Solar PV Panels: Complete Guide to Home Solar Electricity

Everything you need to know to make the most of solar photovoltaic panels, from how they work to how much they cost
by Build It
14th October 2019

Solar electric panels have long been a popular renewable technology among self-builders and renovators. Thanks to a mixture of government incentives and falling technology prices, demand for solar photovoltaics (PV) has boomed over the last decade.

The once-generous Feed-In Tariffs (FITs) have now been dropped, and a VAT hike on the products themselves (from 5% to 20%) came into effect on 1st October this year. Yet the allure of this renewable option is still strong, especially when combined with battery storage.

Despite uncertainty over replacement tariffs and VAT rates, this is an area of growth and change – not just with more aesthetically pleasing products coming to the market, but the tech industry working to improve panels’ efficiency.

How do solar panels work?

Solar photovoltaic panels transform free energy from the sun into electricity. This is then converted from a DC current to an AC current via an inverter, to make it suitable for household use.

The panels capture energy from the sun and convert it into DC electricity via groups of photovoltaic (PV) cells. These are made up of multiple layers of semi-conducting material (typically silicon). When daylight falls upon the cells, an electric field is generated across them.

The stronger the sunshine, the more electricity is produced. DC current isn’t useable in the domestic setting, so the panels are hooked up to a small but highly efficient inverter, which is the heart of your solar system. . This is usually installed inside your home, though micro versions are now available that fit onto the back of individual panels.

What size solar panels do I need?

Solar electric panels are rated according to their performance in optimum conditions (i.e. direct sunlight during summertime, or – to get technical –under full solar radiation). This is expressed as kilowatts peak (kWp)

A typical domestic setup will be rated somewhere between 3.5kWp and 4kWp – but the exact specification will depend on a variety of factors, such as the size of the house, number of occupants and current/future electricity needs.

Depending on the system you use, you can expect to require around 8m2 of roof space per kWp.

As a rule, 1kWp of solar PV panels installed on a south-facing roof at a good pitch will provide around 800-1,000kWh of electricity per year.

This will vary according to your home’s location in the UK – a well-oriented roof in the Devon countryside is likely to get more sun than an inner-city equivalent in Liverpool, for example.

On average, a 3.5kWp setup might deliver 2,800 to 3,500kWh of electricity – which equates to the majority of an average household’s annual electricity requirement. But don’t expect to rely on the panels all year round.

How efficient are solar electric panels?

As you’d expect, output is highest from May to August, and while the panels will still provide electricity in winter, they’re likely to deliver only 10% of their annual total over this period.

The lifespan of a solar PV array is considered to be around 25 years, but the inverter is likely to need replacing at least once during that period. You can maximise its durability by ensuring it’s properly specified to suit your setup and maximise electricity production.

According to the Energy Saving Trust, a 3kWp system would typically need an inverter rated between 2,400 and 3,300 Watts.

Solar Panels and Battery Storage

Solar PV’s drawback is that, for many households, there’s a mismatch between when electricity is generated and when it’s needed.

“Normally, you’re using energy in the morning and late afternoon/evening, but you generate it from 9-10am until late afternoon, when most people are away from home. So all you are doing is providing energy to the grid, which is not very useful, because it is cheaper to use what you have produced on site,” explains Ulrich Setzermann of Lumenaza, a software company for the renewable energy market.

Learn more: Guide to Battery Storage for Solar PV

Adding a battery storage unit to your setup allows you to save that excess energy for later, and manufacturers include Tesla and Sonnen.

Although a battery will make the most of your solar PV panels, it won’t come cheap. Battery costs (from around £1,700-£4,000 depending on capacity) should be weighed against payback time.

A solar battery will be most beneficial if you use all your electricity outside of daylight hours, for example for charging a car overnight.

Where can solar panels be installed?

Most domestic solar photovoltaic panels are fitted to the property’s roof, ideally one that’s on a south-facing elevation and offers a 30 to 40° pitch to maximise the array’s exposure to light. Fitting the PV units is a fairly painless process, and most jobs can be completed in two or three days.

If you’re building a home from scratch, then you can make allowances for the above in your architectural brief.

“Considering solar when you’re in the early design phase makes a lot of sense, as you can make sure your solar electric panels are orientated and fitted as efficiently as possible,” says Steve Springett, chief product officer at Tonik Energy.

“There’s also an opportunity to save relative to retrofitting solar panels. Scaffolding and mobilisation make up the majority of the installation cost – the panels themselves are relatively affordable. So it’s a great opportunity to get value for money.”

On-roof solar PV

There are two main options for installing the panels. The first is an ‘on-roof’ system, where the modules are fitted in a mount that’s anchored to the rafters. This method is particularly suited to retrofits, as it involves minimal disruption.

If your roof is already tiled, these will need to be lifted, the anchors fixed and the tiles replaced before the mount is attached. With some slate roofs, it’s preferable to leave the covering in situ and drill through to provide a fixing. In both cases, the panels sit slightly proud of the covering to provide a ventilation gap.

In-roof solar PV

The second option is an ‘in-roof’ or integrated system, which is often preferred for self-builds or big renovation projects where the structure is already being re-roofed. With this method, the panels are fitted on the same plane as the covering for a sleeker finish, with a flashing kit used to provide waterproofing.

Price-wise, Steve says in-roof systems are now about the same as stick-on panels, as installers have realised that in-roof is just as quick to put in.

There are helpful new products coming through, such as Easy Roof’s extension to its tray system that will now allow Velux windows to directly abut in-roof PV installations.

Solar tiles

You can even choose to install individual solar tiles or slates, available from suppliers such as Solarcentury. As with any PV system, ventilation is important, so your installer may counter-batten to provide good airflow.

Solar tiles are an interesting alternative to panels, having the advantage that they are virtually invisible, and can even pass muster in conservation areas. This comes at a greater cost, though – perhaps 50% more than a panel system.

Some products are designed to work with traditional tiles, such as GB-Sol’s solar slates (as the name suggests, they are compatible with standard slates). New to the market is the same company’s collaboration with recycled plastic tile manufacturer Envirotile: together they’re making both solar and non-solar versions of the same tile.

Ground-mounted solar pv

Alternatives include ground-mounted panels (which literally sit in a frame in your garden) or flat roof installations. In the latter case, the modules are usually fitted at a greater pitch than the roof to maximise the amount of strong daylight to which they’re exposed.

New ground-mounted products that move to track the sun’s path can deliver an efficiency advantage over static systems. Heliomotion, winner of a Build It award last year for Best Home Technology Product, is GPS-driven system. The company claims it can deliver 30-60% more energy per year than roof panels with the same dimensions.

Because they’re raised above the existing pitch, this type of installation may require planning permission.

Will solar electric panels suit my home?

Some homeowners are put off solar PV simply because they don’t have a south-facing roof. However, a well-specified setup can actually fulfil a significant proportion of your household’s electricity requirements even in an easterly or westerly configuration.

Similarly, while a 30 to 40° roof pitch is best, panels can still soak up enough light at other angles.

And don’t assume that, just because you live in an area that’s often overcast, you won’t get enough sunlight for the panels to function.

Bright, direct sunshine is certainly an advantage, but photovoltaic cells can generate plenty of electricity just from diffused daylight, so they’ll continue to operate on cloudy days.

Do, however, watch out for potential sources of shade that might diminish the performance of your panels. Common culprits include nearby trees, overlooking buildings and chimneystacks.

It’s surprising how often the design of a new home fails to account for this; though it is of course more difficult to avoid with existing buildings.

How much do solar panels cost?

According to the Energy Saving Trust, the average domestic solar PV system is 4kWp (kilowatt peak – the amount it will generate when working optimally) and costs about £6,200.

Be sure to get quotes in from several installers, and look carefully at the detail of what they’re providing for the price. Not every solar system is equal – so look into the efficiency ratings of the specified products versus the initial cost and potential tariff payments to ensure you’re getting good value.

The Energy Saving Trust operates an online calculator that can help you get an idea of the potential returns of installing generic PV panels through bill savings. Remember, Feed In Tariff payments are no longer available for solar electric panels.

Once your panels are installed, it’s important to keep an eye on performance – if only to ensure you’re using your solar electric to its fullest potential.

A smart meter should be fitted to monitor both how much power your household is using and how much is being fed back to the national grid. It may even be able to track how much money you’ve saved through wireless or web-based monitoring programs.

The Smart Export Guarantee

The government’s Feed-in Tariff, which paid out to homeowners both for the generation and for the excess electricity their solar PV systems produced, closed to new applicants on 31 March 2019. Its replacement, the Smart Export Guarantee (SEG), is currently being rolled out, and will be delivered via energy suppliers. Companies with more than 150,000 customers are required to offer at least one export tariff.

They must implement the SEG before 1 January 2020 and it’s up to suppliers to decide the exact amount that customers will receive for each kWh of electricity exported. Early indications suggest that it’ll be around 5p.

The Solar Trade Association’s website currently has a league table of tariffs. At the time of writing they are very thin on the ground (E.on is one of the few providers), but more will come on board as the compliance deadline approaches.

You may need a smart meter to qualify. As with the old Feed-In Tariff, installations must be certified through the Microgeneration Certification Scheme or a suitable equivalent, such as Solar Keymark.

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