Energy in buildings: Unit economics
Which home upgrades pay back fastest, in rupees
What this tab covers What it costs to move a unit of energy this way: capital and operating costs, tariffs, utilisation and how it compares with the alternatives.For households the question is cost per useful unit of heat, air or backup power. Efficient devices usually cost more upfront and pay back through lower bills; rooftop solar and solar water heaters are the clearest examples, especially with subsidies.
Key costs
| Measure | Value | Note |
|---|---|---|
| Solar water heater, 100 litres per day | Rs 18,000-25,000 INR per system (Delhi, recent)2 | Life 15-20 years; Delhi rebate Rs 6,000 |
| Rooftop solar benchmark cost, India | About Rs 50,000 (calculated: Rs 30,000 subsidy is 60% of cost per kW up to 2 kW) INR per kW (2024)4 | Subsidy Rs 30,000 for 1 kW, Rs 60,000 for 2 kW, Rs 78,000 for 3 kW or more |
| Rooftop solar, Germany | 900-1,500 EUR per kWp for a 10 kWp system (end 2025)5 | Including installation |
| Rooftop solar yield, India | At least 1,200 (calculated: 3 kW expected to give over 300 units a month) kWh per kW per year4 | One unit is one kWh |
| BLDC fan saving | About 120-140 (calculated: 40-45 W saved for 10 h a day, 300 days) kWh per fan per year3 | About Rs 840-950 a year at an assumed Rs 7 per kWh (calculated) |
| Home inverter losses, India (illustrative) | About 11 billion kWh per year nationally7 | Estimate in an advocacy letter by an inverter-industry founder, assuming about 30 million units; not an official figure |
Where the money goes
| Item | Share or amount |
|---|---|
| Subsidy share of a 2 kW Indian rooftop system | 60% of benchmark cost4 |
| Wasted energy in a conventional home inverter at part load (advocacy estimate) | 12-20% of throughput7 |
Worked example, cooking: assume electricity at Rs 7 per kWh and LPG at Rs 900 for a 14.2 kg cylinder (both assumptions). The cylinder holds about 181 kWh (calculated, at 46 MJ per kg), so LPG fuel costs about Rs 5 per kWh. Delivered to the food, induction at 85% costs about Rs 8.2 per useful kWh and gas at the US-test 32% about Rs 15.5 (calculated). Indian LPG burners are rated by a different test and score higher than 32%, so the real gap is smaller; at an assumed 60% the two cost about the same. The choice then turns on tariffs, subsidies and the cost of the hob.
Worked example, water: a family using 100 litres of hot water on 250 days a year needs about 1,000 kWh (calculated). If a solar heater supplies most of that instead of an electric geyser, it saves roughly Rs 7,000 a year at Rs 7 per kWh (calculated), paying back a Rs 20,000 system in about three years.
Sources
- FAQ: Solar water heating systems Energy Efficiency and Renewable Energy Management Centre, Government of NCT of Delhi, 2024
- EESL unveils 5 star rated ceiling fans with BLDC technology in India Energetica India, 2022
- Cabinet approves PM-Surya Ghar: Muft Bijli Yojana Prime Minister's Office, Government of India, 2024
- Photovoltaics Report (updated 14 July 2026) Fraunhofer Institute for Solar Energy Systems ISE, 2026
- An open letter: BEE, make inverter efficiency standards mandatory (Kunwer Sachdev, founder of Su-Kam; advocacy, not independent data) kunwersachdev.com (personal blog), 2026