What Happens to Solar Hot Water in Cloudy Weather or Winter?
Quick Answer: Your solar hot water system will still collect heat whenever there is useful sunlight, even in winter or on cold, cloudy days. However, there will be times when there isn't enough solar energy to provide all the hot water you need.
Solar hot water systems work remarkably well in southern Spain, but what happens when the sun isn't shining?
Does solar hot water work in winter?
Yes. Solar thermal collectors don't need hot weather to work — they need sunlight.
Even during winter, the collector can heat the water whenever there is any sunshine. The amount of heat produced will naturally be lower than during the long, sunny days of summer, but the system continues to contribute useful heat.
In Alicante and Murcia, winter sunshine can still provide a significant amount of the hot water needed.
What happens on cloudy days or there just isn't enough sunshine?
Cloud reduces the amount of solar energy reaching the collector, so cold water will not heat as quickly or to the same temperature as it would on a sunny day and will eventually turn cold.
However, cloudy does not necessarily mean no solar gain. The collector can still receive some sunlight through gaps in the clouds and produce useful heat.
The real problem comes when cloudy or poor weather continues for several days and the stored water gradually turns cold as hot water is used. This is why all our solar hot water installations have a backup method of heating the water.
The backup heater only needs to make up the difference between the heat provided by the sun and the temperature needed to give the amount of hot water you require.
The immersion heater we fit is a standard electric heating element that you will find in any typical domestic hot water tank.
Will I run out of hot water?
Normally, no - provided the backup heating system is correctly set and switched on and you manage your water usage wisely.
After several days of poor weather, the solar system may have contributed very little heat, but the backup heater ensures you still have hot water.
My real life take
This is real life experience with my own system: I normally leave my electric heater disconnected at the mains. I sense when the solar water temperature is not hot enough, so what do I do? I go and check the weather forecast! If it says sunshine tomorrow, I do nothing. If it says cloudy, I plug the heater in. As soon as sunshine returns I disconnect it again. That way I know I am not wasting money heating the water unnecessarily.
How often will I need the backup heater?
This depends on the weather, the size of the system and the amount of hot water your household uses.
During the summer, there will usually be no need for backup heating. During winter, or after several days of poor weather, the backup may be needed more often.
In southern Spain, long periods of bad weather are relatively uncommon, so the system provides a large proportion of your hot water throughout the year.
My real life take
Using my electric disconnect routine gives me a good idea of how many days I use the standby heater in a year. I don't have a scientific result but I would say between 15 and 20 days in total.
Is solar hot water still worthwhile in winter?
Most definitely! A solar hot water system is designed to provide useful heat throughout the year, not just during summer. Yes there will be cloudy days, but even in December, Alicante receives on average, enough solar energy to heat your water.
The bottom line
Solar hot water doesn't need sunshine every day to be worthwhile. On sunny days, the system provides most or all of your hot water. On cloudy days it may provide less, and after prolonged poor weather the backup heater can make up the difference.
Once the sun comes back, the solar system goes back to doing what it does best — heating your water for free.
What to do next
Still unsure about Solar Hot Water, but ready to find out more about solar hot water for your home? Contact us today to arrange a free, NO OBLIGATION site visit. We can look at the available space and explain what your options are.