This page is being restored. Return to the homepage.

Frequently Asked Questions

Can a solar hot water system provide all of a household’s hot water? A properly sized solar hot water system provides 50–80 percent of a household’s annual hot water energy, not 100 percent. The solar fraction is intentionally designed below 100 percent because sizing for 100 percent solar supply would require a very large and expensive system that overproduces in summer while still falling short in winter. All systems include backup heating to fill the gap.

What happens to a solar hot water system on cloudy days? On cloudy days, solar collectors produce reduced output — flat-plate collectors generate 10–30 percent of their clear-day output on overcast days; evacuated tube collectors perform somewhat better. The backup heater (electric or gas) automatically activates when the stored water temperature falls below the thermostat setpoint, ensuring continuous hot water availability regardless of weather.

How do you prevent a solar water heater from overheating in summer? Summer overheating (stagnation) is managed differently by system type: drainback systems automatically drain the collectors when the pump stops, eliminating stagnation risk. Glycol systems rely on high-temperature glycol formulations that withstand stagnation temperatures of 350°F+ but require glycol condition monitoring. Some systems use electronic controls that activate a dump valve or circulate water to release excess heat.

How do you know if a solar hot water system is working correctly? A differential temperature controller should show the collector temperature significantly higher than the tank temperature on sunny days (10–15°F differential triggers the pump). A system working correctly will maintain tank temperatures of 120–140°F on sunny days without backup heater contribution. Consistently low tank temperatures on clear sunny days indicate a collector, pump, or plumbing problem.

Further Reading from Authoritative Sources