Solar Panels Complete Guide – What Are Solar Panels & is a Solar PV System Right for You?

What Size Solar Panels do I Need for My Home?

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.

Photo: Rof f Wylam / shutterstock

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.

What is Battery Storage & Should I Install This Alongside a Solar PV System?

Adding battery storage will increase the percentage of energy used at home rather than exported to the grid, but it will substantially increase your initial costs when installing a solar PV system, so won’t reduce your payback period. Instead, you can improve the equation by doing energy-intensive things, such as washing clothes, during the daytime when your solar panels are generating power.

Battery storage can, however, be used to buy and store grid electricity at night, which is cheaper. This can then be used during peak hours instead of importing electricity at full rate. This is a game-changer in terms of the economics of battery storage, but in and of itself doesn’t mean that PV is any more sustainable.

your guide to solar battery storage

Where Can I Install a Solar PV System?

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 systems

There are two main options for installing the panels. The first is an ‘on-roof’ solar PV 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.

Fitted on the roof of a contemporary new home, this solar array was manufactured by Solarwatt. Eco Energy Environment both supplied and installed the setup

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 systems

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 solar panels are fitted on the same plane as the covering for a sleeker finish, with a flashing kit used to provide waterproofing.

Solar PV tiles

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

Solex’s solar roof tiles function as electricity generators, as well as providing a roof covering that blends seamlessly with the design of the house. The tiles can be fitted to cover the whole roof covering from edge to edge, all the way from the eaves to the ridge and out to the verge and hips/valleys. They also interlock seamlessly with natural slate roof tiles if you only want to install solar panels on one section of the roof

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.

Ground-mounted solar PV systems

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.

Bifacial solar PV panels form the roof of this property’s carport in Wiltshire. Pure Energy UK also installed battery storage and an EV charger as part of the setup

New ground-mounted products that move to track the sun’s path can deliver an efficiency advantage over static systems.

How Much Do Solar Panels Cost? What to Budget for a Solar PV System in 2025

Solar PV systems are measured in terms of kWp – kilowatts peak. This is the amount of energy that a solar PV system can generate when operating at full power, ie in the middle of a sunny day. Costs are normally expressed in £/kWp. So, if a typical domestic solar PV system was 4kWp and cost £6,000, we’d say the cost is £1,500 per kWp. This includes not just the collectors, but the inverter, controls, wiring and installation, too.

Surprisingly, it’s quite difficult to nail down average costs of a solar PV system. They seem to have risen in recent years before falling back a little recently – but data is sparse. Plenty of websites claim that a 4kWp system costs between £5,000 and £6,000, but the database of approved installers under the Microgeneration Certification Scheme (MCS) suggests the current cost is over £2,000 per kWp.

In theory, it should cost less to install a solar pv system if you’re building a new house than if you’re retrofitting the panels on an existing one, as scaffolding is already in place and you have ready access to wiring routes etc. Built-in solar PV systems (BIPV), where the collectors form part of the roof covering, should offer a small saving in deducted roof finish materials to offset the capital cost of the system.

It’s also worth bearing in mind that the inverter is expected to need replacing at least once during the solar PV system’s lifetime. Again, data is sparse but it’s wise to factor in this additional cost from the outset.

How Long Will It Take to See a Return on Investment on My Solar PV System?

The amount of time it takes for an investment in a solar PV system to pay back through reductions in energy bills is known as the payback period. Of course, there’s a difference between cash spent now and money saved in the future – you’d rather have £1 now than £1 in 10 years’ time, because that money will go further today.

Future savings should therefore be discounted to arrive at the present value to compare them with capital cost. Using the Treasury’s current Green Book rate of 3.5%, £1 in a decade’s time should be treated as the equivalent of 70p now. So, the value of future payments will gradually diminish over time. After 20 years, the value is less than half. However, electricity prices are expected to rise gradually, so this might well offset the interest rate.

As such, payback times are normally expressed in terms of simple payback – cost divided by savings based on the current price of electricity. This sum is dependent on:

• How much energy your solar panels produce (kWh) for each kWp of capacity (assuming the array is positioned at the optimal angle and orientation).
• How much of your self-generated electricity you use at home, displacing grid consumption, and how much is exported to the grid.

• The amount that your energy supplier pays for exported electricity.
• Whether or not your solar PV system is combined with battery storage.

If you’re out most of the day and don’t have battery storage to store the electricity generated by your solar PV system, the bulk of the energy produced is exported to the grid. It’s not straightforward to say how much you’ll be paid for this, as the Smart Export Guarantee scheme only stipulates the rate should be above zero. To get the best tariffs there are many conditions attached, and rates can vary according to time of day. Generally, the export rate is much lower than what you pay to buy your electricity in.

So, in the example (below), with no allowance for the cost of inverter replacement, the payback period would be about 15 years. After that, you start to see a net return against your initial outlay. This can be compared to the average life of a PV system, commonly quoted as 25 years. Therefore, you can expect the system to pay back within its lifetime. Factor in inverter replacement and the payback would be only just within the expected lifetime.

How Long Will Your Solar PV System Last?

The power output generated by any solar pv system falls slightly each year, but this drop is thought to be less than 1% per annum. The rate of degradation may differ between manufacturers and solar panels, so it’s essential to check independently verified data on long term performance before deciding which solar PV solution best suits your home. At 0.8% per annum, the fall in output would only be 18% by year 25, but this still has a modest impact on the payback equation.

Additional content by Build It expert, Nigel Griffiths. This article has been updated in February 2025.