Light as a form of energy needs converting to heat or electricity in order to be of use in running a home. Solar thermal panels convert light to heat (more specifically, hot water) and photovoltaic (PV) panels convert it to electricity.

Light is available to us every day, to a greater or lesser extent, and both thermal and PV panels produce energy every day, to a greater or lesser extent. So how do you choose between the two? First of all, we need to look at how both systems work.

The Quick Read

  • Solar PV panels (which generate electricity) need more roof space than solar thermal (which generate hot water) — a 4kWp PV system requires 32m² of unshaded roof space
  • The price of PV panels has fallen in recent years; now there is little to choose between PV and solar thermal in terms of capital cost. They provide a similar return, too
  • The roof pitch and orientation are particularly critical when it comes to citing solar PV
  • In retrofit projects, PV is typically easier to install than solar thermal; the latter requires a hot water cylinder

How Does Solar Thermal Work?

Solar thermal systems come in two forms: flat plate and evacuated tubes. Both produce hot water, yet evacuated tubes are more efficient than flat plate — and more expensive. Flat plate panels can be mounted on-roof (i.e. fixed on top of existing tiles) or in-roof (i.e. replacing roof tiles to maintain the overall roof shape), or be ground-mounted on a purpose-built frame; tubes can be similarly mounted, but cannot be mounted in-roof.

A typical solar thermal system will comprise of:

  • two flat panels (4m² in total size), or 20 to 30 tubes;
  • a fluid to deliver the heat;
  • pumps;
  • a large (200 litre or larger) hot water cylinder;
  • and a control system.

The cost will be in the order of £3,000-£5,000 (including VAT at 5%). The total price will depend on factors such as whether tubes or flat plates are specified, the mounting arrangement and the supplier.

A domestic system will typically produce 1,800kWh to 2,000kWh per year. The Renewable Heat Incentive (RHI) pays 19.2p/kWh – which is around £345 per year – increasing with RPI, and will also save around £100 in gas bills.

One factor to bear in mind is installation. This is relatively straightforward when starting from scratch with the heating and plumbing in a new build. However, solar thermal systems need a hot water cylinder, and they need to be separated from an existing space heating system if they are to benefit from the RHI (more on this later) — which complicates retrofit installations.

And Photovoltaic Panels?

Photovoltaic technology is far more complex than solar thermal, meaning that there is more variation in appearance, performance and price between panels. This also means that there is more room for research and development; as a result, the price of PV has plummeted in the past few years.

The Feed-in Tariff (FiT) is an important factor when it comes to PV. Under the FiT structure, 4kWp is the largest solar PV system that can typically be installed on a domestic property. If you opt for a 4kWp system you will need 32m² of unshaded roof space.

It will cost in the range of £6,000-£7,000 (including VAT at 5%) and will produce between 3,000kWh and 3,400kWh per year, depending on the quality of the system and where you are in the country. The Feed-in Tariff pays 13.88p per kWh (as of 1 January 2015), and so a typical system will return about £450 each year, increasing with RPI. It will likely save in the region of £160 on electricity bills. The export tariff is currently 4.2p/kWh, so the system could produce a further £130 from exported excess electricity.

There are a lot of claims made about how much Grid electricity consumption a PV system will save. The major factor is how the house is occupied. A normal work/school occupation pattern implies that there is no one at home to use the electricity when it is produced during daylight. Those working from home or retired can, obviously, use a lot more of the electricity produced by PV.

For budgeting purposes, a figure of 40% of the total production is the generally accepted norm. So around 1,280kWh will be used from a 4kWp system that produces 3,200kWh per year.

Why Orientation and Inclination are Critical

It’s important to consider that with solar PV, the kWp (i.e. 4kWp) is the rating of the system, not its output. The ‘p’ stands for peak and denotes the maximum that the system can produce in ideal conditions.

For instance, the system might produce 4kW at midday on 21 June, on a cloudless day and where the system is installed facing exactly true south at an inclination of 42° from horizontal. For the rest of the year it will produce something less than 4kW — mostly a lot less. With PV, the performance starts to drop off quite sharply with an orientation north of south-east or south-west.

The same is true for solar thermal, but to a lesser extent. Solar thermal systems, especially in evacuated tube form, are more tolerant of orientations other than south, and there are flat plate panels available specifically for flat roofs, too. Having said that, there must be some ‘south’ in the orientation. Anything north of due east or west is of no use.

What Can the Energy be Used for?

Solar thermal systems produce only hot water. The Government imposes further restrictions, in that RHI eligibility rules require that the hot water can only be used for sanitary purposes; that is, it has to come out of a tap or shower. It cannot be used for space heating or for heating swimming pools.

That presents an obvious problem with how the hot water cylinder distinguishes between sanitary and heating uses. There are one or two cylinders that can do this, such as the Chelmer Ecocat, but if you do not have one of these, this can mean significant changes to an existing boiler and plumbing system.

PV systems usually produce fairly small amounts of electricity and that is only in daylight hours. So, again, the uses are limited typically to low-load, continuously used items like fridges and freezers. PV is, however, increasingly being used to heat water using a control system such as Immersun or 4Eco, which diverts excess electricity to the immersion heater.

The problem is that the immersion heater needs at least 3kW to power it and the PV system will seldom produce that. There are ways round this issue through the use of transformers to enable 110v rather than 240v (try These systems cost anywhere between £50-£400 plus installation.

A good PV array with a good control system is said to meet the whole of a modern house’s heating and hot water needs through spring and summer. It’s quite a claim, but for an additional cost of £400+ it’s a better idea than a solar thermal system.

Is a Combined System (PVT) Worth Considering?

Installing both PV and solar thermal is difficult to justify in financial terms alone. A better alternative is PVT (photovoltaic thermal). As the name implies, this system produces both electricity and hot water from the same panel.

The capital cost of such systems is about the same as solar thermal and PV combined to achieve the same amount of energy, and starts at around £5,000. As yet, the technology only benefits from the FiTs scheme, not RHI, but there are adaptations and additions to the technology that make it an exciting opportunity for new build projects.

So Which Should You Choose?

The suitability of the roof is the big issue. Investing in a compass and looking at the roof at different times and in different light conditions will help you to establish the orientation and the likelihood of shadowing. Websites such as are a useful tool in calculating what the efficiency impacts are likely to be.

The fall in the price of PV in recent years means that now, in terms of upfront costs, there is little to choose between PV and solar thermal — the capital cost of both is similar, and both offer a return on investment of a little over 11%. Both have broadly the same maintenance issues and both are subject to similar failures (pumps on the solar thermal system and inverters on the PV system).

PV needs a lot more roof space, more careful orientation, and adds absolutely nothing (being generous) to the aesthetic value of the house. Thermal systems take up less space, orientation is less critical, but they are still not pretty to look at.

And yet, solar energy is constant and predictable. Solar systems also have a long life — 25 years plus. There is no good reason why a newly built house cannot design in some form of solar system. There is more possibility that an existing roof might not be suitable, but even that is rare; adapting an existing plumbing system for a solar thermal system is more likely to be an issue.

In the Know

  • Planning consent for a system mounted on an existing roof is not usually needed, but it is always worth checking. Usually a phone call (seldom a quick call) to the local planning authority is all that is needed. If the building is listed or in a Conservation Area then consent is almost certainly needed.
  • Solar tiles are designed to be used in place of ordinary roof tiles. A tile system will typically cost about twice as much as an equivalent panel system to produce the same amount of heat energy. As a consequence, tile systems are normally only considered where panels are not an option for aesthetic or planning reasons (i.e. Conservation Areas and listed buildings).

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