Glossary of Solar and Building-Science Terms

These are the words the rest of the book is using, in the sense this book is using them. A manufacturer's brochure may shade a definition. When a product sheet and this list disagree, the product sheet is the fact for that product and this list is the fact for these guides.

Difficulty

Level

Beginner

Time

Look up the word in front of you

Skills

None

Cost estimate

Free

If a project needs a tool, the guide that uses the word will say so. This page is only the definitions.

Materials

Item	Qty	Notes
The guide you were already reading	1	Come back to the word in context. A definition makes the most sense next to the page that used it.

The short list

ACH. Air changes per hour. How many times the room's volume of air is replaced in an hour. A fan rating, a leak, and a ventilation target are three different uses of the same unit.

Absorptance. The fraction of sunlight a surface turns into heat. Flat black is high. A selective coating is high in absorptance and low in emittance.

Azimuth. The compass direction a surface faces. In the usual photovoltaic-tool convention for the northern hemisphere, 180 degrees is true south. Confirm the tool's zero before you type. Magnetic south is not true south.

BTU. British thermal unit. A household measure of heat. A million BTU is the comparison unit on the fuel page. About 3,412 BTU make one kilowatt-hour.

Bypass diode. A diode built into a photovoltaic module to skip a shaded substring so the rest of a series string can keep a current. It limits damage. It does not make shade free. See the shade guide.

Charge controller. The device between a panel and a battery that keeps the battery from being boiled or overcharged. A panel clamped to battery posts is not one.

Closed loop. A solar hot-water circuit filled with antifreeze, usually propylene glycol, separated from potable water by a heat exchanger. It is the option that survives freezes. The choice guide sets the conditions.

COP. Coefficient of performance. Heat delivered divided by electricity used. A heat pump can be above 1 because it moves heat. The number falls in cold weather.

Derate. A factor below 1 that stands for heat, dust, wiring, and inverter losses. It is a planning haircut, not a measured test of your roof.

Drainback. A hot-water layout that empties the collector back to a reservoir when the pump stops, so the collector is not left full in a freeze or, if you designed it that way, in stagnation. Every pipe has to slope, and the system has to finish emptying when the power fails.

Emissivity. How well a surface radiates infrared. A selective absorber wants this low. A thermal camera lies on low-emissivity surfaces because they reflect. Sometimes written emittance when it is a measured number on a coating.

GHI. Global horizontal irradiance. Sun on a flat surface, beam plus sky. It is not yet the sun on your tilted roof. See the radiation guide.

Head. For a stream, the vertical drop you can put a pipe on. For a pump, the vertical lift plus the friction of the pipe. A waterfall you cannot reach is neither.

Heating degree day. A count of how far and how long the outdoor air sits below a base temperature. Base 65 F is a convention, not a law of your house. The degree-day guide is the longer version.

Inverter. The box that makes alternating current from direct current. A string inverter does it for a series group. A microinverter does it at a module. See the layout comparison.

Low-e. A family of low-emissivity coatings on glass. The words are not a U-factor and not a solar heat gain coefficient. Ask for those numbers. South heating glass often wants the high-gain version, not the low-gain version sold for cooling.

Net metering. A tariff pattern in which exports offset consumption, sometimes at the retail rate and sometimes not. As of 2026 the rate is on your utility's schedule, not in this glossary. The economics guide is the reading list.

Peak sun hour. One kilowatt-hour of sun per square meter. A day with 4.5 peak sun hours received the energy of 4.5 hours at full sun, spread across the real day. It is not the length of the daylight.

R-value. Resistance to heat flow. Higher holds heat better. The first inch of a bare wall matters more than the last inch on an already thick one. The insulation guide works through that.

Rapid shutdown. A code feature that lets firefighters de-energize a roof array. It is a product and inspection question. It is not optional because the array is small, where the code requires it.

SHGC. Solar heat gain coefficient. The fraction of solar heat admitted through glazing. High helps a winter heater. Low helps a window you are trying not to bake.

Stagnation. What a glazed collector does when the sun is on it and the fluid or the air is not moving. Temperatures get high enough to punish the wrong plastic. PEX does not belong in that collector.

String. Modules wired in series so their voltages add and they share one current. Shade on one module can limit the group. It is also the word for that whole series circuit.

Thermosiphon. Flow driven by hot fluid rising and cooler fluid sinking, with no pump. A water heater that uses it wants the collector below the tank. An air heater that uses it wants a path that can rise. Neither forgives a high spot that traps a bubble or a backwards slope.

TMY. Typical meteorological year. A constructed "ordinary" year used for planning. Not a forecast, and not a promise that next year will match. See the radiation guide.

U-factor. Heat flow through an assembly per degree of temperature difference. Lower loses less. For the same layer, U and R are reciprocals, and a window's U-factor may be the whole unit or only the center of the glass. Ask which.

Voc. Open-circuit voltage of a photovoltaic source, the voltage when almost no current is flowing. It rises in the cold. A string that was fine on a hot afternoon can be over a limit at dawn in January. Neither this glossary nor the wiring guide picks the string length. An electrician does, using the cold number.

1. Use the guide's meaning

   If a definition here and a sentence in a guide ever diverge, the guide you are building from is the one to follow, and this page should be read as the short version.

2. Ask a product sheet for the number

   Low-e, selective, and solar are names. U-factor, SHGC, absorptance, and emittance are numbers.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

• https://www.builditsolar.com/References/Calculators/
• https://www.builditsolar.com/References/Source/TMYWeather/TMYWeather.htm
• https://www.builditsolar.com/References/Source/RadOnColProgram/RadOnCollector.htm
• https://www.builditsolar.com/References/SketchUp/SketchUpEx.htm
• https://www.builditsolar.com/References/Suppliers/SolarHeating.htm
• https://www.builditsolar.com/References/HowMuch/HowMuch.htm
• https://www.builditsolar.com/References/BestRefsRS.htm