About the Geothermal vs Air-Source
Both geothermal and air-source heat pumps move heat rather than generate it, but they draw that heat from very different sources, and that difference shows up directly in the electric bill. This calculator compares the annual operating cost of a ground-source (geothermal) system against an air-source system for the same heating load, based on each system's efficiency. It's meant for comparing ongoing energy costs, not the upfront installation price, which differs substantially between the two technologies.
How It Works
Enter your home's annual heating energy need in kWh, the Coefficient of Performance (COP) for an air-source and a ground-source system, and your electricity rate. The calculator divides the heating load by each system's COP to find how much electricity it would actually consume, since a higher COP means more heat delivered per unit of electricity, then multiplies by the electricity rate to get each system's annual cost. The difference between the two costs is shown as the annual savings from choosing geothermal.
Formula & Methodology
COP is floored at 0.1 in the calculation to prevent a division error if a very low or zero value is entered, though realistic heat pump COPs are well above that floor, typically 2.5 to 4.5 depending on climate and equipment. Because geothermal systems draw heat from underground temperatures that stay far more stable year-round than outdoor air, their COP tends to hold up better in cold weather, which is where air-source systems lose the most efficiency.
Examples
Typical single-family home
With a 15,000 kWh heating load, an air-source COP of 3.0, a ground-source COP of 4.2, and electricity at $0.16/kWh, the air-source system costs about $800 a year to run versus about $571 for geothermal, a savings of roughly $229 a year.
Larger home in a colder climate
A 25,000 kWh heating load with a lower air-source COP of 2.8 (reflecting reduced cold-weather performance), a ground-source COP of 4.0, and electricity at $0.14/kWh puts the air-source cost at about $1,250 a year against $875 for geothermal, a savings of about $375.
Advantages
- Isolates the efficiency difference between the two heat pump types as a direct dollar figure, using the homeowner's own heating load and electricity rate.
- Makes clear how sensitive operating cost is to COP, which is useful when comparing quoted equipment specs from different installers.
- Provides the annual savings number needed as one input to a longer-term payback calculation against installation cost.
Common Mistakes
- Comparing operating cost savings alone and concluding geothermal is the better choice without weighing the much higher installation cost of the ground loop.
- Using a single, static COP for air-source systems when real-world seasonal average COP is often lower than a manufacturer's peak-condition rating.
- Entering a heating load in kWh of fuel or gas equivalent rather than kWh of actual heat delivered, which is the unit the calculator assumes.
Edge Cases to Watch For
- Both COP inputs are clamped to a minimum of 0.1 internally, so an entry of 0 won't crash the calculation, but it will produce an unrealistically inflated cost rather than an error.
- The comparison covers operating cost only; it doesn't include the higher installation cost of drilling or trenching a geothermal ground loop, which is typically the larger factor in a real payback decision.
- A fixed COP is used for the full year even though air-source COP in particular tends to drop during the coldest weather, so the air-source cost shown may understate true winter operating cost.
Common Use Cases
- Homeowners deciding between a geothermal and air-source system who want to see the operating cost gap for their specific home.
- HVAC contractors illustrating the efficiency argument for geothermal to a client alongside installation quotes.
- Anyone building a payback timeline who needs an annual operating savings figure as a starting input.