What is Cost Per BTU?
Cost per BTU is a metric that tells you how much you spend to produce a single British Thermal Unit of energy. A BTU is the standard unit for measuring thermal energy in heating and cooling systems, and understanding your cost at this level lets you make apples-to-apples comparisons between different fuel types and energy sources.
Whether you are running a household furnace on natural gas, heating with propane, or relying on electric resistance heat, each fuel delivers a different number of BTUs per dollar spent. Knowing your cost per BTU helps you:
- Save money by identifying the most cost-effective energy source available in your area.
- Improve efficiency by tracking energy performance over time and spotting waste.
- Plan ahead by budgeting for seasonal rate changes and fuel price fluctuations.
How to Calculate Cost Per BTU
The formula is straightforward:
[\text{Cost Per BTU} = \frac{\text{Total Cost}}{\text{Total Energy (BTU)}}]
Where:
- Total Cost ($) is the amount you paid for the energy (your bill, fuel purchase, etc.).
- Total Energy (BTU) is the total energy content of what you consumed, expressed in BTUs.
If your energy is reported in larger units, convert to BTUs first:
- 1 kBTU = 1,000 BTU
- 1 Therm = 100,000 BTU
- 1 MMBTU = 1,000,000 BTU
Calculation Example
Scenario: Your natural gas bill for January was $400 and you consumed 800 therms.
First, convert therms to BTUs:
[800 \text{ therms} \times 100{,}000 = 80{,}000{,}000 \text{ BTU}]
Now apply the formula:
[\text{Cost Per BTU} = \frac{400}{80{,}000{,}000} = 0.000005]
That works out to $0.000005 per BTU, or about $0.005 per kBTU (half a cent per thousand BTUs).
Comparing Common Heating Fuels
The real power of cost per BTU is side-by-side fuel comparison. Here are typical BTU contents for common residential fuels:
| Fuel | Unit | BTU per Unit |
|---|---|---|
| Natural Gas | 1 therm | 100,000 |
| Propane | 1 gallon | 91,500 |
| Heating Oil | 1 gallon | 138,500 |
| Electricity | 1 kWh | 3,412 |
| Wood Pellets | 1 ton | 16,500,000 |
| Cord Wood | 1 cord | 20,000,000 |
To compare, divide the price you pay per unit by the BTU content of that unit. The fuel with the lowest cost per BTU delivers the most heat for your dollar.
Key Points to Remember
- Always convert energy to the same unit (BTUs) before comparing different fuels.
- Appliance efficiency matters. A furnace rated at 95% AFUE delivers only 95% of the fuel's BTU content as usable heat, so factor in your equipment efficiency when making real-world comparisons.
- Prices vary by region and season, so recalculate periodically to ensure you are still using the most economical option.