Solar Power FAQs


What is solar PV?

Solar photovoltaics (PV) are materials that convert light directly into electricity, using no moving parts.  It is made out of semiconducting materials, most usually silicon.

What are solar panels made of?

The heart of the panel is made of wafers of semiconducting material, usually silicon – 95% of the market for PV is silicon panels.  Other compounds such as gallium arsenide and copper indium diselenide are available.  Plastic PV is just coming to the market although conversion efficiencies are still low.  In future, materials called perovskites look to be a promising low-cost solar conversion technology.

Apart from the solar material itself, modules also contain a glass front surface, an aluminium frame and polymer resins to seal the panel and prevent water getting inside.

What is a kWp?

The size of a solar system is measured in kWp (kilowatt peak). It is the amount of power produced under standard laboratory test conditions, which broadly equate to bright sunshine.  So a 2 kWp system will produce 2 kW of electrical power in bright sunshine.   For standard modules of 16% efficiency, each kWp takes up about 6.25m2 of roofspace.  More efficient modules are available, and because they convert light more efficiently, they take up less space.  1kWp of high efficiency 20% modules only take up 5m2 of roofspace.

How much electricity is produced by solar panels?

The kWh produced by a solar array depends on the size of the PV array, and the amount of light received (which depends on location and orientation). In terms of location, a solar panel in the UK will produce between 700 kWh (Scotland) to over 1000 kWh (Cornwall) for every kWp of panels installed. We will model the output of your proposed system at the design stage.

What angle is best for solar panels?

Solar PV produces electricity from all light that lands on the surface. The majority of electricity is produced from direct sunshine, but light from the sky (diffuse light) and reflected from the surroundings (albedo) is also important.  The former is dependent on orientation, but the others produce electricity at any angle.

orienttion, solar, output, optimal, angle

Adding all this together, the optimum orientation is facing due south at a pitch of 30 degrees.  However, because of the contribution of diffuse light from the sky, there is a very large hotspot of high performance.  Roofs between SW to SE, and 10 to 50 degrees will be producing 95% of optimal.  East or west facing arrays will also produce about 85% of optimal.  So whilst south facing orientations are best, almost any roof will generate substantial amounts of electricity.

Where can I put solar panels?

Generally, solar panels can be installed on any roof or free terrain with enough space to accommodate them, whilst trying to avoid shady places.

Solar can be installed on virtually all rooftypes – tiles roofs, slate roofs, flat roofs, trapezoidal metal roofs, standing seam roofs, and even integrated with turf roofs.

It is possible to install on other structures as well – we see solar carports, canopies, walkways, louvres, facades and pergolas.

Solar panels can of course also be installed on the ground as well, from back-garden arrays to solar fields.

Does shading affect my solar PV?

Shading can be very detrimental to solar PV performance.  First, any shade will reduce the amount of light landing on an array and reduce output.  Second, because panels are wired together in a chain (called a string) the performance of that string is only as good as its weakest link.  So shade, (eg from a chimney) can affect not just the one panel that is shaded, but the whole string.  As designers we use software to estimate the effects of shading for a particular building, and modify the design to minimise shade.

Can I run my house off solar panels?

This really depends on how much electricity you use within the home.  An average house uses about 3500 kWh of electricity per year, but with energy efficient lighting and appliances it is possible to use half of this (1750 kWh per year.)  A typical rooftop array in the UK is 3 kWp in size, and produces about 2500 kWh per year.  So yes, it is possible to produce more than you use on an annual basis, and you can be a net exporter of electricity.  The caveat here is that generation does not perfectly coincide with demand in the house – PV produces less electricity in winter, and of course none at all at night-time.

We should also point out, that the above is electricity for lights and appliances only -it does not include demand for hot water or space heating (which are typically fuelled by gas in the UK, but not always).  Increasingly we are seeing customers wishing to use solar to meet demand from electric vehicles as well, which is an additional electrical load.

Why should I get solar Panels?

The main advantages are that electricity produced is zero carbon, and therefore does not contribute to climate change.  The electricity is free once the system has been paid for, and you will be paid a premium for all the electricity you produce called the feed-in tariff.  Over the lifetime of the system, the cost of electricity produced is far lower than purchasing from the grid.  There is the satisfaction of generating your own electricity, and not being as reliant on your electricity supplier anymore.

Are solar PV safe?

If installed correctly in accordance with IET regulations the PV is perfectly safe.  The same could be said of any electrical circuit.

Can you buy flexible solar panels?

Yes, you can. Flexible solar panels are lightweight and versatile, and useful if a portable solution is needed. They can even be found in rolls.  However, they are not well suited to most rooftop applications apart from standing seam metal roofs, to which they can be bonded.  Flexible panels are most commonly used for portable applications such as caravans, boating and camping.

How much energy does it take to make a solar panel?

An entire solar PV system including panels and inverters, will repay the energy needed to manufacture it within 3 years operating in the UK.

What is an inverter?

The inverter is the brains of the system.  It converts the DC electricity produced by the solar panels into AC electricity for use within the building.  It also matches the phase of the AC electricity produced to the phase of the grid, ensuring complete compatibility and the ability to export excess back to the wider electricity network.

How can solar PV affect my roof warranty?

This is an issue for commercial properties rather than residential, and depends on the mounting system you use on the roof you have. There are roofing manufactures that produce solar PV mounting systems too. If you use a solar PV mounting system produced by the same roof manufacturer then the warranty should not be affected, and there are the cases where your roof warranty get extended for another 25 years to match the warranty of the solar panels.

However if this is not the case, and this happens most of the times you can lose the roof warranty by installing solar panels.

Do pigeons nest under solar panels?

Pigeons nesting under your panels and fouling your property is one of the most frustrating issues faced by solar households. But it has easy solution, and it isn’t to kill them!

Pigeons, as other birds, are always looking for perfect places for nesting. The most preferable places are covered ones, where they can leave the eggs on safe, and the best option is a warm one to avoid freezing during the winter. That’s the reason that under your solar panels, which are orientated to receive all the solar energy which they can, are the perfect place for them.

To prevent this, solar panel manufacturers have designed extra covers to surround your panels, leaving out of under them any kind of bird without disturb the panel function at all.

Do solar panels requite maintenance?

This is one of the most common questions about PV. If you are investing in solar energy you need to know if we are going to be paying more money to repair them.
Generally PV does not need maintenance during its effective life. Unlike other technologies, solar PV has no moving parts, so there is nothing to wear out or need lubricating.  Solar panels are warrantied to work for 25years, and may last even longer than that.

Solar panels have a surface coating on them to help water and dust run away, and they are considered self-cleaning if pitched at greater than 10 degrees.

However, in some locations there is too much dust, plant debris (leaves etc) or bird excrement, so the panels can get dirty. You can clean it with hot water if you want to keep them perfectly clean, but use no abrasives or chemicals, as these will damage the specialist coating.

Do solar panels affect the price of my house?

The value of your property is very much down to the opinion of the prospective buyer. However, solar PV will increase the energy performance rating of the property (EPC rating), and we are seeing a correlation between energy performance and house price for properties in the same location – that is to say an efficient home is seen as more desirable.

If you move out of a property that has had solar PV installed at the very least the new owner/occupier will benefit from reduced energy bills which should be seen as a plus. However the quality of the installation is also important. To give yourself the best chance of increasing the value of your property with solar it is important that you look for a reliable installer who will be available in the future to respond to any questions or issues.

How do solar panels work?

Without getting too deep into the quantum physics of semiconductors (though do ask if you are interested!) solar photovoltaic cells consist of a positively charged and a negatively charged layer of silicon. As the sunlight hits the solar cells, it frees the electrons from the negatively charged silicon. The negatively-charged free electrons are attracted to the opposite surface of the cell.  Collector wires on the front and back of the cells complete the electrical circuit and allows a flow of electric current, via your appliances!

How long do solar panels take to install?

This will depend on the size of the installation. Typically a residential installation of solar PV will take 1 day to complete, though more complex systems may take longer. This excludes the installation of scaffolding which will typically take 1 day to erect in advance and 1/2 a day to remove for a residential installation.

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Batteries FAQs


How does a solar battery work?

A solar battery is storage for the excess electricity generated by your PV system, and a way for you to then use your own solar electricity at times when the sun isn’t shining.

During the day there are moments when your panels are generating too much energy to be used within the property, and conventionally  this excess electricity would be exported back to the grid. That changes with a battery system. Now, when you wouldn’t  be using all your generated electricity this will go into the battery, stored ready to be used as soon you will need it. When the sun goes down, you will start to use the electricity in the battery instead of taking it from the grid.

What are the benefits of batteries Storage

The main benefit is that you can store the excess electricity produced from your solar PV array during the day and use the electricity at night. Instead of getting 5p/kWh from your supplier for exporting electricity back to the grid, you use it yourself, thereby saving you the 15p/kWh you would have had to pay to import grid electricity.  So it reduces your energy bills.  There are other benefits to batteries

  • many people find it rewarding to import as little grid electricity as possible;
  • to be more self-reliant;
  • distrust of electricity utilities;
  • a buffer of energy in an emergency.

Where does the energy of my panels go first – the house or the battery?

The appliances in the house will use the electricity first – any leftover will go to the battery. When the battery is full you export back to the grid in the usual way, or you can divert this power to heat the hot water or into electric vehicles.

How big does my battery need to be to supply all my home energy?

Most houses use around 10 kWh of electricity per day. If you have solar, some of that energy will come directly from the solar panels in the day. So around 7 kWh of storage would be enough to run your house all night. We recommend a bit of a buffer for a few rainy days as well as potential for the battery to degrade over time. A battery of 10 kWh of storage would be more than sufficient for most residential applications. However, this can vary depending on your demand, so good battery system design is all about sizing the battery according to your circumstances.

If I don’t use all the battery energy during the night will I have it the next day?

Yes, if you don’t use it all that energy will remain in the battery for use the next day.  However, because the battery is partially charged already, there will be less space in the battery to capture the next day’s solar energy.  The best battery economics come when you are fully charging and discharging the battery every day, so that you are fully utilising the battery storage capacity you have paid for.

Will a battery take me completely off-grid?

Possibly, but it’s not something we recommend in the UK. If the battery works off-grid, it can potentially take you off grid too. However, you will need a really big solar PV system to supply enough energy in the winter, or some other form of back-up generation.  For this reason we don’t recommend going off grid in the UK.

In other locations, such as southern USA where solar resource is much more even over the course of a year, going off-grid is increasingly possible.

Can a battery work off grid in the UK if there’s a power cut?

Yes, there are battery systems that can operate off grid in the event of a power cut, and then reconnect when the grid comes live again.  . There are a limited number of options for grid tied battery systems that can operate in the event of a power cut but Victron offer this capability.  This is because of the electrical regulations that govern all generation equipment connected to the grid.  Put simply, if there is a power cut, operatives need to work on the wires safely, and they can’t have generators making it live.  In a power cut, the battery system needs to discharge to your house, but not anything at all to the grid, and this control as to be absolutely 100% reliable.

Does the Tesla Powerwall work off grid?

The Tesla Powerwall does not currently work off grid in the UK, for the regulatory reasons described above. Tesla have developed this capability and it operates in other countries, but they are working on a solution with the regulators to bring an off-grid compatible Powerwall to the UK.

Can I have a battery without solar/ charge  it directly from the grid?

It certainly is possible for AC coupled batteries to simply charge from the grid using off-peak cheap electricity.  Currently this includes “Sonnen”, “LG chem” and “Maslow” and we expect Tesla to follow suit shortly.

How much is a Tesla Powerwall?

£6,935 when install with a solar PV array, and £8,600 when installed itself as a retrofit to an existing solar installation. The differences here are the amount of VAT payable (5% when part of a solar system, 20% when retrofitted), and the project management costs required.

What is the return on investment for batteries?

This is a complicated calculation as it depends how the household uses the battery, and if they manage to fully utilise the battery capacity on a daily basis.  The payback of a well-sized battery system is of the order of the lifetime of the battery unit itself (10 -15 years).

We are seeing customers wishing to utilise more of their own solar generation, and becoming more independent from the grid.  A battery is a broadly cost neutral way of doing this, but until prices reduce (and they are reducing rapidly) they aren’t a means of generating a huge financial return on the investment.

How Long will batteries last?

Most modern, lithium ion batteries have a 10 year warranty, (for example Tesla Powerwall and LG Chem batteries). As this type of residential solar battery technology is relatively new to the market there is little evidence of what will happen after 10 years. Although the batteries could last 20 years or longer we recommend budgeting for a replacement after the 10 year warranty is up. Some of the older manufacturers like Sonnen have a 10 year warranty but an expected 20 year lifespan. The cost per kWh of storage are much higher for batteries like Sonnen as these batteries are predicted to achieve more cycles.

What size are batteries?

This will vary depending on the make and capacity of the battery, but can be quite large, and we need to think carefully about where to locate them. Some commonly used models include:

  • Tesla Powerwall 13.5kWh, 1150mm x 755mm x 155mm
  • LG Chem 9.8kWh, 744mm x 206mm x 907mm
  • Sonnen 4-8kWh, 660mm x 1400mm x 360mm

Is my battery fireproof?

Yes, all batteries meet stringent fire safety regulations, and can be used indoors.

How do I dispose of my batteries?

After the life span of the batteries the battery  will be picked up by the manufacturer of the batteries and dispose them for you.  They have to meet WEEE regulations which govern the disposal of electrical goods, in the same way you would for a washing machine etc.

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Electric Vehicle Charging FAQs


How long will it take to charge my car?

It depends on the battery size in your car and the power of your charge point. Typically plug-in hybrids (e.g. Golf GTE, Prius) charge at a rate of 3.7kW, fully charging within 3 hours. Fully electric cars (e.g. Tesla) can charge at 7.4kW on a single phase supply but have a much bigger battery, a 75kWh Tesla Model  S would take 10 hours to fully charge. A 3.7kW charge charges at a rate of approximately 12mph, a 7.4kW charge would charge at a rate of approximately 24mph.

Can I charge my EV with solar panels?

Yes, there are a couple of ways to do this.  One is to use the dedicated Zappi charger, which diverts excess generation into an electric vehicle.  In this way, what would have been exported back to the grid is now providing you with zero-carbon mobility.  However, you would need to have your EV parked up at home during the day.

The alternative is to use a battery system to store this energy, and then discharge this into your EV in the evening/night-time.  The battery would also cover your household loads one the sun has gone down.

Can I charge my car directly from my solar (no inverter required?)

No. Electric vehicles charge from an AC supply.

Do different EVs require different chargers?

There are two standard connectors which different models of EV use (Type 1 and Type 2), the market is moving to using Type 2 as standard but charge points are available in either and there are also Type 1 to Type 2 adaptor cables.

How much is an EV charge point?  Are there grants available?

The cost of a 7.4kW charge point is under £1,000 supplied and installed, an OLEV grant covering half of this cost is available so you can expect to pay £400-500 inc. VAT.

How much can I charge my neighbour for using my EV charge point?

Joju can supply EV charging solutions which allow you to charge others to use a charge point you install. The rate you set is up to you.

 Will EV chargers blow my fuseboard?

Joju will carefully check your existing electrics to ensure that you don’t blow your main fuse. In some instances we may suggest you install a 3.7kW charger rather than 7.4kW. Usually this is not an issue.

How long will it take to install an EV charger?

An installation will typically take half a day. If your fuse box is far from the charge point location or cables need to be run through internal rooms, the installation may take a little longer.
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