Solar & Heat Pump Glossary
Plain-English definitions of the home-energy terms you'll meet when shopping for solar or a heat pump — from AFUE to 25D. Every entry links to a deeper guide where one exists.
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- 25C credit
- The Energy Efficient Home Improvement Credit — 30% of cost up to $2,000/year for a qualifying air-source heat pump (plus up to $1,200 for other measures). Non-refundable, resets annually, no carryforward. See the credit guide.
- 25D credit
- The Residential Clean Energy Credit — 30% of cost with no cap for solar, batteries and geothermal heat pumps, carrying forward to future years, through 2032. See the solar tax credit guide.
A
- AFUE
- Annual Fuel Utilization Efficiency — the seasonal efficiency rating for furnaces and boilers, expressed as a percentage of fuel converted to useful heat. A 95% AFUE furnace wastes 5% of its fuel. Unlike a heat pump, a combustion system can never exceed 100% AFUE.
- Air-source heat pump (ASHP)
- A heat pump that exchanges heat with outdoor air to heat and cool a home. The most common residential type, it qualifies for the 25C federal tax credit (30% up to $2,000). See our ASHP vs GSHP guide.
- Air-to-water heat pump
- A heat pump that heats water rather than air, supplying radiators, underfloor heating or a hot-water cylinder. Useful for replacing a boiler in a home with hydronic heating; see our older-homes guide.
- Azimuth
- The compass direction a solar array faces. In the US, due south (180°) is ideal, though southeast to southwest orientations lose only modest production. Covered in our roof suitability guide.
B
- Balance point
- In a dual-fuel system, the outdoor temperature below which the system switches from the heat pump to the backup furnace — typically 25–40°F, set by your local energy prices. See our dual-fuel guide.
- BTU
- British Thermal Unit — the standard unit of heating and cooling capacity. Heat pump and AC capacity is measured in BTUs per hour, or in tons (12,000 BTU/h = 1 ton). See the sizing guide.
C
- CEE tier
- The Consortium for Energy Efficiency's efficiency tiers, used to define which heat pumps qualify for the 25C federal tax credit. A unit must meet the CEE highest tier for its region to earn the 30% credit.
- Cold-climate heat pump
- A heat pump engineered to maintain capacity and useful efficiency well below freezing, often listed on the NEEP Cold Climate ASHP list. Essential for northern climates; see our cold-climate guide.
- COP
- Coefficient of Performance — the ratio of heat delivered to electricity consumed at a given moment. A COP of 3 means 300% efficiency. Air-source units run COP 3–4 in mild weather; geothermal holds COP 4–5. See SEER2 vs HSPF2.
D
- Degradation
- The gradual, predictable decline in a solar panel's output, typically about 0.5% per year. After 25 years a panel still produces ~85–88% of its original output. Covered in our lifespan guide.
- DSIRE
- The Database of State Incentives for Renewables & Efficiency (dsireusa.org) — the authoritative, ZIP-searchable catalog of every US state and utility energy incentive. The best place to find your exact rebates.
- Dual-fuel system
- See Hybrid heat pump. A system pairing a heat pump with a backup furnace, switching at the balance point.
- Ductless mini-split
- A heat pump with indoor wall- or ceiling-mounted heads connected to an outdoor unit by a small refrigerant line, requiring no ductwork. Ideal for retrofits, additions and zoning; see ducted vs mini-split.
E
- EER2 / EER
- Energy Efficiency Ratio (updated to EER2 in 2023) — cooling efficiency measured at a single hot temperature (95°F), important in very hot, dry climates. See SEER2 vs HSPF2.
G
- Geothermal heat pump
- Also called ground-source (GSHP) — exchanges heat with the stable temperature of the ground via buried loops. Highly efficient (COP 4–5) and eligible for the uncapped 30% 25D credit. See the geothermal calculator.
H
- HEEHRA / HEAR
- The High-Efficiency Electric Home Rebate / Home Electrification and Appliance Rebates program — IRA-funded, state-administered rebates up to $8,000 for heat pumps, for income-qualified households. See our rebates guide.
- HSPF2 / HSPF
- Heating Seasonal Performance Factor (updated to HSPF2 in 2023) — a heat pump's seasonal heating efficiency. Higher is better; aim for 8.1+ for high-efficiency. Detailed in SEER2 vs HSPF2.
- Hybrid heat pump
- A dual-fuel system combining an electric heat pump with a gas (or propane/oil) furnace, using the heat pump most of the year and the furnace on the coldest days. See our dual-fuel guide.
I
- Interconnection
- The utility's approval and metering process that allows a solar system to connect to the grid and export power. A required step in the permitting process.
- Inverter (HVAC)
- A variable-speed compressor that modulates output rather than cycling fully on and off, delivering steadier temperatures, higher efficiency and quieter operation. The defining feature of premium heat pumps.
- Inverter (solar)
- The device that converts the DC electricity from solar panels into the AC electricity a home uses. Types include string inverters and microinverters.
- IRA
- The Inflation Reduction Act of 2022 — the law that locked the 30% clean-energy tax credit through 2032 and funded heat pump rebates. See our IRA guide.
K
- kW / kWh
- A kilowatt (kW) is a unit of power (system size); a kilowatt-hour (kWh) is a unit of energy (what you're billed for). A 7 kW solar array producing for 1,400 hours yields about 9,800 kWh per year.
L
- Load calculation (Manual J)
- The industry-standard method for calculating a home's precise heating and cooling needs, accounting for insulation, windows, air leakage and size. Proper sizing depends on it; see the sizing guide.
N
- NEM 3.0
- California's revised net-metering policy, which sharply cut the value of exported solar power and increased the value of batteries. Explained in our NEM 3.0 guide.
- Net metering
- A billing arrangement that credits solar owners for electricity exported to the grid, effectively using the grid as a battery. Policies vary by state; see how net metering works.
P
- Payback period
- The time it takes for cumulative energy savings to equal the net cost of a system. Solar paybacks commonly run 6–12 years; estimate yours with the payback calculator.
R
- Reversing valve
- The component that lets a heat pump switch between heating and cooling by reversing the direction of refrigerant flow. Its switching can cause a normal soft 'clunk'; see our noise guide.
S
- SEER2 / SEER
- Seasonal Energy Efficiency Ratio (updated to SEER2 in 2023) — a heat pump or AC's seasonal cooling efficiency. Higher is better; aim for 16+ for high-efficiency. See SEER2 vs HSPF2.
- Short-cycling
- When a system rapidly turns on and off, usually because it's oversized. It wastes energy, worsens comfort and shortens equipment life — a key reason not to oversize a heat pump.
- SREC
- Solar Renewable Energy Certificate — a tradable credit earned per megawatt-hour of solar generated, sold in certain state markets for extra income. See our SREC guide.
T
- Ton (cooling)
- A unit of heat pump/AC capacity equal to 12,000 BTU/h. A typical home needs 2–5 tons depending on size and climate; see the size calculator.
V
- Variable-speed compressor
- See Inverter (HVAC). A compressor that modulates output for efficiency, comfort and quiet operation.