It is one of the most common questions we hear from Devon homeowners, and the honest answer is: it depends. The number of panels your home needs is shaped by how much electricity you use, how your roof faces, whether you plan to add a battery, and whether your future energy use will change with things like an electric vehicle or a heat pump.
This guide gives you the framework to understand the answer, with Devon-specific figures throughout.

Start with your electricity use, not your bedroom count

Bedrooms are a rough proxy, but your electricity bills are a much better guide. The standard UK figure for a three-bedroom home with two or three occupants is around 2,700 to 3,300 kWh per year. In practice, a three-bedroom Devon household with people working from home, an electric shower, and a tumble dryer might use closer to 3,500 kWh, while a couple in the same house with more modest habits might use 2,500 kWh.

According to Ofgem’s Typical Domestic Consumption Values, the average UK medium household uses around 2,700 kWh of electricity per year. That is the baseline, but your own 12-month total from your bills or smart meter is always the more useful number.

The reason this matters for sizing is straightforward. A system sized to cover 2,500 kWh of annual use will leave a 3,500 kWh household buying significantly more grid electricity than expected. Getting this figure right at survey stage is one of the most important things your installer should do.

What system size does a three-bedroom Devon home typically need?

For a household using 2,700 to 3,300 kWh per year with a good south or south-west facing roof, a 3.5 to 4.5 kWp system is usually the right starting point.

A typical 3-bedroom UK home needs 10 to 12 panels at around 4 kWp to cover most of its electricity usage. With modern panels now commonly rated at 400 to 450 watts each, that 4 kWp system translates to between 9 and 11 panels depending on the specific panel your installer specifies.

A useful way to think about it is this:

Smaller three-bed, two occupants, lower usage (around 2,500 kWh/year): 8 to 10 panels, around 3 to 3.5 kWp
Average three-bed, two to three occupants (around 2,700 to 3,300 kWh/year): 10 to 12 panels, around 4 to 4.5 kWp
Larger three-bed, higher usage or working from home (around 3,500 kWh/year): 12 to 14 panels, around 4.5 to 5 kWp

These are starting points for a south-facing roof with minimal shading. Your actual system may differ once roof orientation, any shading from chimneys or trees, and roof space have been assessed at survey stage.

How Devon’s sunshine changes the picture

This is where Devon homeowners have a genuine advantage over most of the country.

Devon receives substantially more sunlight than the great majority of UK counties, with Exeter receiving 1,237 kilowatt-hours per square metre on average per year, which is 13% above the national average. Sunsave Exeter sees 1,552 hours of sunshine annually, with peak months like May averaging 6.6 hours of daily sunshine.

In practical terms, a 4 kWp system in Devon generates more electricity each year than the same system would in the Midlands or the North of England. A 4 kWp solar panel system in the UK will produce an annual output of around 3,400 kWh in average UK irradiance, and your system will generally produce more in a sunnier place like Devon or Cornwall.

A well-specified 4 kWp system on a good south-facing Devon roof typically generates somewhere between 3,200 and 3,800 kWh per year. For a household using around 3,000 kWh annually, that means the panels are generating close to or more than the home’s total electricity use across the year, even before accounting for any battery storage.

The important caveat is that solar generation and household consumption do not happen at the same time. Panels generate most of their electricity between 10am and 4pm on sunny days. Most households use more electricity in the mornings and evenings. Without a battery, a significant proportion of what the panels generate leaves the house as export to the grid, rather than being used on site.

Does roof orientation matter as much as people think?

South-facing is ideal, but it is not the only option that works well.

A south-facing roof at a pitch of around 30 to 40 degrees captures the most solar radiation over the year. East or west-facing roofs produce around 15 to 20% less annually than a south-facing equivalent. That is meaningful, but it does not make a system financially unviable in Devon, where the irradiance advantage absorbs a good deal of that difference.

An east-west split across a ridged roof, with panels on both sides, can actually improve self-consumption by spreading generation more evenly across the day, reducing the midday peak and extending the generation window into early morning and late afternoon. For households at home during the day, this can work particularly well.
North-facing roofs are generally not suitable. The output drop is too significant to justify the installation cost.

Shading is the factor that catches people out most often. A chimney stack, a dormer window, or a nearby tree that shades even part of the roof for a few hours per day can reduce output disproportionately. This is why a proper shading assessment at survey stage is not optional, it is essential.

Should you add a battery, and how does it affect the system size?

Adding a battery does not change how many panels you need, but it changes how much of what those panels generate you actually use yourself.
Without storage, a typical household self-consumes 30 to 50% of what their solar panels generate. With the right battery size and smart charging, many homes reach 60 to 85% self-consumption.

The electricity you use from your own panels is worth considerably more than electricity you export to the grid. You are saving the full unit rate on every kilowatt-hour you self-consume. Electricity exported under the Smart Export Guarantee earns a fraction of that. So increasing self-consumption from 35% to 70% has a significant impact on the financial return from your system.

For a typical three-bedroom Devon home, a battery in the range of 5 to 10 kWh is usually the right starting point. A 5 kWh battery suits a household with moderate evening usage and a smaller 3 to 4 kWp system. A larger 10 kWh battery suits households with higher evening demand, or those pairing solar with an EV charger or planning to add a heat pump. For UK homes with 4 to 6 kWp solar panels and typical usage, a 5 to 10 kWh battery offers the best balance of additional savings to additional cost.

Devon’s long summer days mean panels often generate a significant surplus during peak months. A battery captures that surplus and puts it to use in the evenings, which is particularly valuable in a county where power cuts in rural areas are a real consideration. A battery-backed system with the right configuration can continue to supply essential circuits during a grid outage, a feature worth asking your installer about at the design stage.

One practical note: if you are considering a battery now or in the future, it is worth discussing inverter choice at the outset. A hybrid inverter is designed to connect battery storage from day one, or to add it later without significant additional equipment. A standard string inverter is less expensive up front but can be more costly to retrofit for storage later.

What changes if you have an EV or a heat pump?

Both add substantially to your electricity use and change the system sizing calculation.

A heat pump for a three-bedroom household typically adds around 3,200 kWh to annual electricity consumption, while an electric vehicle and charger adds around 2,000 kWh on average.

If you already have an EV or plan to get one in the next few years, sizing a system around your current electricity use alone will leave you short. A household using 3,000 kWh today that adds an EV and a heat pump over the next three years could be using 8,000 kWh or more annually. A 4 kWp system sized for today’s usage will cover a far smaller proportion of that future demand.

The right response is not necessarily to install the largest possible system now. It is to design a system that accounts for your likely future use and uses an inverter that can accommodate additional panels or battery capacity later. This is a conversation to have at survey stage, and a good installer will ask about your plans rather than simply sizing for current consumption.

A quick self-check before getting a quote

A few questions worth thinking through before you book a survey:

Do you know your annual electricity use in kWh? It will be on your bill or smart meter data.
Which direction does your main roof face?
Are there any chimneys, dormers, or trees that shade the roof at any point during the day?
Do you have or plan to get an EV, a heat pump, or any other high-consumption equipment in the next few years?
Do you use more electricity in the evenings than during the day?

If you can answer most of these, you will have a much more productive conversation with your installer and are less likely to end up with a system that is too small, or oversized in ways that add cost without adding proportionate value.

Get a free Devon solar survey from M&E Contrax

M&E Contrax is an MCS-accredited, NICEIC-approved solar installer based in Newton Abbot. We design and install solar panel systems for Devon homes and businesses, with every system sized to your actual usage, your roof, and your plans for the future. All electrical installation work is carried out by our in-house NICEIC-accredited engineers.

If you want a clear, honest assessment of what solar could do for your home, we offer a free no-obligation survey and quote. No national call centres, no generic packages.

MCS Accredited | NICEIC Approved Contractor | Based in Newton Abbot, Devon | Full support and aftercare