Solar water heating explained

Solar water heating is one of the longest-established forms of renewable energy for homes. It's a mature technology with a solid track record — but it's not the right fit for every household. Understanding what it does and doesn't do will help you decide whether it's worth exploring further.

What solar water heating is — and isn't

Solar water heating (also called solar thermal) uses panels or tubes on your roof to capture heat from sunlight and transfer it to your domestic hot water. It heats water — not your home's radiators, and not electricity for your sockets.

This is different from solar PV panels, which generate electricity. PV panels and solar thermal collectors can look similar on a roof, but they work in completely different ways and are used for different purposes. If you're interested in generating electricity at home, see our guide to solar panels.

How a solar thermal system works

The basic components are simple:

• Collectors — mounted on the roof, these absorb heat from sunlight. There are two main types: flat-plate collectors (a glazed insulated panel) and evacuated tube collectors (a row of glass tubes, each with a heat-absorbing rod inside). Evacuated tubes tend to be more efficient, particularly in cooler or cloudier climates.
• A pump and controller — a small pump circulates a heat-transfer fluid (usually a water-glycol mix) between the collectors and the cylinder. A controller starts the pump when the collectors are warmer than the cylinder.
• A hot water cylinder — the heat from the collectors warms the water inside the cylinder. The cylinder typically has two coils: one connected to the solar circuit and one connected to your boiler or immersion heater.
• A backup heater — when solar input isn't enough (at night, in winter, on grey days), the boiler or immersion heater tops up the water to the temperature you need. This backup is essential — a solar-only system would not reliably deliver hot water year-round in most climates.

In practice, the system runs quietly and automatically. On a good summer day it can meet most or all of your hot water needs from solar alone. In winter it typically pre-heats the water, so the backup heater does less work.

Is your home suitable?

Solar thermal works best when several conditions are met:

• Roof aspect. In the northern hemisphere, a south-facing (or within about 45 degrees of south) roof captures the most solar energy. East or west-facing roofs can still work but produce noticeably less heat. North-facing roofs are generally not suitable.
• Roof pitch. A pitch of roughly 20–50 degrees suits solar thermal well. Shallower or steeper roofs can work with adjusted collector angles, but efficiency falls at the extremes.
• Shading. Any significant shading from trees, chimneys or neighbouring buildings across the collector area will reduce output substantially. Assess shading at different times of day and year before committing.
• Available roof space. A typical family system needs around 2–4 square metres of collector. Your roof must be structurally sound enough to carry the additional weight.
• Hot water demand. Households that use a lot of hot water (larger families, daily baths, frequent showering) get more value from a solar thermal system than households with modest demand.
• Existing cylinder. Solar thermal requires a compatible hot water cylinder — usually a twin-coil cylinder with a dedicated solar coil. If you currently have a combi boiler with no cylinder, adding solar thermal is more involved, as a cylinder must be fitted. An installer can advise on what your existing setup needs.
• Climate. Solar thermal works in most climates, including cooler northern European ones. The output is lower than in sunnier climates, but still meaningful.

Pros and cons

Pros:

• Cuts the energy used to heat your domestic hot water, lowering bills and emissions.
• Mature, well-understood technology with good long-term reliability — collectors typically last 20–25 years or more.
• Works silently with minimal maintenance (an annual check of the fluid level and pressure is usually enough).
• Reduces dependence on your gas or electric backup for hot water, particularly in summer.

Cons:

• Significant upfront installation cost — prices vary considerably by country, system size and installer. Get multiple quotes and research current local incentives or grants.
• Heats water only, not space heating — your boiler or heat pump still handles radiators and underfloor heating.
• Output is weather-dependent and seasonal. Backup heating is always needed.
• Requires a hot water cylinder, which isn't always present in homes with combi boilers.
• Payback period varies widely depending on your hot water use, your existing fuel costs, and your local climate. It may be longer than other investments.

The solar PV + immersion diverter: a growing alternative

If you already have or are planning solar PV panels, a solar immersion diverter (sometimes called a solar PV diverter or power diverter) offers a low-cost way to heat your hot water with surplus solar electricity. When your PV panels are generating more electricity than you're using, the diverter redirects that surplus to your immersion heater rather than exporting it to the grid for very little return.

This approach is simpler to install than a full solar thermal system, uses your existing cylinder (if you have one) and has a much lower upfront cost. The trade-off is that it only works when your PV panels are generating surplus power — typically on summer days. For a household already committed to solar PV, it's often a very cost-effective hot water solution.

Getting it installed

Solar thermal installation requires a qualified installer — the work involves plumbing, electrical connections and working at height. Look for installers who are certified or accredited under the relevant scheme in your country (MCS in the UK, for example). Accreditation is often required to access government incentive payments.

• Get at least three quotes and check what is and isn't included (cylinder replacement, scaffolding, commissioning).
• Ask each installer to show their certification and provide references from recent jobs.
• Check what incentives or grants are available in your country or region — these change regularly, so verify with current official sources rather than relying on any fixed figures.
• Ask about the system's expected annual output and how the backup heater will be controlled.

Checklist before you decide

• Check your roof faces roughly south (or east/west with clear sky exposure) and isn't significantly shaded.
• Confirm you have, or can fit, a compatible hot water cylinder with a solar coil.
• Assess your household's daily hot water use — larger demand means better return.
• Research current incentives or grants available in your country or region from official sources.
• Get at least three quotes from certified/accredited installers.
• Ask each installer about expected annual output for your specific roof and usage.
• Consider whether a solar PV + diverter approach might suit you better if you already have or plan PV panels.
• Check that the backup heater will automatically maintain 60°C for Legionella prevention.