Natural gas and propane are both clean-burning fossil fuels, but they differ in emissions, cost, and infrastructure. Natural gas emits 117 pounds of CO₂ per million BTU. Propane emits 139 pounds of CO₂ per million BTU — a 19% difference that matters for emissions reporting and fuel switching decisions.
Cost comparison requires converting to common units. Natural gas is priced per therm (100,000 BTU). Propane is priced per gallon (91,500 BTU). This guide covers the conversion math, EPA AP-42 emissions factors, efficiency differences, and infrastructure considerations.
CO₂ Emission Factors and Calculation Method
EPA AP-42 Chapter 1 emission factors: natural gas at 117.0 lb CO₂ per million BTU, propane at 139.0 lb CO₂ per million BTU. These are based on complete combustion and are independent of equipment efficiency.
A facility burning 50,000 therms of natural gas per year produces 585,000 lb (292.5 tons) CO₂. The same heat from propane: 695,000 lb (347.5 tons) CO₂. Switching from propane to natural gas reduces CO₂ by 15.8%.
For greenhouse gas reporting, also account for methane and nitrous oxide emissions, though these are minor compared to CO₂. Convert to CO₂ equivalent using GWP factors: CH₄ × 25, N₂O × 298.
Natural gas: Therms × 0.1 × 117 = lb CO₂
Propane: Gallons × 0.0915 × 139 = lb CO₂
Quick factors: 1 therm NG = 11.7 lb CO₂
1 gallon propane = 12.7 lb CO₂
Natural Gas vs Propane Emissions Calculator
Compare CO2, NOx, and total emissions between natural gas and propane for heating and process applications. See emissions per MMBtu delivered and annual totals based on your usage.
Cost Per Million BTU Comparison
Natural gas at $1.20 per therm: $12.00 per million BTU. Propane at $2.50 per gallon: $27.32 per million BTU. At these prices, natural gas costs 56% less per unit of heat.
Equipment efficiency is essentially identical between fuels when using the same technology. A modern condensing furnace operates at 95% to 97% AFUE on either fuel.
Where efficiency diverges is in older equipment. Replacing a 78% AFUE natural draft furnace with a 96% condensing unit saves 17% to 19% on fuel. The fuel switch and equipment upgrade decisions should be evaluated together.
Infrastructure and Conversion Considerations
The biggest factor is often infrastructure availability. Natural gas pipeline extensions cost $30 to $100 per linear foot. A one-mile extension can cost $158,000 to $528,000, making propane the clear choice in rural areas.
Propane requires on-site tank storage. Leased tanks lock you to one supplier. Owned tanks cost $1,500 to $4,000 residential, $5,000 to $15,000 commercial, plus installation.
Equipment conversion between fuels requires different orifice sizes. Conversion kits cost $50 to $200 per appliance. A whole-house conversion runs $500 to $1,500 in parts and labor. Some equipment is not field-convertible.