Solar Electric Panels Explained

Everything you need to know about photovoltaic panels, from how they work to how much they cost
by Build It
7th August 2013

Solar electric panels have long been a popular renewable technology among self-builders and renovators, and take-up has soared in recent years.

Thanks to the introduction of the Feed-In Tariff (FIT), solar photovoltaics can now not only save you money on ever-rising utility bills, but also pay you back for every unit of electricity you generate.

How do solar panels work?

Fundamentally, solar electric panels transform daylight into useable energy, thereby taking advantage of a free resource to help you save money on ever-rising utility bills.

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. This is usually installed inside your home, though micro versions are now available that fit onto the back of individual panels.

The inverter is the heart of your solar system. It converts the DC into mains AC electricity that can power your household appliances, as well as controlling and optimising the performance of your 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.

If you’re taking advantage of the Feed-In Tariff, you’ll be able to offset the cost of the energy you buy during the winter months with the payments for generation and export when you’re producing surplus electricity.

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.

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.

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 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.

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.

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

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.

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?

Since the Feed-In Tariff was launched in 2010, uptake of solar PV panels has soared. As a result, costs are tumbling – a 3.5kWp system that 12 months ago might have cost you close to £10,000 can now be had for anywhere between £5,000-£9,000.

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 and FITs payments.

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.

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