Geothermal and Solar: The Full Energy Independence Stack
If heat pumps and solar is a natural combination, geothermal heat pumps and solar is the next level up: two systems that work together to address nearly all of a home's energy needs — heating, cooling, and electricity — from two sources that are free once the systems are installed.
Geothermal heat pumps use the stable temperature of the earth (a few feet below the surface, ground temperature in Illinois stays around 50–55°F year-round) as both a heat source in winter and a heat sink in summer. Air-source heat pumps work with outside air temperatures that swing between -10°F and 95°F. Geothermal works with a constant 50–55°F. This gives geothermal a significant efficiency advantage over air-source heat pumps, especially in extreme weather.
How Geothermal Works
A ground loop — a network of pipes buried in the ground — circulates water (or a water/antifreeze mix) that absorbs or releases heat to the earth. In winter, the loop fluid picks up heat from the ground and carries it inside, where the geothermal heat pump concentrates and distributes it. In summer, the process reverses: the system extracts heat from inside the building and deposits it in the cooler ground.
Ground loops are installed horizontally (in horizontal trenches if you have the land area) or vertically (in bored wells if land area is limited). Vertical well systems are more common for residential properties with smaller lots.
Like air-source heat pumps, geothermal systems are electric — they move heat using a compressor and a refrigerant cycle, powered by electricity. This is why combining geothermal with solar is particularly powerful: solar generates the electricity that runs the geothermal system.
The Efficiency Advantage
Geothermal heat pumps typically operate at a coefficient of performance (COP) of 3.5–5.0 or higher. This means for every unit of electricity consumed, the system delivers 3.5–5 units of heat energy. Compare this to an air-source heat pump (COP 2–4 in mild conditions, lower in extreme cold) and a gas furnace (which converts fuel to heat with losses).
The efficiency advantage is most pronounced during Illinois winters. When outdoor temperatures drop to 0°F or below, air-source heat pumps struggle and may rely heavily on backup resistance heat. Geothermal draws from the stable 50–55°F ground and maintains high efficiency regardless of outdoor temperature.
How Geothermal Affects Solar Sizing
Like any heat pump, geothermal increases your home's electrical consumption because you're now using electricity to heat the home instead of gas. The solar system needs to be sized to cover this added electrical load.
However, because geothermal is more efficient than air-source heat pumps, the added electrical load per unit of heat delivered is lower. For the same heating output, a geothermal system typically requires less electricity than an air-source heat pump, which means the solar system size needed to offset it is somewhat smaller.
SPM models the full energy picture — geothermal heating load, solar production, and net electricity balance — when designing a combined geothermal and solar system.
The Installation Sequence
Geothermal and solar can be planned together and installed in a coordinated sequence, or phased. Because the geothermal installation involves ground work (trenching or drilling), it's typically done before solar to avoid any interference with landscaping or ground area near the house.
The right sequence:
- Energy efficiency assessment — windows, insulation, envelope — to reduce the heating/cooling load before sizing geothermal
- Geothermal system sizing and installation (ground loop + indoor unit)
- Solar system sizing to cover the home's full electrical load including geothermal
- Solar installation
Is Geothermal Right for Your Property?
Geothermal requires drilling or excavation — not every property is suitable, and installation cost is higher than air-source heat pumps. The economic case improves with:
- Long-term ownership — geothermal has high upfront cost but very low operating cost over decades
- Existing gas heating that would be eliminated — eliminating the gas bill improves the economics
- High current heating costs — the bigger the bill, the more the geothermal savings matter
- Poor air-source heat pump performance profile — cold climates benefit more from geothermal's ground-temperature advantage
- Combined with solar — the solar electricity that powers the geothermal is effectively free
Frequently Asked Questions
How does geothermal compare to air-source heat pumps in Illinois winters?
Geothermal maintains consistent high efficiency regardless of outdoor temperature. Air-source heat pumps (including cold-climate models) become less efficient as outdoor temperatures drop. For Illinois winters with regular sub-zero days, geothermal has a meaningful efficiency advantage. Whether that advantage justifies the higher upfront installation cost is a site-specific calculation.
Can I combine geothermal with an existing solar system?
Yes, with a caveat: if your existing solar system was sized for your pre-geothermal electrical load, it will likely be undersized after geothermal is added. You'd either need to expand the solar system or accept that you'll purchase some electricity from the grid. SPM can evaluate your existing solar system's capacity and what expansion would be needed to cover a geothermal system.
What does geothermal installation involve?
Vertical loop installation involves drilling wells (typically 150–400 feet deep, depending on your home's heating load and soil conditions), installing loop piping, connecting to the indoor heat pump unit, and integrating with your home's existing ductwork or hydronic distribution system. It's a significant installation project that requires a geothermal contractor with drilling capacity. SPM handles the full installation.
Interested in geothermal and solar together? Learn about SPM's geothermal service or schedule a whole-home energy assessment.