
Which site conditions could change the solar savings result?
The daytime load profile matters most: weekly offs, a single shift, seasonal production or a planned shutdown all move solar from self-use to export or curtailment, where it is worth less. Shading, soiling, tilt and orientation change the generation itself. Your tariff structure changes the value: a time-of-day tariff nets solar within each time block, a kVAh-billed tariff settles in kVAh, and fixed and demand charges stay — in Punjab, for example, the prosumer keeps paying fixed and demand charges under net metering (PSERC Grid Interactive Rooftop Solar Photo Voltaic Systems Regulations, 2021, reg 12.3(g), consolidated up to the 3rd Amendment, 2026). The sanctioned load or contract demand can cap the plant allowed under net metering — in Punjab, up to the sanctioned load or contract demand and at most 500 kWp, measured on the AC inverter rating (reg 4.2(i)). And a grid-tied plant shuts down when the grid fails, so frequent outages reduce what it delivers.
What should I send an engineer for a project-specific solar review?
Twelve months of electricity bills; interval load data (15- or 30-minute) from the main meter or a logger, covering a representative period; the roof layout or drawings with photographs of obstructions; the single-line diagram with your sanctioned load or contract demand; your tariff category and DISCOM; any yield report or quotation you already hold; and planned changes in load or shifts. If you have run this calculator, you can attach your inputs to the optional review request below — it shows exactly what it would send before you send it.
Why does the calculator sometimes say there is no unique IRR?
IRR is the discount rate at which the present value of the cash flows is zero. When the yearly cash flows change sign more than once — a replacement year that turns a positive year negative, for example — there can be more than one such rate, or none, and quoting the first one a solver finds would be inventing a number. The calculator says so and you judge the project on NPV at your own discount rate. When IRR is unique it is useful: it compares the plant with any other use of the same capital, which payback cannot do because payback ignores everything after the money comes back.
Why is the discounted payback longer than the simple payback?
Simple payback divides the project cost by the year-one net benefit. It treats a rupee in year eight like a rupee today and ignores degradation, rate changes and replacements. Discounted payback works through the yearly cash flows after discounting each one at your rate, so with a positive discount rate it comes later. The calculator shows both, and the payback on the undiscounted yearly cash flows between them.
Are these numbers a quotation?
No. They are your inputs put through a stated method. Generation, self-consumption and the export settlement need site data — a yield study, interval meter data and your DISCOM's terms — and the project cost needs a quotation for your roof. Secured Engineers' published planning rate for turnkey commercial and industrial rooftop solar is ₹28,000 per kW of installed DC capacity, excluding GST, within a stated scope; a firm price follows a site survey.
Optional: ask an engineer to look at it
The result above is yours with no form. If you want an engineer to check it against your drawings, schedules or bills, send a request. Nothing is sent until you press the button.
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