Ethan BrooksBy Ethan Brooks

Updated November 2026

Thermal emittance vs solar reflectance comes down to two different jobs. Solar reflectance measures how much sunlight a surface bounces back, mostly in the visible and near-infrared range. Thermal emittance measures how efficiently a surface gives off heat as infrared radiation. For a solar water heater, you usually want high solar absorptance and low thermal emittance, which means the surface should be a strong absorber of sunlight and a poor radiator of heat.

Why the two properties pull in opposite directions

A surface that reflects sunlight is one that is not absorbing that energy. Dark, matte coatings absorb most of the solar spectrum, which is why they reflect very little. But those same dark coatings often have high thermal emittance, so they lose heat quickly to the sky once they warm up. A collector absorber plate that is excellent at catching sunlight can still lose a lot of that energy back out as infrared if its emittance is high.

Shopping for one? See our guide: Thermal vs Photovoltaic Solar Power (2026): Which Is Better for Your Home?.

This is why selective absorber coatings were developed. They are engineered to have high absorptance in the solar band and low emittance in the infrared band. Manufacturers typically specify both values, and the gap between them is what determines how well the plate performs at the temperatures a water heater actually reaches.

Factors that change the numbers

  • Operating temperature. Emittance losses rise steeply as absorber temperature climbs. A low-emittance coating matters most when the collector runs hot, such as in a sealed glazed box during summer.
  • Surface condition. Dust, water spotting, and oxidation raise effective emittance and lower absorptance. Owners often report that a cleaned collector recovers noticeable output.
  • Glazing. A cover sheet changes the picture. Low-iron glass with an anti-reflective treatment passes more sunlight to the absorber, while the glass itself blocks much of the outgoing infrared.
  • Coating durability. Selective coatings can degrade with heat cycling, moisture, or UV exposure. A coating that starts at good values but drifts over years is a real concern for DIY builds.

What to do about it

  1. Check the datasheet for both solar absorptance and thermal emittance. If only one number is listed, ask the supplier for the other.
  2. For flat plate collectors, look for a selective coating rather than plain black paint if your climate has cold nights or you want performance in shoulder seasons.
  3. Keep the absorber clean and avoid painting over selective surfaces with ordinary high-emittance paint.
  4. Use a glazing layer to cut convective and radiative losses, especially in cool or windy locations.
  5. Insulate the back and sides of the collector box, since conduction losses often outweigh radiative losses at lower temperatures.

What we’d use

For a DIY absorber upgrade, a matte black high-temperature spray paint is a common starting point for low-cost experiments, though it will not match a true selective surface. Owners who want better emittance often look at a selective black absorber sheet cut to size. View on Amazon

For glazing, tempered low-iron solar glass or polycarbonate panels sized to your collector frame are widely used in DIY builds. View on Amazon

FAQ

Is low thermal emittance always better for a solar collector?

For the absorber, usually yes, because it keeps captured heat from radiating away. But the coating must still absorb sunlight well, so the best choice is high absorptance paired with low emittance.

Does high solar reflectance mean a surface is cool?

Not necessarily. A surface can reflect a lot of sunlight and still get warm, and it can also be heated by absorbed infrared. Reflectance alone does not tell you the surface temperature.

Should I consult a professional before building a high-temperature collector?

Yes. Water heaters that reach high temperatures can cause scalding and pressure hazards. A licensed plumber or solar installer should review pressure relief, tempering valves, and electrical backup before you connect anything to your home supply.

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Ethan Brooks

About Ethan Brooks

Ethan Brooks is part of our editorial team and reviews and edits our solar water heaters guides, checking product details against manufacturer specifications and owner feedback.