Montana State University

MSU Extension Forestry

Estimate Space Heating Demand and Effective Costs


Jane Smith heats her house with natural gas. Her heating appliance is a condensing furnace rated at 95% AFUE (annual fuel utilization efficiency).

STEP 1 - Summarize your Energy Data

She looks through her monthly utility bills to find out the amount of fuel used and what she paid for it. Common units for natural gas are "therms" or "ccf" (hundred cubic feet).
Her lowest monthly use of natural gas was in August, at 5 therms. Assuming that at this time of the year no natural gas is needed to heat the home, this amount is an estimate of what is used every single month of the year for other heating applications like water heating, cooking, etc.

Depending on the fuel type used for heating, select the "units" here:

Fuel Units Fuel Cost Cost/Fuel Unit Fuel Units for Space Heating Cost for Space Heating
January $
February $
March $
April $
May $
June $
July $
August $
September $
October $
November $
December $

Find the lowest value in the Fuel Units column. This should be a summer month, when no or very little energy is required for space heating, and establishes a "base load" from water heating, cooking, etc.
Enter this value here


STEP 2 - Calculate the Unit Cost of your current Fuel

After calculating, she can see in the "TOTALS" line that she used 431 therms and paid $388 for space heating that year. She enters these amounts here, and clicks "Calculate".

Amount of Fuel used Amount charged Cost per Fuel Unit
$ $

STEP 3 - Calculate the Costs per Million BTU (British thermal units)

She selects "Natural Gas (therms)", and the cell "Fuel Heat Content per Unit" is populated with the value "100,000" as one therm of natural gas contains that many BTU's.
She finds that she pays $9.00 per Million BTU.

LPG/Propane (gallons)
Natural Gas (therms)
Electricity (kWh)
Wood Pellets (tons)
Firewood, 20% MC (cords)
Corn (kernels) (tons)
#2 Heating Oil (gallons)
Kerosene (gallons)
Coal, Bituminous (tons)

Fuel Heat Content per Unit Cost per Million BTU

STEP 4 - Estimate your actual Energy Demand and effective Costs for Space Heating

She selects "Condensing Boiler/Furnace (95% AFUE)". Her "Primary Energy Use" (from STEP 2) is 43.1 Million BTU. Since her furnace converts only 95% of the energy contained in the fuel into useable heat, the actual Heat Energy delivered to the home is 40.9 Million BTU at an effective cost of $9.48 per Million BTU.

Air-source Heat Pump (1.55 COP)
Ground-source (Geothermal) Heat Pump (3.3 COP)
Electric Baseboard (100%)
Condensing Boiler/Furnace (95% AFUE)
ENERGY STAR qualified Boiler/Furnace (85% AFUE)
Standard Boiler/Furnace (75% AFUE)
(Vented) Room Heater (80% AFUE)
EPA-certified Catalytic Wood Stove (72% AFUE)
EPA-certified Non-Catalytic Wood Stove (65% AFUE)
EPA Phase 2 Hydronic Heater (75% AFUE)
Conventional Hydronic Heater (45% AFUE)
Conventional Wood Stove (54% AFUE)
Fireplace (20% AFUE)

Primary Energy Use Conversion Efficiency Heating Energy Demand
mmBTU % mmBTU

The amount of energy used for space heating in the specified time period is approximately
mmBTU, at an effective cost of $ per mmBTU.