11.1 Renewable Energy Basics: Solar PV, Solar Thermal, Geothermal, and Wind
Key Takeaways
- Solar photovoltaic (PV) modules produce DC electricity that an inverter converts to AC; residential arrays are sized in kW-DC and typically produce roughly 1,000 to 1,700 kWh per installed kW-DC per year depending on climate, orientation, and shading.
- Peak PV output requires an unshaded roof plane facing near true south at a tilt close to the site latitude; shading is the single most damaging site defect because it can drag down output far out of proportion to the shaded area.
- Solar thermal collectors heat water directly rather than making electricity and typically supply 50% to 80% of annual domestic hot water, using drainback or glycol loops for freeze protection.
- A ground-source (geothermal) heat pump is an efficiency measure, not a generator: it moves heat between the house and stable ground temperatures at a COP of roughly 3 to 5, which is why it belongs in the mechanical scope rather than the generation scope.
- Small wind is rarely viable on residential lots because a turbine needs sustained wind and a tower reaching roughly 30 feet above anything within about 500 feet, which zoning and lot size usually prevent.
11.1 Renewable Energy Basics: Solar PV, Solar Thermal, Geothermal, and Wind
Quick Answer: Renewable systems belong at the end of the home performance sequence, not the beginning, because every kilowatt-hour of load removed by air sealing, insulation, and right-sized equipment is a kilowatt-hour the array never has to generate. Solar photovoltaic (PV) modules convert sunlight to DC electricity that an inverter converts to household AC; residential arrays are sized in kW-DC and typically yield roughly 1,000 to 1,700 kWh per installed kW-DC per year across U.S. climates. Solar thermal collectors heat water instead of making electricity and commonly cover 50% to 80% of annual domestic hot water. A ground-source (geothermal) heat pump is not a generator at all — it is a very efficient heat mover operating at a COP of roughly 3 to 5 against stable ground temperatures. Small wind is technically real but rarely site-viable in residential settings.
Where Generation Sits in the Sequence
Section 10.2 established the loading order that governs a whole-house scope of work. Renewables occupy the final position in that sequence for a reason worth stating precisely, because exam items test it directly.
An array is sized to the load it must serve. Reduce the load first and every downstream number shrinks:
+---------------------------------------------------------------------------+
| WHY GENERATION COMES LAST: THE SIZING CASCADE |
+---------------------------------------------------------------------------+
| |
| BEFORE ENVELOPE WORK AFTER ENVELOPE WORK |
| ------------------------ ----------------------- |
| Heating + cooling load: HIGH Load cut 30% to 50% |
| | | |
| v v |
| Heat pump sized LARGE Heat pump sized SMALL |
| | | |
| v v |
| Annual kWh consumption HIGH Annual kWh consumption LOW |
| | | |
| v v |
| PV array sized LARGE ($$$$) PV array sized SMALL ($$) |
| |
| Same comfort. Same net energy outcome. Much lower capital cost. |
+---------------------------------------------------------------------------+
The building science point is not that renewables are optional. It is that generation capacity is the most expensive way to buy a kilowatt-hour, so it should be the last purchase, sized against a load that has already been minimized.
Solar Photovoltaics (PV)
How the System Works
A PV module contains semiconductor cells that release electrons when struck by photons, producing direct current (DC). Because household circuits and the utility grid run on alternating current (AC), every system needs an inverter:
- String inverter: One central inverter serves a series string of modules. Lowest cost, but the string's output tends to follow its weakest module, so partial shading hurts badly.
- Microinverters: One small inverter per module, converting to AC at the roof. Each module produces independently, which limits the damage from localized shading and simplifies per-module monitoring.
- DC power optimizers: Per-module electronics paired with a central inverter — a middle path that recovers much of the shading tolerance of microinverters.
Sizing and Production
Arrays are rated in kilowatts DC (kW-DC) at standard test conditions. A typical residential system falls in the 4 kW to 12 kW range. Annual production depends on location, orientation, tilt, shading, and system losses, and runs roughly 1,000 to 1,700 kWh per kW-DC per year across U.S. climates — the low end in cloudy northern coastal regions, the high end in the desert Southwest. NREL's PVWatts calculator is the standard free tool for producing a site-specific estimate.
Worked example. A home uses 9,600 kWh per year after efficiency work. In a location that yields about 1,300 kWh per kW-DC annually:
Had the same home been left at its pre-retrofit 14,000 kWh per year, the array would need to be about 10.8 kW-DC — roughly 46% more equipment, roof area, and capital for exactly the same net result.
Site Assessment Factors
| Factor | What to Look For | Why It Matters |
|---|---|---|
| Azimuth (orientation) | True south is optimal; southeast through southwest is good | Off-south orientation reduces annual yield; east or west facing planes lose a meaningful share |
| Tilt | Roughly equal to site latitude, commonly matching roof pitch | Maximizes annual incident radiation; low-slope roofs need ballasted tilt racking |
| Shading | Trees, chimneys, plumbing vents, neighboring structures, and future tree growth | The most damaging single defect; on a string system one shaded module can suppress the whole string |
| Roof age and structure | Remaining shingle life and framing capacity | Modules last 25+ years; never place an array over a roof that needs replacement in five |
| Electrical service | Panel bus rating and available breaker space | Interconnection has ampacity limits; an undersized panel becomes an unbudgeted upgrade |
Grid Interconnection
Most residential PV is grid-tied. The compensation rule matters enormously to the economics and varies by state and utility:
- Net metering credits exported kWh at or near the full retail rate, effectively running the meter backward.
- Net billing / export rate structures credit exports at a lower, often time-varying wholesale-like rate. Several states have moved in this direction, which shifts the economics toward self-consumption and toward pairing PV with storage.
A standard grid-tied inverter is required to stop exporting during a grid outage so it cannot energize lines that utility crews believe are dead. That is why a plain grid-tied array does not keep the lights on in a blackout unless it includes islanding-capable equipment and storage.
Battery Storage
Batteries store energy; they do not create it. They serve two purposes: shifting self-consumption from midday production into evening use, and providing backup power during outages. They add cost and round-trip losses, so they should be recommended for a stated purpose rather than reflexively.
Solar Thermal (Solar Water Heating)
Solar thermal is frequently confused with PV on written exams. The distinction is simple: PV makes electricity; solar thermal makes hot water.
- Flat-plate collectors: A glazed insulated box with an absorber plate and fluid tubing. Durable, lower cost, the common residential choice in moderate climates.
- Evacuated tube collectors: Glass tubes under vacuum, which cuts convective loss and performs better in cold and diffuse-light conditions at higher cost.
Freeze protection defines the system type in any climate that freezes:
- Drainback systems let collector fluid drain by gravity into an indoor reservoir whenever the pump stops, so there is nothing outdoors to freeze. They require continuous downward pitch in all collector piping.
- Closed-loop glycol (indirect) systems circulate a propylene glycol/water mix through a heat exchanger in the storage tank. The glycol needs periodic testing and eventual replacement.
- Direct (open-loop) systems circulate potable water through the collector and are suitable only in non-freezing climates.
A well-designed residential system typically supplies 50% to 80% of annual domestic hot water, with a conventional water heater serving as the backup for cloudy stretches and winter. Note the interaction with Section 10.1: as heat pump water heaters have become inexpensive and reach UEF ratings above 3.0, many programs now find an HPWH delivers comparable savings at lower installed cost and lower maintenance than solar thermal.
Ground-Source (Geothermal) Heat Pumps
A ground-source heat pump (GSHP), often marketed as "geothermal," exchanges heat with the earth instead of with outdoor air. Below roughly six feet, soil temperature stays near the local annual average year-round — far warmer than winter air and cooler than summer air — so the heat pump works against a much gentler temperature difference than an air-source unit.
- Horizontal loops are trenched over a large area and cost less where land allows.
- Vertical bores drill several hundred feet and suit small lots at higher cost.
- Open-loop (pump-and-dump) systems use well water directly and require an adequate, legally permitted water source and discharge.
Typical heating COP runs about 3 to 5, meaning three to five units of heat delivered per unit of electricity consumed. The classification point BSP candidates must get right: a GSHP generates no energy. It moves heat with high efficiency. It belongs in the mechanical systems scope alongside furnaces and air-source heat pumps, not in the generation scope alongside PV.
Small Wind and Other Sources
- Small wind turbines require sustained average wind and a tower tall enough to reach clean air — a common rule of thumb is roughly 30 feet above any obstruction within about 500 feet. Suburban lots rarely satisfy the wind resource, the tower height, or local zoning simultaneously, so wind is usually a rural-acreage option.
- Biomass and wood heating is renewable in the fuel sense but reintroduces every combustion safety and particulate concern covered in Chapter 8. If a home has a wood or pellet appliance, it belongs in the CAZ assessment.
- Micro-hydro requires a year-round stream with usable head and flow and applies to a vanishingly small number of homes.
BPI Exam Tips & Common Traps
[!CAUTION] PV is not an efficiency measure. Solar panels do not reduce a home's energy use; they offset it with on-site generation. An item that asks for the most cost-effective way to cut a home's heating load is asking about the envelope, not about an array.
[!IMPORTANT] Geothermal is a heat pump, not a generator. If a question lists "renewable generation options," a ground-source heat pump is the distractor. It moves heat at a COP of 3 to 5; it produces no kWh.
[!TIP] Solar thermal vs. solar PV. Thermal = hot water, rated by fraction of DHW load served. PV = electricity, rated in kW-DC and annual kWh. A question mentioning drainback, glycol, or evacuated tubes is a solar thermal question.
[!TIP] "Reduce before you produce" in one sentence. Sizing generation against an unimproved load buys a larger, more expensive system to deliver the same result — which is why the BSP sequence puts health and safety first, the envelope next, mechanical right-sizing after that, baseloads after that, and generation last.
A contractor proposes installing a ground-source heat pump and describes it to the homeowner as "on-site renewable generation that will produce your own energy." From a building science standpoint, what is wrong with that description?
A home consumed 14,000 kWh per year before retrofit. Comprehensive air sealing, insulation, and a right-sized heat pump reduce annual consumption to 9,600 kWh. At a regional yield of about 1,300 kWh per kW-DC per year, how does the required solar PV array size change, and what principle does this illustrate?
An assessor is evaluating a home in a cold climate for a solar water heating system. Which system description correctly matches a freeze-protection strategy to its operating principle?