6.2 Cost per useful kWh: electricity, LPG, PNG, diesel, biomass
A heat pump does not create heat by burning. It moves heat already present in the outside air into water, like a fridge, but using the heat it rejects instead of the cold it makes. The electricity drives the compressor; the ambient air supplies the rest.
Combustion appliances can never deliver more heat than the fuel contains. Their efficiency is always below 100%. A heat pump's COP is above 1 because the free ambient heat is not on the bill.
Typical reference values used in this course
| Energy source | Example price | Typical NCV | Energy per unit (kWh) | Typical efficiency / COP |
|---|---|---|---|---|
| Electricity (resistance geyser) | ₹9/kWh | — | 1.0 per kWh | 0.95 |
| Electricity (air-source heat pump, hot water 55–60 °C) | ₹9/kWh | — | 1.0 per kWh | COP 3.5 (typical range 3–4.5) |
| LPG | ₹100/kg | ~46 MJ/kg | ~12.8 per kg | 0.85 |
| PNG (piped natural gas) | ₹75/scm | ~34 MJ/scm | ~9.5 per scm | 0.85 |
| Diesel (HSD) | ₹100/L | ~36 MJ/L | ~10.0 per L | 0.80 |
| Biomass briquettes | ₹8/kg (example) | ~15 MJ/kg | ~4.2 per kg | 0.70 |
scm means standard cubic metre, the gas volume at reference conditions. HSD means high-speed diesel. Biomass values vary widely with moisture content.
Cost per useful kWh (example rates, typical efficiencies)
| Source | Cost per kWh of fuel | ÷ efficiency | Cost per useful kWh |
|---|---|---|---|
| Heat pump (COP 3.5) | ₹9.00 | 3.5 | ₹2.57 |
| Biomass briquettes | ₹1.90 | 0.70 | ₹2.72 |
| LPG | ₹7.81 | 0.85 | ₹9.19 |
| PNG | ₹7.89 | 0.85 | ₹9.29 |
| Electric resistance geyser | ₹9.00 | 0.95 | ₹9.47 |
| Diesel | ₹10.00 | 0.80 | ₹12.50 |
Biomass looks cheap on fuel alone. It needs storage space, manual feeding, ash removal and emission control. Its supply and moisture also vary by season, which affects real efficiency.
Worked example — LPG in a Chennai hostel kitchen water heater
- Energy in 1 kg LPG ≈ 46 MJ ÷ 3.6 = 12.8 kWh.
- Cost per kWh of fuel = ₹100 ÷ 12.8 = ₹7.81.
- Divide by efficiency 0.85: ₹7.81 ÷ 0.85 = ₹9.19 per useful kWh.
- Heat pump: ₹9 ÷ 3.5 = ₹2.57. The heat pump heat costs about 28% of LPG heat.
Remember
- Heat pump cost per useful kWh = tariff ÷ COP.
- At typical COP, heat pump heat costs roughly a quarter to a third of LPG, PNG or diesel heat.
- Biomass can be cheap but carries hidden handling costs.
- Recalculate whenever fuel prices change.
a heat pump turns 1 kWh of electricity into about 3–4 kWh of useful heat, so its cost per useful kWh is a fraction of the tariff.
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