Heating Cost Calculator

The Heat-Loss × Degree-Day Method

The HDD method scales your design-condition heat loss (the worst-case BTU/hr at the coldest expected outdoor temperature) up to a full-season estimate. The key insight: heat loss is proportional to the temperature difference between inside and outside, and HDD captures the integrated temperature difference over the season. Annual energy = heat loss × 24 × HDD ÷ design ΔT. Divide by equipment efficiency to get fuel consumption.

Fuel Energy Content

Natural gas: 100,000 BTU per therm (approximately 1 CCF or 100 cubic feet). Propane: 91,500 BTU per gallon. Heating oil #2: 138,500 BTU per gallon. Electric: 3,412 BTU per kWh. Hardwood cord (128 cu ft): about 24 million BTU. Wood pellets: about 16 million BTU per ton. Multiply by appliance efficiency to get delivered heat.

Typical Efficiency Values

Old non-condensing gas furnace: 70-80% AFUE. Modern standard gas furnace: 80-82% AFUE. Condensing gas furnace: 92-98% AFUE. Oil boiler: 83-87% AFUE. Propane furnace: same as gas. Electric resistance: 100%. Heat pump COP 2.5 = 250% "effective AFUE", COP 3.5 = 350%, COP 4.0 (inverter mini-split) = 400%. Wood stove: 60-75% EPA-certified. Pellet stove: 70-83%. Fireplace insert: 60-70%.

Comparing Fuels

The cost per million BTU delivered is the apples-to-apples comparison. At typical current US prices: natural gas $16-18/MMBTU, heating oil $30-35/MMBTU, propane $35-45/MMBTU, electric resistance $40-50/MMBTU, heat pump $15-25/MMBTU (depending on COP and electric rate), wood $10-25/MMBTU (varies widely), pellets $20-30/MMBTU. Natural gas wins where it's available; heat pumps win in mild to moderate climates with reasonable electric rates.

Typical HDD and Annual Heating Cost by City

HDD values are 30-year climate normals (base 65 °F) from NOAA. Annual cost shown is for a 2,000 sq ft house with 40,000 BTU/hr design heat loss at ΔT = 60 °F, using a 95% AFUE natural gas furnace at $1.50/therm and a COP-3.0 heat pump at $0.15/kWh.

Note: these are rough estimates using average utility rates as of 2024. Your actual cost depends on local fuel prices (which can vary 2-3× regionally), your specific heat loss, thermostat setback habits, and equipment condition. Heat pump costs rise sharply in very cold climates because COP drops at low outdoor temps — a cold-climate inverter HP holds up much better than a standard unit below 20 °F.

Cost per Million BTU — Quick Reference (2024 US averages)

• Natural gas at $1.50/therm, 95% AFUE: $15.80 / MMBTU delivered
• Heat pump at COP 3.0, $0.15/kWh: $14.70 / MMBTU delivered
• Cord wood at $300/cord, 70% stove efficiency: $17.90 / MMBTU delivered
• Pellets at $320/ton, 82% stove efficiency: $24.40 / MMBTU delivered
• Heating oil at $4.00/gal, 87% AFUE: $33.20 / MMBTU delivered
• Propane at $3.00/gal, 95% AFUE: $34.50 / MMBTU delivered
• Electric resistance at $0.15/kWh, 100%: $43.90 / MMBTU delivered

Frequently Asked Questions

Where do I find HDD for my location?

NOAA Climate Data Online, degreedays.net (free), or your local utility's website.

Should I use design heat loss or average heat loss?

Design — the HDD method scales it down automatically. Using average would double-count the reduction.

Why does my actual gas bill differ?

Real use depends on thermostat setbacks, internal gains (cooking, people, appliances), solar gain, infiltration variation, and pilot light losses. ±15% is normal.

How do I calculate HDD in metric (K·days)?

Multiply °F-days by 5/9. 6,000 HDD (65 °F base) ≈ 3,333 K·days (18 °C base). The calculator switches automatically.

What is heat pump efficiency for this calc?

Use seasonal COP × 100. A cold-climate heat pump with seasonal COP 2.8 goes into this calculator as 280% efficiency.