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Tamil Nadu's industrial sector has a complicated relationship with diesel generators. On one hand, DG sets are the lifeline that keeps factories running during power cuts -- and scheduled and unscheduled outages remain a reality in many industrial areas. On the other hand, diesel generation is the single most expensive way to produce electricity, running at Rs 18-28 per unit depending on load factor, fuel price, and generator efficiency.
This analysis presents a detailed, numbers-driven comparison between rooftop solar and diesel generators for Tamil Nadu industries. We are not suggesting you disconnect your DG set tomorrow -- power cuts still happen after sunset, and solar alone cannot replace a generator for 24/7 reliability. But we will show exactly how much money a solar installation saves by reducing your DG runtime, and why a hybrid solar-diesel approach is the financially optimal strategy for most factories.
The True Cost of Diesel Generation
Most factory owners underestimate their DG running cost because they only track fuel expense. The true per-unit cost includes fuel, maintenance, overhaul, and depreciation.
Per-Unit Cost Breakdown (50 kVA DG Set)
| Cost Component | Monthly Cost | Units Generated (200 hrs/month) | Per-Unit Contribution |
|---|---|---|---|
| Diesel fuel (8 litres/hr x Rs 90/litre) | Rs 1,44,000 | 6,000 units | Rs 24.00 |
| Lubricant oil and filters | Rs 4,000 | 6,000 units | Rs 0.67 |
| Routine maintenance | Rs 3,000 | 6,000 units | Rs 0.50 |
| Major overhaul (amortised) | Rs 2,500 | 6,000 units | Rs 0.42 |
| Operator salary (shared) | Rs 5,000 | 6,000 units | Rs 0.83 |
| Depreciation (Rs 8 lakh over 10 years) | Rs 6,667 | 6,000 units | Rs 1.11 |
| Total | Rs 1,65,167 | 6,000 units | Rs 27.53 |
Rs 27.53 per unit. Compare this with TANGEDCO grid power at Rs 6-8 per unit or solar at Rs 2.50-3.50 per unit.
Even a smaller, more efficient DG set running at optimal load produces electricity at Rs 18-22 per unit. There is no scenario where diesel generation is cost-competitive with either grid power or solar on a per-unit basis.
Solar Generation Cost for Industries
Per-Unit Cost (100 kW Rooftop Solar System)
| Cost Component | Value |
|---|---|
| System cost | Rs 42,00,000 |
| Accelerated depreciation tax saving (Year 1) | -Rs 4,20,000 |
| Net effective cost | Rs 37,80,000 |
| Annual generation | 1,53,000 units (Year 1) |
| 25-year total generation | 35,50,000 units |
| Annual maintenance | Rs 30,000 |
| 25-year total maintenance | Rs 7,50,000 |
| Inverter replacement (Year 12-15) | Rs 3,00,000 |
| 25-year LCOE | Rs 1.36/unit |
| 10-year LCOE | Rs 2.73/unit |
Head-to-Head: 10-Year Cost Comparison
Scenario: Factory with 100 kW Demand, 3 Hours Daily Power Cuts
Option A: Continue with Diesel Generator Only
| Year | DG Runtime (hours/year) | Fuel + Maintenance Cost | Total Annual Cost |
|---|---|---|---|
| 1 | 1,095 | Rs 11,87,000 | Rs 11,87,000 |
| 2 | 1,095 | Rs 12,46,000 | Rs 12,46,000 |
| 3 | 1,095 | Rs 13,08,000 | Rs 13,08,000 |
| 4 | 1,095 | Rs 13,73,000 | Rs 13,73,000 |
| 5 | 1,095 | Rs 14,42,000 | Rs 14,42,000 |
| 6-10 | 1,095/year | Escalating | Rs 80,50,000 |
| 10-Year Total | Rs 1,46,06,000 |
(Assumes 5% annual diesel price increase)
Option B: Install 100 kW Solar + Keep DG for Night/Monsoon Backup
| Component | 10-Year Cost |
|---|---|
| Solar system (net of AD benefit) | Rs 37,80,000 |
| Solar maintenance (10 years) | Rs 3,00,000 |
| Reduced DG runtime (1 hour/day instead of 3, daytime hours covered by solar) | Rs 48,69,000 |
| TANGEDCO grid savings (solar offsets grid consumption during operating hours) | -Rs 50,00,000+ |
| Net 10-year cost of solar + reduced DG | Rs 39,49,000 |
10-Year Savings
| Metric | DG Only | Solar + DG Hybrid | Savings |
|---|---|---|---|
| 10-year electricity/generation cost | Rs 1,46,06,000 | Rs 39,49,000 | Rs 1,06,57,000 |
| Average cost per unit (DG hours) | Rs 24+ | Rs 8-10 (blended) | Rs 14-16/unit |
| Carbon emissions (10 years) | 450+ tonnes CO2 | 150 tonnes CO2 | 300 tonnes CO2 avoided |
The Hybrid Solar-Diesel Strategy
Complete DG elimination is not realistic for most Tamil Nadu factories today because:
- Power cuts occur at night (no solar generation)
- Monsoon months reduce solar output
- Some processes cannot tolerate even brief interruptions during cloud cover
The financially optimal strategy is a solar-diesel hybrid approach:
Daytime (6 AM - 6 PM): Solar Priority
- Solar system handles all daytime power needs
- Surplus solar feeds back to grid (net metering) or charges batteries
- DG remains on standby but rarely starts during sunny hours
- DG runtime reduction: 60-80% during daytime
Evening/Night (6 PM - 6 AM): Grid + DG Backup
- Grid power is primary source
- DG starts only during actual power cuts
- If battery storage is included, even evening DG runtime reduces further
Monsoon Months: Grid + DG + Reduced Solar
- Solar generation drops 20-30% during monsoon
- Grid remains primary; DG covers power cuts
- Still 40-50% less DG runtime than a no-solar scenario
Beyond Cost: Other Advantages of Solar Over Diesel
1. Noise Elimination
Diesel generators produce 75-95 dB of noise -- equivalent to standing next to a highway. Solar systems are completely silent. For factories near residential areas (increasingly common as cities expand into industrial zones), this eliminates noise complaints and potential regulatory issues.
2. Air Quality
A 50 kVA DG set running 3 hours daily emits approximately 15 tonnes of CO2 per year, plus particulate matter, NOx, and SOx. Industrial estates with multiple DG sets create localised air quality problems. Solar eliminates all direct emissions during generation.
3. Maintenance Simplicity
| Maintenance Aspect | Diesel Generator | Solar System |
|---|---|---|
| Daily maintenance | Oil check, coolant check, belt inspection | None |
| Weekly maintenance | Battery check, fuel filter, air filter | None |
| Monthly maintenance | Load test, cleaning, fluid top-up | Panel cleaning (15 minutes) |
| Annual maintenance | Major service, injector calibration | Inverter check, wiring inspection |
| Major overhaul | Every 3,000-5,000 hours (Rs 50,000-1,50,000) | None (inverter replacement at Year 12-15) |
| Operator needed | Yes (full-time or shared) | No |
4. Fuel Price Independence
Diesel prices have increased from Rs 55/litre (2015) to Rs 90/litre (2026) -- a 63% increase in 11 years. Every diesel price hike directly increases your generation cost. Solar generation cost is fixed at installation and never increases.
5. Space Utilisation
A 100 kW DG set requires a dedicated ground-floor room with ventilation, exhaust, fuel storage, and sound insulation. A 100 kW solar system uses your rooftop -- space that is currently unused and generating no value. Solar turns a dead asset (roof) into a productive one.
Battery Storage: The DG Replacement Path
While today's solar-diesel hybrid is the practical choice, battery storage costs are declining rapidly. Within 3-5 years, adding battery storage to your solar system may make DG sets obsolete for all but the heaviest industrial applications.
Current battery economics (2026):
- Lithium-ion battery cost: Rs 8,000-12,000 per kWh
- A 100 kWh battery (enough for 1-2 hours of 50 kW load): Rs 8-12 lakh
- Payback vs DG fuel savings: 3-5 years
As battery prices continue to fall, the hybrid solar-battery configuration will progressively replace diesel generators entirely. Installing solar now positions you to add batteries later when economics improve.
Getting Started: Industrial Solar Assessment
Every factory's DG usage pattern, power cut frequency, load profile, and roof characteristics are different. A proper assessment includes:
- DG runtime analysis: How many hours per day/month does your DG run?
- Load profiling: What is your daytime vs nighttime load split?
- Roof assessment: How much rooftop solar capacity can you install?
- Financial modelling: Year-by-year comparison of current costs vs solar hybrid costs
- Implementation planning: Phased installation if budget requires
Use our solar savings calculator for an initial estimate, or contact Tristar for a comprehensive industrial solar-diesel analysis. We serve factories and industrial units across Tamil Nadu's major industrial corridors including Coimbatore, Tirupur, Chennai, and Salem.
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