The efficiency of energy transformation for various heating systems varies significantly based on the fuel source, technology, and specific design. Understanding these efficiencies is crucial for evaluating both environmental impact and operational costs.

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Efficiency of Energy Transformation

Coal heaters typically have a lower efficiency compared to other modern heating systems. Traditional coal furnaces can range from 50% to 70% efficient, meaning a significant portion of the energy in the coal is lost as heat up the chimney or through incomplete combustion.[1] Newer, more advanced coal boilers might achieve slightly higher efficiencies, but they still face challenges with emissions and ash disposal.

Pellet heaters offer improved efficiency over traditional wood or coal burning. Modern pellet stoves and boilers can achieve efficiencies ranging from 75% to 90%.[2] This higher efficiency is due to controlled combustion, automated fuel feeding, and often secondary combustion chambers that burn off gases that would otherwise be lost.

Wood heaters, particularly older models, can have efficiencies as low as 40% to 60%.[3] However, modern EPA-certified wood stoves and boilers, designed with advanced combustion technologies, can reach efficiencies of 70% to 85%.[4] Factors like wood moisture content and proper operation significantly impact actual efficiency.

Electric heaters are often considered 100% efficient at the point of use.[5] This is because all the electrical energy consumed is converted into heat. However, this statement can be misleading when considering the overall energy chain. The electricity itself is generated at power plants, which have their own efficiencies (e.g., coal-fired power plants might be 30-40% efficient, natural gas plants 40-60%). Transmission and distribution losses further reduce the overall efficiency from source to end-use.

• Marble electric heaters (often radiant panels) and oil radiators (electric oil-filled heaters) are both types of electric resistance heaters. As such, their efficiency at the point of use is 100%.[5] They convert all the electrical energy they consume directly into heat. The difference lies in how they distribute that heat (radiant vs. convective) and their thermal mass, which can affect comfort and how long they retain heat after being turned off.

Gas heaters (natural gas or propane) typically have efficiencies ranging from 80% to 98% for modern condensing furnaces and boilers.[6] Older, non-condensing models might be in the 60-80% range. Condensing units recover additional heat from the exhaust gases, significantly boosting efficiency.

Geothermal heating systems (ground-source heat pumps) are highly efficient. They don't generate heat by burning fuel but rather move existing heat from the ground into a building. Their efficiency is measured by the Coefficient of Performance (COP), which is the ratio of heating output to electrical input. Geothermal systems typically have COPs ranging from 3.0 to 5.0.[7] This means for every unit of electricity consumed, they deliver 3 to 5 units of heat. This translates to an effective efficiency of 300% to 500% when compared to direct electric resistance heating.

Inverter heaters (air-source heat pumps with inverter technology) are also very efficient. Like geothermal systems, they move heat rather than generate it. Inverter technology allows the compressor to vary its speed, optimizing performance and efficiency across different operating conditions. Air-source heat pumps typically have COPs ranging from 2.5 to 4.5.[8] This means an effective efficiency of 250% to 450%. Their efficiency can decrease in very cold climates as the temperature difference between the air and the desired indoor temperature increases.

Heat exchange systems with pumps generally refer to heat pumps (air-source, ground-source, or water-source). As discussed above, these systems are characterized by their high COP, indicating their ability to move significantly more heat energy than the electrical energy they consume. Their efficiency is fundamentally higher than combustion-based systems because they are not limited by the Carnot efficiency of converting chemical energy to heat.

The general formula for efficiency ($\eta$) for combustion-based systems is: $\eta =\frac{\mathrm{\text{Useful Heat Output}}}{\mathrm{\text{Energy Content of Fuel Input}}}\times 100\%$

For heat pumps, the Coefficient of Performance (COP) is used: $\mathrm{\text{COP}}=\frac{\mathrm{\text{Heat Output}}}{\mathrm{\text{Electrical Energy Input}}}$ And the effective efficiency can be considered as $\mathrm{\text{COP}}\times 100\%$.

Prices per kW for Heating in Serbia (as of 2025-10-20)

Determining exact, real-time prices for heating per kW in Serbia is complex due to fluctuating energy markets, different tariff structures, and regional variations. However, based on recent trends and available data, here's an estimated breakdown. These prices are indicative and subject to change. It's important to note that "price per kW" can refer to the cost of the fuel required to produce 1 kW of heat, or the cost of electricity for electric heaters. For comparison, we will focus on the cost of the energy input required to deliver 1 kWh of useful heat, accounting for efficiency.

• Coal: The price of coal in Serbia varies significantly by type (lignite, brown coal) and supplier. Assuming an average price of €0.10 - €0.15 per kg for lignite and an energy content of approximately 15 MJ/kg (4.17 kWh/kg), and an efficiency of 60%, the cost per kWh of useful heat would be approximately: Cost per kWh = $\frac{\mathrm{\text{Price per kg}}}{\mathrm{\text{Energy content per kg}}\times \mathrm{\text{Efficiency}}}$ = $\frac{€0.125/kg}{4.17kWh/kg\times 0.60}\approx$ €0.05 per kWh.[9]

• Pellets: Wood pellets are generally more expensive per unit of energy than raw wood but offer higher efficiency. With an average price of €0.30 - €0.40 per kg and an energy content of 18 MJ/kg (5 kWh/kg), and an efficiency of 85%, the cost per kWh of useful heat would be: Cost per kWh = $\frac{€0.35/kg}{5kWh/kg\times 0.85}\approx$ €0.08 per kWh.[10]

• Wood: The price of firewood varies greatly depending on the type of wood, moisture content, and region. Assuming an average price of €60 - €80 per cubic meter (approximately 400 kg/m${}^3$) for seasoned wood, an energy content of 15 MJ/kg (4.17 kWh/kg), and an efficiency of 70%, the cost per kWh of useful heat would be: Cost per kWh = $\frac{€70/400kg}{4.17kWh/kg\times 0.70}\approx$ €0.06 per kWh.[11]

• Electricity: Electricity prices in Serbia are tiered and depend on consumption and time of day (day/night tariffs). For residential consumers, the average price can range from €0.07 to €0.15 per kWh depending on the tariff zone and consumption block.[12] Since electric heaters are 100% efficient at the point of use, the cost per kWh of useful heat is directly the electricity price: €0.07 - €0.15 per kWh.[12]

• Gas (Natural Gas): Natural gas prices for residential consumers in Serbia are regulated. Assuming an average price of €0.04 - €0.06 per kWh of natural gas, and a modern condensing boiler efficiency of 95%, the cost per kWh of useful heat would be: Cost per kWh = $\frac{€0.05/kWh}{0.95}\approx$ €0.053 per kWh.[13]

• Geothermal Heating: Geothermal systems use electricity to run the compressor and pumps. With a COP of 4.0 and an electricity price of €0.10 per kWh, the cost per kWh of useful heat would be: Cost per kWh = $\frac{\mathrm{\text{Electricity Price}}}{\mathrm{\text{COP}}}$ = $\frac{€0.10/kWh}{4.0}\approx$ €0.025 per kWh.[7]

• Inverter Heaters (Air-Source Heat Pumps): Similar to geothermal, these use electricity. With a COP of 3.5 and an electricity price of €0.10 per kWh, the cost per kWh of useful heat would be: Cost per kWh = $\frac{\mathrm{\text{Electricity Price}}}{\mathrm{\text{COP}}}$ = $\frac{€0.10/kWh}{3.5}\approx$ €0.029 per kWh.[8]

Summary of Estimated Prices per kWh of Useful Heat in Serbia:

• Coal: €0.05 per kWh
• Wood: €0.06 per kWh
• Pellets: €0.08 per kWh
• Gas (Natural Gas): €0.053 per kWh
• Electricity (Direct): €0.07 - €0.15 per kWh
• Geothermal Heating: €0.025 per kWh
• Inverter Heaters (Air-Source Heat Pumps): €0.029 per kWh

These figures highlight that while direct electric heating can be expensive, heat pump technologies (geothermal and inverter) offer significantly lower operational costs per unit of heat due to their high efficiency.

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