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Solar Thermal

Introduction to solar heat and hot water

Solar Panels

Solar Panels

Solar water heating (known as solar thermal) systems capture the free heat from the sun and use it to heat up water for use in the home. It’s a simple process:

  • panels on your roof absorb heat from the sun – they are known as the collector
  • the water in the panels heats up
  • this hot water is pumped through a coil in your cylinder
  • which transfers the heat to the water in the cylinder

Is solar thermal suitable for my home?

The ideal situation for solar panels is facing due south, although they are effective facing anywhere between south east and south west. As a rule of thumb you need between 1 and 2 m2 of collector (solar panels) per person living in the house.

Most panels are mounted on a roof, but they can also be mounted at ground level. It is important that they get direct sunlight. To get the best results they should be at an angle between 20 and 50 degrees from horizontal (most pitched roofs fall within this bracket).

If you have an electric shower it won’t use your solar hot water. Similarly cold-fill dishwashers and / or washing machines heat the water they use. In this situation, solar water heating is not suitable unless you use the bath for most of your washing and bathing, as you won't be able to use much of the solar hot water you generate.

Solar panels are compatible with most existing hot water systems. However, you will need a new cylinder with two coils. Ideally it should be big enough to hold two days worth of hot water. 

Solar hot water with combi boilers is more difficult, but still possible. If you have a combi boiler it is important to check with the manufacturer that it will accept pre-heated water.

If your present system is gravity fed, it will need a control (such as a valve and pump) for the hot water circuit so the panels can work effectively in winter when the boiler is running for central heating.

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How much hot water from solar thermal panels?

Solar Panels

Solar Panels

Solar thermal panels should provide most of your hot water from April to September, and make a worthwhile contribution in the months on either side of that period. Outside of that estimates vary depending on who you ask.

Solar panels will provide about 60 per cent of a household's hot water needs, if well-installed and properly used according to Energy Saving Trust field trials.

The reality will depend on a variety of factors:

  • How much hot water your household or business uses. The higher the usage, the more benefit you get from a solar thermal system. 
  • How much interest you take in how the system works and adapt to make the most of the free hot water (ie having showers in the evening rather than the morning). The sun isn’t as reliable as a timer clock.
  • The size of your cylinder. Many cylinders only hold enough water for a day’s supply of hot water, so a day or two of cloud and rain will mean you have to turn on the boiler or immersion heater.
  • How you programme your back up heating. If your control panel does not allow you to programme the hot water and central heating separately, you may not get the maximum benefit from the solar panels when the heating is turned on. By only boosting the hot water once the sun has gone down, you maximise opportunity for solar heating.
  • Adequate insulation of both cylinder and pipes carrying hot water.
  • Allowing hot water temperature to vary. If you do not need high temperatures all the time, you will have less need for back-up heating. You will also reduce heat loss. However, it is important to make sure your cylinder reaches more than 60 degrees centrigrade at least once a week to avoid risk of Legionella.

The EST identified a huge range of performance, with the best system producing 98 per cent of the household's hot water, and the worst just 9 per cent. The median across all systems was 39 per cent, so it is important to take action to maximise your solar gain.

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What type of solar thermal panel is best?

There are two types of solar thermal panel: flat plate panels and evacuated tubes.

Flat plate panels consist of an absorber plate in an insulated metal box. The top of the box is glass or plastic, to let the sun’s energy through, while the insulation minimises heat loss. Lots of thin tubes carry water through the absorber plate heating it up as it passes through.

Instead of a plate, evacuated tube collectors have glass tubes containing metal absorber tubes, through which water is pumped. Each tube is a vacuum (the air is ‘evacuated’ hence the name), which minimises heat loss.  

The Energy Saving Trust field trial found little difference in performance between the two. For many people the decision is a matter of aesthetics.

Research from Swiss-based Solartechnik Prufung Forschung shows that the best performing collectors are more than twice as efficient as the worst ones. It found that the most effective  flat plate collectors are made in Austria and Germany and the best evacuated tubes in Switzerland and Northern Ireland. Six of the 10 worst performing evacuated tube collectors are made in China.

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What does solar thermal cost?

The cost of installing solar a solar thermal system will depend on the type and quality of the panels, whether you need scaffolding, and how easy it is to integrate into your existing plumbing system. 

As a guide, average systems are likely to cost between £3,000 and £5,000 according to the Energy Saving Trust. A typical system, including a well-insulated twin coil cylinder, cost around £4,500 according to the Solar Trade Association.

Research commissioned by DECC and published in February 2013 found the following average costs per kW:

0-5kW system - £2,060; 5-10kW - £1,199; 10-20kW - £1,025 

The cost of scaffolding and the new cylinder are both significant, so a cost effective time to install would be when you need a new hot water cylinder, or when you are having repairs done on the roof (or at the same time as a solar PV installation).

Domestic Renewable Heat Incentive (RHI)

The domestic renewable heat incentive is a boiler replacement scheme, aiming to bridge the gap between the cost of a replacement oil or LPG boiler and the cost of a renewable heating system. Solar thermal panels are an eligible heating system. 

The rate is 19.2p per kWh index-linked and paid quarterly in arrears for seven years. 

To apply you will must have a green deal assessment (unless you are a self builder, in which case an EPC is required), and install loft and cavity wall insulation if it gets a green tick on the green deal advice report.

A typical 4 square metre installation with a 250 litre cylinder will get £2,347 over the 7 year life of the incentive according to figures from the Solar Trade Association (£335 a year). This calculation is based on the new MCS standard, so make sure your installer has adopted it. Click the link to read full details about the domestic RHI.

RHI for solar thermal can be claimed in conjunction with RHI for either a heat pump or a biomass boiler. This is because neither work at their highest efficiency when heating domestic hot water - especially when space heating is turned off. Those installing both technologies will be paid the full deemed heat - in effect being paid twice for the solar heated water.

Non-domestic Renewable Heat Incentive (RHI) is for all other installations. It is available for installations with a capacity of less than 200kWth. The tariff rate will pay 9.4p per kWh of heat generated. The tariff will be index-linked, and paid for 20 years. The heat generated must be measured by an approved meter. Click the link for answers for more information about the non-domestic RHI.

Panels are available for DIY installation, but buying them this way means that you have to pay the full rate of VAT (instead of the 5 per cent rate) and, more significantly, won't be eligible for the renewable heat incentive. Some people make their own from old radiators and the like. CAT publishes a book Solar Water Heating: a DIY Guide to help the enterprising.

Well installed, and properly-used, solar hot water systems will provide around 60 per cent of the hot water a home needs. Typical savings from a well-installed and properly used system in the EST field trial (published October 2011) were £55/year when replacing gas and £80/year when replacing electric immersion heating. 

Find a solar thermal installer. 

Planning permission for solar panels?

Solar panels are generally considered ‘permitted development’ in England and Wales as long as they do not protrude more than 200mm from the wall or above the roof slope; and are not higher than the highest part of the roof.

They are permitted in a conservation area as long as the panels are not installed on a wall that fronts a highway. However, if you live in a listed building you will have to apply for planning permission. Up-to-date advice is available on the government’s planning portal.

Stand alone solar is permitted development as long as it is less than 4m high, more than five meters from the boundary, and the surface area of the panels does not exceed nine square meters; or any dimension of its array does not exceed three meters. In a conservation area, it must not be nearer to the highway than it is to the dwelling. If it is in the grounds of a listed building you will need to apply for planning permission.

Click to see the full text of the most recent changes to permitted development.

For those of you living in Wales, from the beginning of September 2009, the Welsh Assembly Government announced new planning rules to encourage householders to install renewable energy equipment. A leaflet has been published to explain the changes - Domestic microgeneration permitted development: A guide for householders.

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Solar thermal calculator

When entering area available below, take into consideration that 4 sq meters is the average size needed for a three bedroom house. Running costs outlined in this calculator comprise the electricity to run the pump plus a service every five years.


More information on solar thermal

From the blog:

Before you install

Solar hot water panels: 7 things to check before you install

Estimated renewable heat incentive payments on a solar thermal installation (July 2013)

Choosing an installer 

Why using solar quote sites may cost your more than going direct

10 tips for avoiding the solar thermal cowboys

Six tips to save you from solar panel cowboys

Practical advice-maintenance

Seven steps for servicing your solar thermal system

Solar Thermal - A guide to life expectancy and maintenance 

Servicing your solar water heating panels

How do I know if my solar thermal panels are working correctly? (Sept 2013) 

More practical advice  

Solar hot water: readers' tip of the month 

5 key ways to get the best from your solar hot water system 

Getting the best out of your solar thermal system

Solar thermal panels are possible with a combi boiler

Solar hot water without changing your cylinder 

How to avoid bugs in your solar hot water

Storing your solar heat 

User control is vital for best performing solar water heating 

Your questions answered

Thermodynamic panels: your questions answered 

Is my solar thermal system safe while I'm away on holiday?

Can solar PV and solar thermal be successfully combined?

Do I need a new cylinder for solar hot water?

Can I use solar thermal panels to heat my house?

Solar thermal panels: flat plate or evacuated tube?

Can I have solar hot water with a combi boiler? 

How much of my hot water can I get from the sun?

Can I use solar thermal for space heating?

Case studies

Solar hot water: Its the best thing I ever did

Getting the best out of solar panels

Planning for solar panels

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