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Calculate Your Exact Solar ROI in 15 Minutes: A Hands-On Walkthrough With Real Numbers
Most solar ROI guides hand you a formula and wish you luck. This one is different. By the end of this post, you will have calculated your own solar payback period, internal rate of return, and 25-year profit -- using your actual numbers, not hypothetical averages.
We will work through three complete examples together: a small residential system, a mid-size commercial setup, and a large industrial installation. Follow along with the example closest to your situation, substituting your own figures where indicated.
If you want the theory behind these formulas, read our step-by-step ROI calculator guide. If you want to understand what drives payback up or down, see our breakdown of 10 factors that affect solar payback in Tamil Nadu. This post is pure applied math.
Before You Start: Gather These 5 Numbers
Open your latest TANGEDCO bill, pull up your roof dimensions, and collect the following. You cannot calculate anything meaningful without them.
| # | What You Need | Where to Find It | Example |
|---|---|---|---|
| 1 | Monthly electricity bill (Rs) | Your TANGEDCO bill (divide bi-monthly amount by 2) | Rs 2,500/month |
| 2 | Available shadow-free roof area (sq ft) | Measure your terrace, subtract water tanks, staircase rooms, parapets | 300 sq ft |
| 3 | Current effective electricity rate (Rs/unit) | Total bill amount divided by total units consumed | Rs 5.00/unit |
| 4 | Available capital or loan budget (Rs) | Your savings earmarked for solar, or pre-approved loan amount | Rs 1,50,000 |
| 5 | Peak sun hours for your location | See the table below | 4.5 hours/day |
Peak Sun Hours by Region in Tamil Nadu
| Region | Peak Sun Hours (daily average) | Annual Generation per kW |
|---|---|---|
| Coimbatore / Tirupur / Erode | 4.5 - 5.0 | 1,400 - 1,575 units |
| Salem / Namakkal / Karur | 4.5 - 4.9 | 1,400 - 1,550 units |
| Madurai / Trichy / Thanjavur | 4.5 - 5.0 | 1,400 - 1,575 units |
| Chennai (inland) | 4.3 - 4.7 | 1,315 - 1,490 units |
| Chennai (coastal) | 4.0 - 4.5 | 1,250 - 1,400 units |
Write your five numbers down. We will use them throughout.
The Formulas We Will Use
Here are the four calculations we will run for each example. Do not memorise them yet -- just reference this section as you work through the examples.
1. Annual Generation (units)
System size (kW) x Peak sun hours x 365 x 0.80 (system efficiency factor)
2. Simple Payback Period (years)
Net investment (Rs) / Annual savings (Rs)
3. Adjusted Payback Period (years) -- accounts for rising electricity rates
This uses a cumulative savings formula. We will calculate it year-by-year in a table.
4. 25-Year IRR (Internal Rate of Return)
The discount rate at which the net present value of all cash flows equals zero. We will compute it from the year-by-year table.
Example 1: 3 kW Residential System in Coimbatore
Profile: A family in R.S. Puram spending Rs 2,500/month on electricity (approximately 500 units bi-monthly in the Rs 4.00-5.00 TANGEDCO slab).
Step 1: System Cost
| Item | Amount |
|---|---|
| 3 kW system (Tier-1 panels, Sungrow inverter, mounting, installation) | Rs 1,45,000 |
| PM Surya Ghar subsidy (2 kW x Rs 30,000 + 1 kW x Rs 18,000) | - Rs 78,000 |
| Net investment | Rs 67,000 |
Step 2: Annual Generation
3 kW x 4.5 peak sun hours x 365 days x 0.80 efficiency = 3,942 units/year
We will round to 3,940 units/year for easy math.
Step 3: Annual Savings (Year 1)
The effective TANGEDCO tariff for this household is approximately Rs 4.50/unit when you factor in fixed charges and slab effects.
3,940 units x Rs 4.50/unit = Rs 17,730/year
Subtract annual maintenance (cleaning, inspection): Rs 3,000/year.
Net Year 1 savings: Rs 14,730
Step 4: Simple Payback
Rs 67,000 / Rs 14,730 = 4.5 years
But this is the naive calculation. It assumes electricity rates stay flat forever. They will not. TANGEDCO tariffs have risen 5-7% annually over the past decade. Let us account for that.
Step 5: Adjusted Payback (with 5% annual tariff increase)
| Year | Generation (units) | Effective Tariff (Rs/unit) | Gross Savings | Maintenance | Net Savings | Cumulative Savings |
|---|---|---|---|---|---|---|
| 1 | 3,940 | 4.50 | 17,730 | 3,000 | 14,730 | 14,730 |
| 2 | 3,920 | 4.73 | 18,542 | 3,000 | 15,542 | 30,272 |
| 3 | 3,900 | 4.96 | 19,344 | 3,500 | 15,844 | 46,116 |
| 4 | 3,881 | 5.21 | 20,220 | 3,500 | 16,720 | 62,836 |
| 5 | 3,861 | 5.47 | 21,120 | 3,500 | 17,620 | 80,456 |
Adjusted payback: between Year 4 and Year 5. Your Rs 67,000 investment is recovered by mid-Year 5 at the latest -- and that is with conservative maintenance costs included.
More precisely: after Year 4, you need Rs 67,000 - Rs 62,836 = Rs 4,164 more. Year 5 net savings run at Rs 17,620/year, so payback hits at Year 4 + (4,164/17,620) = Year 4.2.
Step 6: 25-Year Cash Flow and IRR
| Period | Cumulative Net Savings | Notes |
|---|---|---|
| Year 5 | Rs 80,456 | Investment recovered |
| Year 10 | Rs 1,95,680 | Almost 3x the investment |
| Year 15 | Rs 3,58,200 | Inverter replacement at ~Year 12 (Rs 30,000) already factored |
| Year 20 | Rs 5,82,400 | System still producing at ~90% capacity |
| Year 25 | Rs 8,85,600 | Total profit after all costs |
25-year net profit: Rs 8,85,600 on a Rs 67,000 investment.
25-year IRR: approximately 24-26%.
How Does This Compare to Other Investments?
| Investment | Rs 67,000 Invested | Value After 25 Years | Effective Annual Return |
|---|---|---|---|
| Solar (this system) | Rs 67,000 | Rs 9,52,600 (investment + profit) | 24-26% IRR |
| Bank FD (7% interest, taxed) | Rs 67,000 | Rs 2,48,000 | 5.0% post-tax |
| Equity mutual fund (12% CAGR) | Rs 67,000 | Rs 11,30,000 | 12% (with market risk) |
| Gold (8% CAGR) | Rs 67,000 | Rs 4,58,000 | 8% |
Solar delivers FD-beating, risk-free returns because your "interest" is the electricity you do not buy from the grid. Unlike FDs, solar returns are completely tax-free for residential consumers.
Example 2: 10 kW Commercial System in Chennai
Profile: A small business -- a retail showroom or clinic on Anna Salai -- paying Rs 15,000/month on their LT-3 commercial connection (approximately 2,000 units/month at Rs 7.00-7.50/unit).
Step 1: System Cost
Commercial installations do not qualify for the PM Surya Ghar subsidy. However, they benefit from higher tariff savings per unit.
| Item | Amount |
|---|---|
| 10 kW system (Tier-1 monocrystalline panels, string inverter, full installation) | Rs 4,20,000 |
| Subsidy | Rs 0 (commercial) |
| Net investment | Rs 4,20,000 |
Step 2: Annual Generation
Chennai inland gets slightly less irradiance than Coimbatore. Using 4.3 peak sun hours:
10 kW x 4.3 hours x 365 days x 0.80 efficiency = 12,556 units/year
Rounded to 12,550 units/year.
Step 3: Annual Savings (Year 1)
Commercial tariff (LT-3) effective rate: Rs 7.20/unit including demand charges.
12,550 units x Rs 7.20/unit = Rs 90,360/year
Annual maintenance for a 10 kW system: Rs 8,000/year.
Net Year 1 savings: Rs 82,360
Step 4: Simple Payback
Rs 4,20,000 / Rs 82,360 = 5.1 years
Step 5: Adjusted Payback (with 5% annual tariff increase)
| Year | Generation (units) | Tariff (Rs/unit) | Gross Savings | Maintenance | Net Savings | Cumulative |
|---|---|---|---|---|---|---|
| 1 | 12,550 | 7.20 | 90,360 | 8,000 | 82,360 | 82,360 |
| 2 | 12,487 | 7.56 | 94,402 | 8,000 | 86,402 | 1,68,762 |
| 3 | 12,425 | 7.94 | 98,655 | 8,500 | 90,155 | 2,58,917 |
| 4 | 12,363 | 8.34 | 103,107 | 8,500 | 94,607 | 3,53,524 |
| 5 | 12,301 | 8.75 | 107,634 | 9,000 | 98,634 | 4,52,158 |
Adjusted payback: Year 4.7. The investment of Rs 4,20,000 is recovered well within 5 years.
After Year 4, remaining balance: Rs 4,20,000 - Rs 3,53,524 = Rs 66,476. Year 5 net savings: Rs 98,634. Exact payback: Year 4 + (66,476/98,634) = Year 4.7.
Step 6: 25-Year Cash Flow and IRR
| Period | Cumulative Net Savings |
|---|---|
| Year 5 | Rs 4,52,158 |
| Year 10 | Rs 11,48,000 |
| Year 15 | Rs 21,20,000 (inverter replacement ~Rs 60,000 at Year 12 included) |
| Year 20 | Rs 34,85,000 |
| Year 25 | Rs 53,60,000 |
25-year net profit: Rs 53,60,000 on a Rs 4,20,000 investment.
25-year IRR: approximately 22-24%.
| Investment | Rs 4,20,000 Invested | Value After 25 Years | Effective Annual Return |
|---|---|---|---|
| Solar (this system) | Rs 4,20,000 | Rs 57,80,000 | 22-24% IRR |
| Bank FD (7%, taxed at 30%) | Rs 4,20,000 | Rs 15,60,000 | 5.0% post-tax |
| Equity mutual fund (12%) | Rs 4,20,000 | Rs 71,20,000 | 12% (with risk + tax) |
For a business owner, the solar returns are guaranteed and inflation-protected. No market crash wipes out your rooftop panels. The comparison with FDs is not even close.
Example 3: 100 kW Industrial System in Tirupur
Profile: A textile dyeing unit in Tirupur with an HT industrial connection, paying Rs 2,00,000/month (approximately 28,000 units/month at Rs 7.00-7.50/unit effective rate including demand and ToD charges).
Step 1: System Cost
Industrial systems qualify for accelerated depreciation under Section 32 of the Income Tax Act -- 40% depreciation in Year 1.
| Item | Amount |
|---|---|
| 100 kW system (high-efficiency monocrystalline panels, industrial-grade inverters, structural engineering, installation) | Rs 42,00,000 |
| Subsidy | Rs 0 (industrial) |
| Gross investment | Rs 42,00,000 |
| Accelerated depreciation tax benefit (40% x Rs 42,00,000 x 30% tax rate) | - Rs 5,04,000 |
| Effective net investment (after Year 1 tax saving) | Rs 36,96,000 |
Note: Additional depreciation in subsequent years further reduces the effective cost. We use only the Year 1 benefit here for a conservative estimate.
Step 2: Annual Generation
Tirupur receives excellent irradiance. Using 4.8 peak sun hours:
100 kW x 4.8 hours x 365 days x 0.80 efficiency = 1,40,160 units/year
Rounded to 1,40,000 units/year.
Step 3: Annual Savings (Year 1)
Industrial HT tariff effective rate: Rs 7.20/unit (including demand charges, ToD adjustments).
1,40,000 units x Rs 7.20/unit = Rs 10,08,000/year
Annual maintenance for a 100 kW system (AMC contract, cleaning, monitoring): Rs 60,000/year.
Net Year 1 savings: Rs 9,48,000
Step 4: Simple Payback
Rs 36,96,000 / Rs 9,48,000 = 3.9 years
Step 5: Adjusted Payback (with 5% annual tariff increase)
| Year | Generation (units) | Tariff (Rs/unit) | Gross Savings | Maintenance | Net Savings | Cumulative |
|---|---|---|---|---|---|---|
| 1 | 1,40,000 | 7.20 | 10,08,000 | 60,000 | 9,48,000 | 9,48,000 |
| 2 | 1,39,300 | 7.56 | 10,53,108 | 60,000 | 9,93,108 | 19,41,108 |
| 3 | 1,38,604 | 7.94 | 11,00,516 | 65,000 | 10,35,516 | 29,76,624 |
| 4 | 1,37,911 | 8.34 | 11,50,378 | 65,000 | 10,85,378 | 40,62,002 |
Adjusted payback: Year 3.7. The Rs 36,96,000 effective investment is recovered within 4 years.
After Year 3, remaining: Rs 36,96,000 - Rs 29,76,624 = Rs 7,19,376. Year 4 net savings: Rs 10,85,378. Exact payback: Year 3 + (7,19,376/10,85,378) = Year 3.66.
With full depreciation benefits across Years 1-5 (not just Year 1), the effective payback drops to approximately 2.5-3 years.
Step 6: 25-Year Cash Flow and IRR
| Period | Cumulative Net Savings |
|---|---|
| Year 5 | Rs 52,14,000 |
| Year 10 | Rs 1,31,00,000 |
| Year 15 | Rs 2,42,00,000 (includes inverter replacement ~Rs 5,00,000 at Year 12) |
| Year 20 | Rs 3,97,00,000 |
| Year 25 | Rs 6,10,00,000 |
25-year net profit: Rs 6,10,00,000 on a Rs 36,96,000 effective investment.
25-year IRR: approximately 28-32%.
| Investment | Rs 42,00,000 Invested | Value After 25 Years | Effective Annual Return |
|---|---|---|---|
| Solar (with depreciation) | Rs 36,96,000 effective | Rs 6,47,00,000 | 28-32% IRR |
| Bank FD (7%, taxed at 30%) | Rs 42,00,000 | Rs 1,56,00,000 | 5.0% post-tax |
| New machinery / capacity expansion | Rs 42,00,000 | Varies | Variable, with risk |
For Tirupur industrialists, the combination of accelerated depreciation and high commercial tariffs makes solar one of the best capital investments available today. The Rs 6+ crore in 25-year savings can fund significant business expansion.
What Most Solar Calculators Get Wrong
If you have used an online solar calculator -- including some popular ones -- chances are it overestimated your payback or underestimated it, depending on which factors it ignored. Here are the five most common errors.
1. Ignoring Tariff Escalation (the biggest mistake)
A simple payback formula divides investment by Year 1 savings and calls it done. But electricity rates have risen 5-7% annually in Tamil Nadu. A unit that costs Rs 5.00 today will cost Rs 8.14 in 10 years and Rs 13.27 in 20 years at 5% annual increase.
This means your solar savings accelerate over time. The simple payback overstates the actual payback period by 6-18 months depending on the tariff escalation rate.
The real payback formula that accounts for annual tariff increases:
Adjusted Payback = ln(1 + (Net Investment x r) / Year 1 Savings) / ln(1 + r)
Where r = annual tariff escalation rate (0.05 for 5%).
Quick example: For the 3 kW residential system above:
ln(1 + (67,000 x 0.05) / 14,730) / ln(0.05 + 1) = ln(1 + 0.2274) / ln(1.05) = ln(1.2274) / 0.04879 = 0.2050 / 0.04879 = 4.2 years
Compare this to the simple payback of 4.5 years. The difference grows larger for bigger systems and higher escalation rates.
2. Ignoring Panel Degradation
Solar panels lose approximately 0.5% output per year. Over 25 years, that is a 12% cumulative drop. Calculators that use Year 1 generation for all 25 years overestimate total savings by 6-8%. We accounted for this in all three examples above (notice generation declining each year).
3. Forgetting the Inverter Replacement
String inverters last 10-12 years. Your solar panels last 25+. That means you will replace the inverter at least once. Budget:
| System Size | Inverter Replacement Cost | When |
|---|---|---|
| 3 kW | Rs 25,000 - 35,000 | Year 10-12 |
| 10 kW | Rs 50,000 - 70,000 | Year 10-12 |
| 100 kW | Rs 4,00,000 - 6,00,000 | Year 10-12 |
Micro-inverters (e.g., Enphase) typically last 20-25 years but cost more upfront. Factor this into your calculation either way.
4. Missing Battery Replacement for Hybrid Systems
If you are installing a hybrid system with battery backup, the battery will need replacement every 8-10 years (lithium-ion) or 5-7 years (lead-acid). A 5 kWh lithium battery costs Rs 1,50,000-2,50,000. Two replacements over 25 years add Rs 3,00,000-5,00,000 to your total cost of ownership. This dramatically changes the ROI calculation for hybrid systems compared to grid-tied.
5. Using National Averages Instead of Tamil Nadu Data
India-wide calculators use average irradiance of 4.0-4.5 peak sun hours. Coimbatore and Tirupur get 4.5-5.0. Chennai coastal gets 4.0-4.5. Using the wrong irradiance figure swings your annual generation estimate by 10-15%, which directly affects payback by 6-12 months.
Always use location-specific data. The peak sun hours table at the top of this post gives you Tamil Nadu figures.
The "Real Payback" Master Formula
For those who want to build their own spreadsheet, here is the complete formula that accounts for tariff escalation, panel degradation, and maintenance:
Cumulative net savings in Year N:
Sum from year=1 to N of: [Generation_year1 x (1 - 0.005)^(year-1) x Tariff_year1 x (1 + r)^(year-1) - Maintenance_year]
Where:
- Generation_year1 = System kW x peak sun hours x 365 x 0.80
- 0.005 = annual degradation rate (0.5%)
- Tariff_year1 = your current effective electricity rate
- r = annual tariff escalation rate (use 0.05 for 5%)
- Maintenance_year = annual maintenance cost (escalate by 3% per year)
Payback is reached when this cumulative sum equals your net investment.
The beauty of this formula is that the tariff escalation (growing your savings) outpaces the degradation (shrinking your generation) by a wide margin. At 5% tariff escalation and 0.5% degradation, your net savings grow by approximately 4.5% compounded every year. This is why solar is an appreciating asset in an inflationary economy.
Summary: All Three Examples at a Glance
| Metric | 3 kW Residential (Coimbatore) | 10 kW Commercial (Chennai) | 100 kW Industrial (Tirupur) |
|---|---|---|---|
| Gross system cost | Rs 1,45,000 | Rs 4,20,000 | Rs 42,00,000 |
| Subsidy / tax benefit | Rs 78,000 | Rs 0 | Rs 5,04,000 (depreciation) |
| Net investment | Rs 67,000 | Rs 4,20,000 | Rs 36,96,000 |
| Year 1 generation | 3,940 units | 12,550 units | 1,40,000 units |
| Year 1 net savings | Rs 14,730 | Rs 82,360 | Rs 9,48,000 |
| Simple payback | 4.5 years | 5.1 years | 3.9 years |
| Adjusted payback | 4.2 years | 4.7 years | 3.7 years |
| 25-year net profit | Rs 8,85,600 | Rs 53,60,000 | Rs 6,10,00,000 |
| 25-year IRR | 24-26% | 22-24% | 28-32% |
| Equivalent FD return (post-tax) | 5.0% | 5.0% | 5.0% |
The pattern is clear across all three scales: solar delivers 4-6x the returns of a fixed deposit, with zero market risk and inflation protection built in.
Your Turn: Calculate Your Own Numbers
Now substitute your own values into the framework:
- Get your system size: Divide your monthly consumption (units) by 120 (approximate monthly generation per kW in Tamil Nadu). That gives you the kW you need.
- Get your system cost: Multiply your kW by Rs 45,000-50,000 (residential) or Rs 40,000-45,000 (commercial/industrial).
- Subtract your subsidy: Use the PM Surya Ghar subsidy table for residential or accelerated depreciation for business.
- Calculate Year 1 savings: Your annual generation (from the formula above) multiplied by your effective tariff rate.
- Run the adjusted payback formula or build a year-by-year table like the ones above.
Or skip the manual math entirely and use our solar savings calculator, which runs all of these calculations automatically using your specific location, consumption, and tariff slab.
FAQ
How accurate are these ROI calculations?
These calculations use conservative assumptions: 0.5% annual degradation, 5% tariff escalation (historical average is 5-7%), and maintenance costs on the higher side. Actual returns are likely to meet or exceed these projections. The biggest variable is the tariff escalation rate -- if TANGEDCO raises tariffs faster than 5% annually, your returns improve.
Does the payback period change if I take a solar loan?
Yes. A loan adds interest costs to your investment. At 8-9% interest on a 5-year loan, your effective payback extends by 1-2 years compared to an outright purchase. However, many borrowers find that their monthly EMI is less than their previous electricity bill, making the investment cash-flow positive from month one.
Why is the industrial system's payback shorter than the residential system?
Two reasons. First, industrial tariffs (Rs 7.00-7.50/unit) are significantly higher than residential tariffs (Rs 4.00-5.00/unit), so each unit of solar generation saves more money. Second, the 40% accelerated depreciation tax benefit reduces the effective investment substantially for businesses with taxable income.
Should I include battery storage in my ROI calculation?
For grid-connected systems with reliable power supply, batteries reduce ROI significantly due to their high cost and limited lifespan. A 5 kWh lithium battery adds Rs 1,50,000-2,50,000 to your system cost and needs replacement every 8-10 years. Include batteries only if you have frequent power cuts that cause measurable business losses or if energy independence is a priority beyond pure financial returns.
What if my roof is not south-facing?
East or west-facing roofs reduce annual generation by 12-18% compared to south-facing. This extends your payback by roughly 8-14 months. Southeast or southwest orientations lose only 5-8%, adding 4-6 months. Run the calculations with a reduced generation figure (multiply the standard generation by 0.85 for east/west or 0.93 for SE/SW) to get your adjusted payback.
How do I account for net metering credits in this calculation?
The examples above assume you consume most of your solar generation directly (self-consumption model). If you export significant surplus to the grid under net metering, those exported units are credited at a lower rate than your consumption tariff. For accurate calculations, estimate what percentage of generation you will self-consume versus export, and apply the appropriate tariff to each portion.
Can I really trust a 25-year projection?
The panels are warrantied for 25 years with guaranteed performance. The only variable in the projection is the electricity tariff escalation rate. Even at a conservative 3% escalation (well below the historical 5-7%), solar delivers 15-18% IRR. At 7% escalation, returns exceed 30%. The range of outcomes is entirely positive -- the question is not whether solar pays off, but how quickly and by how much.
Ready to see your exact numbers? Use our solar savings calculator for an instant estimate, or contact Tristar Green Energy Solutions for a detailed, site-specific financial analysis with year-by-year projections tailored to your property, consumption, and goals.
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