Phone (+34) 96 583 7808 Mobile (+34) 615 452 586 Email info@solarpowersystems.es Address Ctra. Moraira-Calpe KM1 03720 Calpe

Solar Water Heating

Solar Water Heating 

Solar water heating is the conversion of sunlight into heat for water heating using a solar thermal collector. A variety of configurations are available at varying cost to provide solutions in different climates and latitudes. Solar Water Heatin is widely used for residential and some industrial applications.

solar water heating roof compact system solar power systems

The parts of a solar water heating system

Solar Water Heating systems are quite easy systems. The can consist out of the following parts:


This is the technical name for the big black panel that sits on your roof. Smaller homes (or ones in hotter climates) can get away with much smaller panels than larger homes (or ones in colder climates); typically collectors vary in size from about 2–15 square meters (~20–160 square feet). Not surprisingly, collectors work most efficiently on roofs that have a direct, unblocked view of the Sun (with few trees or buildings in the way). Broadly speaking, there are two types of collectors known as flat-plate and evacuated tube.

Flat-plate collectors

Flat plates are the simplest collectors: at their most basic, they’re little more than water pipes running through shallow metal boxes coated with thick black glass. The glass collects and traps the heat (like a greenhouse), which the water running through the pipes picks up and transfers to your hot water tank.

Evacuated tubes

These are a bit more sophisticated. They look like thicker versions of fluorescent strip lights, but work more like vacuum flasks. Completely empty (hence the name “evacuated”), they collect and trap the heat from sunlight. This flows to a collecting device (sometimes called a manifold) at the top (or at one end) through which water or another fluid flows, carrying the heat to the hot water tank. Unlike flat-plate collectors, they don’t let as much heat escape back out again, so they’re more efficient. However, since they’re a bit more hi-tech and sophisticated, they are sometimes more expensive.

Hot water tank

There’s no point in collecting heat from your roof if you have nowhere to store it. With luck, your home already has a hot-water tank (unless you have a so-called gas “combi” boiler that makes instant hot water) that can be used to store heat from your collector; it’s a kind of “hot water” battery that you heat up at conveniently economic times (usually at night) ready for use during the day. If you don’t have a hot-water tank, you’ll need to have one fitted. The more people in your household, the bigger the tank you’ll need. A typical tank for a family home might be about 100–200 liters (30–60 gallons).

Heat exchanger

Typically, solar panels work by transferring heat from the collector to the tank through a separate circuit and a heat exchanger. Heat collected by the panel heats up water (or oil or another fluid) that flows through a circuit of pipes into a coppercoil inside your hot-water tank. The heat is then passed into the hot water tank, and the cooled water (or fluid) returns to the collector to pick up more heat. The water in the collector never actually drains into your tank: at no point does water that’s been on your roof exit through a faucet!


Water doesn’t flow between the collector and the tank all by itself: you need a small electric pump to make it circulate. If you’re using ordinary electricity to make the water flow, the energy consumed by the pump will offset some of the advantage of using solar-thermal power, reduce the gains you’re making, and lengthen the payback time. Cleverly, some solar-thermal systems use solar-electric(photovoltaic) pumps instead, which means they are entirely running on renewable energy. A good thing about a design like this is that the solar pump is most active on really sunny days (when most hot water is being produced) and less active on cold, dull days (when, perhaps, you don’t want your solar panel to be working at all).

Control system

If it’s the middle of winter and your roof is freezing cold, the last you thing you want is to transfer freezing cold water into your hot water tank! So there is also generally a control system attached to a solar-thermal panel with a valve that can switch off the water circuit in cold weather. A typical control system may incorporate some or all of the following: a pump, flowmeter, pressure gauge, thermometer (so you can see how hot the water is), and thermostat (to switch off the pump if the water gets too hot).

How does Solar Water Heating work?

Here’s a simple summary of how rooftop solar hot-water panels work:


  1. Sun heats water flowing in a circuit through the collector (the panel on your roof).
  2. The water leaving the collector is hotter than the water entering it and carries its heat toward your hot water tank.
  3. The water doesn’t actually enter your tank and fill it up. Instead, it flows into a pipe on one side of the tank and out of another pipe on the other side, passing through a coil of copper pipes (the heat exchanger) inside the tank and giving up its heat on the way through.
  4. You can run off hot water from the tank at any time without affecting the panel’s operation. Since the panel won’t make heat all the time, your tank will need another source of heating as well—usually either a gas boiler or an electric immersion heater.
  5. The cold water from the heat exchanger returns to the panel to pick up more heat.
  6. An electric pump (powered by your ordinary electricity supply or by a solar-electric (photovoltaic) cell on the roof keeps the water moving through the circuit between the collector and the water tank.
solar water heating solar power systems scheme

How good is solar water heating?

“… One of the most effective and efficient steps the government can take is to encourage the use of solar hot-water systems—a well-developed and relatively low-tech method for using the sun’s energy.”

Larry Hunter, The New York Times (Op Ed)

In pure efficiency terms, solar-thermal panels are over three times as efficient (50 percent or so) at harvesting energy as solar-electric (photovoltaic) panels (typically around 15 percent), but that doesn’t mean they’re three times better: it all depends what you want from solar energy. If you live in the kind of family home where people are taking baths and showers all the time, especially in summer, solar thermal makes perfect sense. A decent system should be able to produce around half to two thirds of a home’s total, annual hot water needs (all your hot water in the height of summer and very much less in winter). The obvious drawback of solar thermal is that it produces nothing but hot water—and you can only do so much with that; unlike photovoltaics, solar-thermal panels can’t help you heat your home or produce truly versatile, high-quality energy in the form of electricity. The typical payback time for solar thermal (when your original capital investment has paid for itself in fuel savings) is about a decade, with a range of 5–15 years (depending on the cost of the fuel you’re saving, how much sun your home gets, and how much hot water you use).

Would you like more information?

Choosing the right system for your requirements can be hard and time-consuming. At Solar Power Systems we would be happy to help you with more information and choosing the right system for your needs. Do not hesitate to contact us or visit our webshop!

Phone (+34) 96 583 7808

Mobile (+34) 615 452 586

Email info@solarpowersystems.es

ShowroomCtra. Calpe-Moraira km1, 03710 Calpe