Not knowing the trick, the homeowner installed a solar water heater and shivered in the winter.

Four years ago, when building a spacious two-story house, Mr. Tuan (Phuc Tho, Hanoi) decided to install a solar energy tank, after being advised by a sales acquaintance about many benefits: One-time investment for long-term use, no monthly electricity bill, no fear of electric shock or fire risks like using a conventional electric or gas water heater. His daughter opened a hair salon in the house, so she had to use hot water regularly. Therefore, he decided to spend more than 10 million to install a 160-liter solar energy tank. "My house has a wide frontage, the front and the surrounding areas are airy, so it receives a lot of sunlight," Mr. Tuan was very excited at that time.

Installed in the summer, his family was very happy because there was plenty of hot water every day. However, in the winter, on sunny days, the water was only lukewarm. On rainy and cloudy days, the water was still cold. His family was forced to install two more electric water heaters, one for household use, and one for washing hair of guests.

"Finally, the solar water heater is considered unnecessary because when it's cold and hot water is needed, the heater doesn't work. The heater only provides hot water during the hot summer sun, the family rarely needs to use it and the water in the stainless steel tank on the second floor is already hot," Mr. Tuan concluded.

Many families in the North, despite having installed solar panels, still have to install water heaters. However, according to Dr. Nguyen Manh Cuong, a lecturer at Ho Chi Minh City National University, this is a way to complicate the problem and is not economical.

"Instead,You just need to install an additional heating device (a resistor)"directly with a solar water heater," Dr. Cuong advised. "In case there is no sun, the solar machine cannot heat the water, the resistor will help it become an indirect water heater. Note, you only supply power to the resistor when the solar radiation is not enough to heat but you need hot water. If you use this device, in winter, you can still save 70% of electricity compared to using a normal electric water heater," Dr. Cuong added.

The solar energy expert also said that the installation of solar resistors is quite simple. This device is currently sold in many stores that supply solar water heaters as well as electrical stores, for a few hundred thousand dong (including a device to protect against electric leakage).

Mr. Toan's family lives in Thai Binh city.A group of 6 people invested more than 10 million a year ago in a solar water heater system, installed on the roof, and then brought water to many locations in need. In addition to bathing, his family also uses hot water for laundry, dishwashing, cooking... Because he has installed an auxiliary heating device, in the winter, his family still has hot water from the solar tank to use comfortably.

Mr. Toan compared to the time when he used an electric water heater, in the summer, his family saved about 40 kWh of electricity, about 80,000 VND and 30,000 VND on cooking gas each month. In the winter, they also saved 30 kWh of electricity per month.

Dr. Cuong commented,Because the operating principle of solar energy machines depends on the lighting time as well as the radiation of the sun.Should be installed in the South (no winter, little and short rainfall, high solar radiation coefficient) more effectively than in the North.In the South, users do not need to install auxiliary heating equipment.

Currently, the use of solar water heaters is suitable for most households, hotels, restaurants, resorts, hospitals, processing factories... On the market, there are many types of machines suitable for household scale with a lifespan of 12-15 years.

Solar water heaters can heat water in a short time, the average heating temperature is 55 - 65 degrees Celsius (even up to 100 degrees Celsius depending on weather conditions and usage). Thermos bottles can keep heat for several days, with an average heat loss of about 4 degrees Celsius/day.

To use solar water heater effectively,Dr. Cuong recommends that consumers should place the device in the roof area (or outdoors) with good sunlight and not be obscured. The direction of the solar panel should best face south with an angle of about 15 degrees is optimal.

The water supply for the machine must be stable because the machine only works when it is supplied with water regularly and continuously. The water must not be contaminated with alum or salt to ensure the machine's lifespan.

Because when using, all the cold water in the water pipe must be drained out to have hot water, so families need to arrange the pipe as short as possible to save water. The pipe should be a dedicated hot water pipe.

"And finally, you should choose a reputable supplier with good after-sales service," Dr. Cuong advised.