frequently asked questions

General Solar FAQs

1. Will solar panels work in the UK given that it’s not very warm? What about in the winter?

Yes, they work well. Solar power depends upon light intensity, not direct sunlight and not heat. So even if it is not bright and there is no direct sunshine, solar panels can still generate power. Solar panels are likely to generate more power in the summer rather than the winter because the sun is stronger and there are more daylight hours.

2. What’s the difference between solar PV and solar thermal?

Solar photovoltaic (PV) systems convert sunlight into electricity using cells made out of semiconductor material. Solar thermal systems convert sunlight into heat energy to heat water, either directly or via another fluid which then heats the water.

3. I have limited roof space - which is best for me?

In terms of energy per m2 solar thermal is 4-5 times more efficient. Solar thermal is able to convert the sun’s energy into heat more effectively than solar PV panels can convert solar energy into electricity. Therefore, from an energy point of view, solar thermal is better on smaller roofs. However, if you hope to benefit from the feed-in tariff (otherwise known as the Clean Energy Cashback scheme), a PV array will be better financially.. To find out more about how you could benefit from the feed-in tariff call us or see feed-in tariff explained (link).

4. How do I know whether my house/building is suitable for solar panels?

See the “Is it for me” section in relation to solar thermal and solar PV

5. What maintenance do the systems need?

Solar photovoltaic (PV) systems don’t have any moving parts, which means that maintaining them is relatively simple. Dust on the panels can affect performance , so the panels to need to be cleaned with a hose or soapy water every 6 months or so. Solar thermal systems have more moving parts (i.e. inside the pump), but they require little maintenance. The anti-freeze in the solar thermal system will need to be replaced every five years or so.

6. Do my roof tiles need to be removed?

No they don’t, as the panels can be mounted on the surface. During installation, some tiles may be temporarily removed but will be replaced by a trained roofer.

7. Will solar panels affect the structure of my roof?

Solar panels have been designed so that they can be mounted on roofs without affecting their structure. However, if your roof is in bad condition, this will need to be addressed before the solar panels can be installed.

8. Do I need planning permission to install solar panels?

Planning permission is usually only required if you are planning to install solar panels on a listed building or if your building is located in a conservation area. However, it is always prudent to check with your local planning officer or local building control.

Solar PV

1. How do I get electricity when it gets dark?

As solar panels generate electricity using sunlight, when it gets dark, no electricity will be generated. You will still be connected to the electricity grid and buy electricity from your usual supplier. Your electricity bills will be reduced under the feed-in-tariff by the amount of electricity you generate.

2. What if there’s a power cut?

Most PV systems are connected to the electricity grid (you need to remain connected because you’ll still need grid electricity at night and you will want to sell any excess electricity you generate to the grid). For safety reasons, if there is a power cut the PV system will automatically shut down too. This means that you will be out of power until the problem is rectified.

3. Does the system need batteries?

Your PV system does not need batteries, although storage systems are available to properties not connected to the grid.

4. How much space will I need inside the house/building?

The solar panels will be mounted on the outside of your home/building. On the inside you will need an inverter (which converts the DC electricity generated into AC electricity which is suitable for all appliances) and a new meter. This is takes up a similar amount of space as your current fuse box.

5. How long do the photovoltaic (PV) systems last?

If your PV system has been correctly designed, installed and maintained, it can last for well over 20 years. Most systems even come with a 20 year performance warranty. In the US, there are some systems which were installed in the 1960s and are still in use today! However, as the years go by, the systems become less efficient at converting sunlight into energy [Note: add annual figures]. The inverters, which convert the DC electricity into AC electricity, need to be replaced approximately every [�–�] years.

6. What happens if something goes wrong?

On the rare occasion that something goes wrong, we offer a warranty of [�–�] years on the installation, which means that we will come out and fix your problem. Panels come with a warranty of a minimum of 20 years, which means that the supplier will be able to provide you with support and arrange a home visit if necessary.

7. What is the difference between a kilowatt (kW) and a kilowatt hour (kWh)?

A kW is a measurement of power - it represents 1,000 watts. A kWh is a measurement of energy generated or used – a 1kw source operating for 1 hour will generate/use 1 kWh.

8. What is a kilowatt peak?

A kilowatt peak is the number of kilowatts that can be generated if a system is working at its peak, which can only be achieved in factory standard test conditions. These conditions cannot really be replicated outside of the factory - the test conditions are used to ensure that a panel is working properly before it leaves the factory. In the UK, a 1 kilowatt peak system facing due south will generate approximately 800kwh of energy per year.

9. How does the feed-in-tariff work?

See our explanation of the feed-in tariff by clicking here.

Solar Thermal

1. What is solar thermal used for?

Solar thermal can be used to heat your domestic hot water, provide pre-heating for your underfloor heating and heat your swimming pool. These can be done separately or one system can run them all, though you will need a slightly different system for each setup.

2. How long do solar thermal systems last?

Solar thermal panels will last you for years. In Europe there are systems over 30 years old that still produce plenty of hot water. All the panels we stock are tested for durability and efficiency so they won’t break down. They come with the European quality guarantee, the Solar Key Mark. The panels come with a 10 year warranty, the cylinder a 5 year warranty and the pump a 2 year warranty. However systems are expected to far outlive this.

3. How big is solar thermal panel, will it fit on my roof and how much space do I need in the house?

A standard panel is around 2.3 m2, meaning that most roofs can easily fit 2-3 panels. If you have a small roof, solar thermal offers you better energy generation efficiency per m2 than solar PV. However it produces heat rather than electricity. Inside the house, you will need space for a hot water cylinder. This is about the same size as your old one if you have one. If you don’t have space for a hot water cylinder, solar thermal may not be for you.

4. What happens when the sun is not out?

When there is no sun, the system may not be able to warm your hot water to the temperature you would like. At this point, your boiler will kick in and provide top-up heat for your tank. At night there is often enough hot water stored in your tank to last for a few days. If lots of people decide to have a bath or shower at the same time, the boiler will top the tank up until morning.

5. Will I run out of hot water?

No, unless your boiler breaks down, because when there is no sun the boiler provides back-up hot water. The only other time you might run out of hot water is when you use lots of water at once and the panels and your boiler can’t produce enough hot water fast enough.

6. What exactly is the difference between the types of panel?

Solar thermal panels come in two varieties: flat plates and evacuated tubes.

Flat plate panels look like a large roof-light window. They consist of a copper tube surrounded by a high absorbance copper plate used to gather and convert solar energy to heat. They are very effective at capturing all types of solar radiation, especially when facing directly south and angled between 30 - 45. They have the benefit of being very hard-wearing and durable.

Evacuated tube panels consist of many long tubes placed alongside each other. Inside each tube is a thin tube of fluid suspended in a vacuum, the suns energy heats the fluid as it travels down the tubes. Evacuated tubes work better than flat plate panels when placed flat or in more awkward locations, they are also able to reach high temperatures suitable for industrial use. However, they are harder to install and tubes can be prone to breaking.

Heat Pumps

1. What is a heat pump?

Our environment stores solar energy at low temperatures all around us. A heat pump takes this low temperature heat and upgrades it to a high temperature heat that can be used to heat water and buildings. A heat pump can also work in reverse to provide cooling during summer months.

2. Which type of heat pump is for me?

Which heat pump to use depends largely on your property:

• A ground source heat pump draws low-grade heat from the earth using pipes laid under the ground. These pipes can be placed in shallow trenches, requiring large areas, or fed down into deep boreholes, often as deep as 120m. Using boreholes requires less space but can be a costly process. Due to the constant temperatures found underground (of 10-14 C), ground source heat pumps have a consistently high performance compared to air source pumps. This makes them capable of meeting all of a building’s heating and hot water requirements.

• An air source heat pump is ideal for situations where ground access is limited or budget is tighter. They have a good operating temperature range and can even extract heat from the air when outdoor temperatures are below zero. However, a back-up heating system may be required.

3. What are heat pumps used for?

Heat pumps can be used in many energy saving applications, including:

• Domestic and industrial hot water heating

• Space heating in underfloor heating systems, as well as through conventional radiators

• Swimming pool heating systems

• Dehumidification

• Space cooling

4. Are they hard to install?

Installing a ground source heat pump requires some work in the garden, either using drilling machinery or tractors for ditch digging. However the internal aspect of the installation is straightforward and can be simply combined with your current plumbing system. Air source heat pumps have a single unit that is easily installed against the property wall. Typical installations take around 5 days to complete.

5. How large are the units?

The size of each unit depends on its output, the greater the output the larger the unit. A standard heat pump for the average 4 bedroom home is around the same size as a small fridge.

6. How efficient are heat pumps?

Heat pumps are considerably more efficient than conventional boilers. A very good boiler runs at 90% efficiency. An air source heat pump, the least efficient, uses 1kW of electricity to produce around 3kW of heat energy making it 300% efficient. The best ground source heat pumps are 500% efficient and the average is around 400%. This results in significant energy savings compared to conventional heating systems. Heat pumps are measured by the Coefficient of Performance (CoP), a CoP of 4 means that the heat pump produces 4 units of heat energy for every 1 unit of electrical energy used.

Mechanical Ventilation with Heat Recovery (MVHR)

1. What does the ‘mechanical’ part mean?

“Mechanical’ simply refers to the fan in the system. Some ventilation systems can be ‘Passive’, usually referring to a vent in the building through which air can flow naturally. By using a low energy fan, the ventilation can be controlled throughout the year and heat can be actively recovered more efficiently.

2. Why do I need to ventilate my building?

Well-built and well-insulated buildings have very few air gaps in the building fabric, meaning inside air can often become stagnant. Stagnation of air can cause concentrations of dust, allergens, house mites, bacteria, mould and funguses to build up with time - all of these put your health at risk. Ventilation is therefore required to avoid this from happening. Ventilation systems without heat recovery waste the precious energy used to heat your home by transferring it straight outside.

3. What’s special about Mechanically Ventilated Heat Recovery (MVHR)?

Installing ventilation with heat recovery allows you to maintain a constant supply of fresh air whilst keeping heat within the building. MVHR systems also filter incoming air, meaning air quality within the home is actually improved compared to outside, which is especially beneficial if you live in a city or suffer from hayfever.

4. How do you stop the bad stuff from coming back in with the fresh air?

The fresh incoming air and the stale outgoing air never actually mix. Instead, they flow through a series of pipes in very close proximity to each other (a “heat exchange”) and the heat is transferred from the warm air to the cold air with up to 94% efficiency.

5. How often is the air changed?

It is usual for all of the air in the house to change every 2 hours.

6. How big is an MVHR unit?

The size of the unit depends on the size of your home but most units are around 1.5m x 0.5m x 0.5m – about the same size as a large fridge. They fit easily in standard cupboards, attics, or a plant room and can even be wall mounted.

7. Where does the ducting go and how big is it?

The ducting, or pipework, can be incorporated discretely into walls, floors and ceilings. Ducting is often placed in the back of cupboards or run behind permanent furniture to keep it out of sight. It comes in a variety of shapes and sizes depending on your property and your preference. In new builds, it is especially easy to ensure the ducting is completely invisible, whilst in renovations the most common choice is a flat box style (2cm x 20cm) that is subtly added to existing walls.

8. Is it noisy?

No. The fan will make a quiet noise, but placing the unit in an unobtrusive location makes this unnoticeable. Using an MVHR also means that there is no need for noisy extractor fans in kitchens and bathrooms.

9. How often will I need to change the filters?

Filters need a quick rinse after six months - this keeps the system working at its most efficient. They should be replaced once a year. Replacing a filter is very simple, much the same as replacing the filter in your Hoover.

10. What happens in the summer when it gets too hot?

During summer months when it is not necessary to recapture heat from the home, the heat exchanger is bypassed. This means that the system simply ventilates the home, improving air quality and sometimes having a cooling effect.

Insulation

1. What does insulation actually do?

Insulation slows down the flow of heat. This keeps heat inside a building during winter months and out during the summer.

2. There are so many types of insulation, how do I know which is right for me?

Choosing the right insulation is not always easy. There are various factors to consider, the most important being ‘how good is the insulating material at preventing heat flow?’ The answer to this question determines the thickness of the insulation required. However, it is also important to consider other issues, including: how much space is taken up, the cost, whether to insulate internally or externally, the energy used in producing the insulation, fireproofing, condensation and lifetime performance. greentomatoenergy can help you resolve these problems by working to your budget and your requirements.

3. What is the R-value?

Insulation is rated using an “R-value”. The R-value is the measure of the insulation material’s ‘resistance’ to heat - the higher the R-value the better the insulation. You may also see insulation rated using the ‘-value’, lambda Value. This is the inverse of the R-value, meaning the lower the value the better the insulation.

4. What is a thermal bridge?

Thermal bridges (sometimes called “cold bridges”) are parts of a building through which heat can escape between insulated areas or as a result of conduction.. Thermal bridges are commonly found around window edges, between walls and the ground, and between joists and walls. Thermal bridges can be a significant source of heat loss in a building and can create cold spots where condensation and damp may occur.

5. Will installing insulation create a lot of mess?

This depends on the scale of the job and the type of insulation used. It is best and easiest to install insulation during a refurbishment or when doing other works. However, it is possible to quickly and cleanly install insulation at other times and in certain places – for example, in the loft or a cavity wall. Using dust free insulation such as polyurethane foam can help reduce mess.

6. Will I have problems with damp?

Correctly installed insulation should not cause problems with damp. Insulation should be water repellent or include a waterproof membrane. Insulation can in fact help resolve problems with damp by preventing condensation on walls.

7. Does insulation change over time?

Some insulation does change a little over time and can lose some of its insulating quality. However, for most insulation such as polyurethane foam and cavity wall insulation, this is a very slow process and is not considered a problem. For less dense insulation such as wool and cellulose there is some settling and it is a good idea to install slightly more to account for the settling over time.

8. Where does insulation go?

Insulation can be installed throughout a building, though it is most easily installed in the loft and cavity walls. It is also important to insulate the floors, around windows and dormer extensions. It is also helpful to insulate thermal bridges (link) where possible.

9. Will insulation change the appearance of my house?

Insulating internal walls will result in some loss of space in each room - how much space is lost depends on the depth of insulation. Redecoration will mean the inside appearance of the property will be exactly as you want it. It is also possible to install external insulation. External insulation is covered by a render meaning the appearance should remain virtually unchanged. However, installing external insulation may require planning permission.

Windows

1. I already have double glazing, why should I change it?

You may not need to change your windows if you already have good double glazing. Double glazing is considerably more effective at preventing heat loss than single glazing. However, old windows that don’t close properly are the most common cause of draughts letting cold air into a building. New double or triple glazing with airtight seals will improve the thermal performance of your property considerably. When building a low energy house, considering triple glazing is vital - the significantly improved thermal performance is extremely important to the overall building performance.

2. How much does triple glazing cost?

Triple glazing adds between 20 – 40% on the cost of double glazing. The exact cost depends on the required performance, the spacers used, the gas fill and other factors.

3. What difference does the glass make?

The type of glass in each window can have a significant effect on the overall performance. Many double and triple glazing windows use low Emissivity (low E) glass that reduces heat transfer. Noise reducing glass is popular in cities and laminated glass helps prevent breakage. It is possible to select different glass types for each window.

4. Why is the gap filled with a gas?

By filling the space between the panes with an inert gas such as argon, the performance of the window can be improved further. Inert gases have a very low transmittance of heat compared to air, which means it acts like a layer of insulation between the glass panes. It is also possible to create a vacuum in the space for a similar effect.

5. What are ‘spacers’ and why are they important?

Spacers are the divider between the glass panes. This width of the spacer determines how much air there is between panes, which affects the thermal performance of the window. Generally, the larger the gap, the better the performance. The spacer material is also very important because metal spacers can create a thermal bridge through which heat can escape. Foam spacers help to reduce this effect, as do fibreglass and other heat resistant materials.

6. What is a U-value?

A U-value is a measure of the heat loss across the entire window, including the frame and the glass. The U-value is measured in watts conducted per square meter¬¬ per degree of temperature difference - W/(m¬¬¬¬2K) See the chart below for guides on window performances and U-values.

Glazing

Average performance (U-value)

High performance

(U-value)

PassivHaus standard (U-value)

Single 5.6 5.0 N.A.

Double 2.2 1.7 N.A.

Triple 1.4 1.0 0.66 – 0.85

7. Will I need planning permission for new windows?

Double and triple glazed windows can be built to match your existing style, whether it be sash or large panes. However, if you live in a listed building or a conservation area and new windows will change the look of your building, you should consult your local planning authority.

Rainwater Harvesting

1. What can rainwater be used for?

Harvested rainwater can be used for toilets, washing clothes, outdoor taps, garden watering and other non-drinking uses. It is possible to purify rainwater for drinking, but this is more complex and costs extra.

2. How much water will I save?

On average, a domestic property can reduce its water consumption by 30-50%. For industry, this figure can be as high as 80%. As water usage becomes increasingly controlled, this could equate to considerable money savings.

3. How much water can I collect?

This depends on the size and angle of your roof and where you live. Flat roofs collect less water than pitched roofs and London collects less water than Manchester. However, in the south-east, the average pitched roof can collect 800 litres of water per m2 a year. Most commonly a system will be sized to match the area of the roof and the capacity for collection. It is possible to undersize a tank and a higher proportion of water will be lost to the drains.

4. Can rainwater harvesting be installed in an old building?

Yes, rainwater tanks can be installed on new and old buildings. A secondary plumbing system will need to be installed to use rainwater in toilets and the garden. It is important to keep rainwater separate from the existing system to prevent contamination of drinking water.

5. Where are rainwater harvesting tanks installed?

Most rainwater tanks are installed underground. The smallest tanks (up to 1000 litres) can be placed in the garden. Most tanks are buried, meaning that installing larger systems requires some digging to lay the tank and piping. An overflow drain will also need to be connected to the tank for times when there is excess rainfall. All this means that installing rainwater harvesting will make some mess in the garden.

6. How do you stop debris from getting into the system?

There are a number of filters in the system including one that prevents large debris from entering the tank and a second that prevents small debris from getting into the pipes. All rainwater harvesting systems have an overflow and a bypass, the overflow allows small flotsam to be washed away and the bypass kicks in after periods of no rain to stop excess dust and chaff from entering the tank.

7. What maintenance is required?

Filters need to be cleaned once every 3 months to ensure a full flow of water; these only require a quick rinse. The overflow ensures the system flushes debris a number of times a year.

8. Will my water be clear?

It is possible that the water may have a slight brown tint, rainwater can pick up some colour when flowing over leaves in gutters, however often this is barely noticeable. If you have a green roof, living wall or permeable pavement with grass, this discolouring will be more noticeable.

9. What happens when there is no rain?

During long periods of no rain the tank may reach a minimum level. At this point the float switch will activate a mains supply to top up the tank until it rains again and the tank is able to fill completely. A correctly-sized system should store sufficient water to supply the property for at least 1 week before a top up is required.

10. What are the environmental benefits?

Rainwater harvesting has numerous environmental benefits:

• Treating water is a highly energy intensive process - by using rainwater where possible, some of this energy can be saved.

• Collecting rainwater reduces the effect of storm water runoff, which can cause flash flooding (and flood damage), as well as problems with city drainage and the contamination of water supplies.

• No chemical treating of water, therefore reducing contamination in the water system.

• More water available for other users.