Seasonal cloud cover, rain, and overcast skies are a reality in many parts of the world. It's the question every buyer asks: "Will a solar water heater actually work on cloudy days?" The honest answer is: yes — with some nuance.

How ETC tubes perform without direct sunlight

Borosilicate vacuum tubes absorb diffuse UV radiation, not just direct sunlight. On a typical overcast day — with limited direct sunshine — you can expect:

Most suitable solar regions worldwide receive 4–6 peak sun hours per day annually. Even in locations with significant seasonal cloud cover, the system produces meaningful output on the majority of days.

The electrical backup element

All Kaldor systems include provision for an optional electrical backup heating element — similar to a standard geyser element installed inside the storage tank. It activates automatically when water temperature drops below a set threshold, ensuring you always have hot water regardless of weather conditions.

Most customers find the backup element activates on only 15–25 days per year even in regions with significant cloud cover.

Annual economics still strongly favour solar

Even accounting for seasonal overcast periods, a well-installed ETC system in a temperate or partly cloudy location saves approximately 65–80% of annual water heating energy costs. The economics remain compelling regardless of climate: payback in 3–6 years depending on local energy costs, followed by 15–20 years of effectively free hot water.

FPC vs ETC on cloudy days

ETC systems significantly outperform FPC in diffuse light conditions because the vacuum insulation works independently of the light source. FPC collectors lose more heat through the glass and absorber plate on cold cloudy days, reducing efficiency. For locations with more than 60 cloudy days per year, ETC is strongly recommended.

See how a Kaldor system performs in your specific climate.

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