What size solar panel do I need?

According to Energy Saving Trust, installing solar panels on your home’s roof could save you up to £545 on your energy bill in 2024 and, depending on how much clean energy your home is producing, earn you up to £642 per year when you sell excess electricity to the grid.

But before you can start seeing those savings, you’ll need to decide which solar panel size and wattage is right for you. Otherwise, you could end up with a system that’s either more expensive or larger than you need or too weak to supply your household with the right amount of green energy.

The typical solar panel in the UK is 350W, which can produce up to 1,128.75Wh of electricity per day – enough to cover almost a sixth of the average UK home’s electricity needs by itself. However, solar panels come in a range of different sizes, with levels of efficiency and power outputs that vary accordingly. So how do you pick the right size and wattage for your home?

Below, we’ll walk you through how big solar panels are, how much electricity they provide and how much roof space you’ll need. We’ll explain what solar panel wattage is, look at how it relates to the interlinked – but distinct – concepts of size and efficiency and even help you calculate exactly how many solar panels your home will need to get started.

How big are solar panels?

You should note that when this guide talks about a solar panel’s size, it’s referring to its physical measurements – its dimensions. As such, solar panel size shouldn’t be confused with solar array (or, if you prefer, solar system) size.

Solar system is the term for a collection of solar panels, rather than the panels individually, and in the UK and abroad, the size of a solar system is often denoted by its power output, which is measured in watts (W) – not in the metres or centimetres of its physical dimensions.

So, if you ask a solar panel installer or supplier in the UK what size your solar array or system should be, they’ll often quote you on the wattage rather than the exact physical dimensions. They’ll be using solar system “size” to refer to the combined total of each solar panel’s wattage or power output.

In the UK, a standard 350W residential solar panel is around 1.89m long, 1m wide and 3.99cm thick and contains approximately 60 solar cells.

This means that a 350W solar panel will take up around 1.89m² of roof space – although more efficient panels can be smaller but produce the same amount of power.

What is solar panel wattage?

Solar panel wattage refers to the amount of power a solar panel can generate under standard test conditions (STC). Measured in watts, solar panel wattage refers to the maximum power output a solar panel can produce when exposed to sunlight.

Common domestic solar panel wattages in the UK include 250W, 300W, 350W and 400W, but even higher wattages are available – and all the best solar panels are at least 400W. (The most powerful solar panel we recommend, the JA Solar JAM72S30 Mono PERC Half-Cell MBB, has a power output of between 525W and 550W.)

Understanding solar panel wattage is vital to picking a solar panel powerful enough to meet your home’s electricity needs.

A 250W panel should, under ideal conditions, produce 250 watt-hours (Wh) for every hour of sunlight it receives. If you multiply 250W by the average daily sunlight hours in the UK – which across all 12 months in 2023 was 4.3 – you get 1,075Wh per day. But you also need to take into account sunlight fluctuations and suboptimal orientation, so to do that, you can multiply 1,075Wh by a modifier of 0.75. Therefore, a 250W panel should generate about 806.25Wh in a day.

The wattage of solar panels and how much roof space is needed

Solar panel wattage	Daily electricity provided	Roof space required (m²)
200W	645Wh	1.30
250W	806.25Wh	1.37
300W	967.5Wh	1.62
350W	1,128.75Wh	1.89
400W	1,290Wh	1.92
450W	1,451.25Wh	2.17
500W	1,612.5Wh	2.55

Given that the average household electricity usage in the UK is 7.5kWh (kilowatt-hours) per day for a medium-use household, you’d need at least 10 250W panels (or fewer panels with a higher wattage) to meet those needs. But what would the power output of a full domestic solar array be?

As they’re made up of multiple solar panels (and, as such, generate a lot of power), solar arrays or systems are measured in kilowatts (kW), with 1kW = 1,000W.

How much electricity your home is capable of generating based on solar system size (wattage if using 350W panels)

Solar system size	Yearly electricity production*	Household size	Roof space required	Number of panels
3kW	2,199kWh	One to two people	15m2	8
4kW	2,826kWh	Two to three people	22m2	12
5kW	3,532kWh	Three to four people	30m2	16
6kW	4,238kWh	Four to five people	37m2	20

What factors affect solar panel wattage?

Some of the factors that affect the wattage of a solar panel include:

• Cell efficiency: expressed as a percentage, this is the proportion of the sun’s light a panel is able to convert into electricity. Cell efficiency is closely related to the type of solar panel, with certain varieties more efficient than others (monocrystalline solar panels, for example, have an efficiency of 15 to 22 per cent, whereas thin-film silicon solar panels have an efficiency of 7 to 10 per cent)
• Type of solar cell or panel: advancements in solar technology – such as PERC (Passivated Emitter and Rear Cell), bifacial cells and perovskite cells – enable smaller panels to produce as much or more green energy as those with larger surface areas
• Number of cells: the number of cells a solar panel contains will affect its wattage. Panels with more cells can generate higher voltages and currents, resulting in more impressive overall wattage outputs. Panels that contain more cells will also have a larger surface area

How does solar panel size affect wattage?

A solar panel’s physical size tends to strongly correlate with its wattage. As a general rule, larger solar panels have higher power output than smaller ones. 

This is because larger solar panels have more surface area, meaning they can accommodate more solar cells. Since solar cells are responsible for capturing sunlight and converting it into electricity, the equation is simple: more cells = more power.

That said, you don’t always have to buy the solar panels with the largest physical dimensions to get the most power – as we’ve discussed, efficiency also plays a role. A smaller panel with more efficient cells can produce as much power as a larger panel with cells that aren’t as efficient. So, while the physical size of a panel does matter, it’s not the be-all and end-all – which is why a solar panel’s wattage and efficiency are more reliable indicators of its performance and capabilities than its physical dimensions.

Does solar panel size impact efficiency and power output?

Yes, solar panel size – that is, its physical dimensions – has an impact on efficiency. In fact, a solar panel’s efficiency can only be calculated by factoring in the amount of space it takes up.

For example, a solar panel rated at 400W with a surface area of 2m² will have an efficiency rating of 20 per cent ([0.4kW/2m²] × 100 = 20 per cent). So a panel’s efficiency is dictated by both its physical dimensions and its power output.

This means that panels with the highest wattage – vis a vis their size – will be the most efficient. So why is panel efficiency so important?

Well, the more efficient your panels are, the more electricity they produce for your home. And the more electricity your home produces, the less you’ll have to rely on the National Grid for the bulk of your household’s energy needs – which means your electricity bills will be lower. Better still, maximising your home’s renewable energy production means you can take advantage of the UK’s solar panel grants – namely, the Smart Export Guarantee (SEG).

Effectively, the SEG allows you to sell any excess electricity your home’s solar panels generate back to the grid for a profit – thus augmenting the savings you’re already getting from lower electricity bills and reducing the amount of time it’ll take to break even on your solar panel installation.

How much a solar panel system could earn using SEG

System size	Estimated installation cost	Energy generated per year	SEG payments (15p/kWh)	SEG payments (10p/kWh)	SEG payments (5p/kWh)
3kW	£4,957	2,199kWh	£243	£162	£81
4kW	£5,942	2,826kWh	£342	£228	£114
5kW	£6,928	3,532kWh	£441	£294	£147
6kW	£7,913	4,238kWh	£540	£360	£180
7kW	£8,899	4,945kWh	£642	£428	£214

How big should my solar array be?

To calculate how large your solar array should be, you’ll first need to crunch some numbers to find out how many panels you’ll need.

To do this, you’ll need a recent electricity bill to understand how much electricity your home currently consumes per day. As an example, take the average figure for the UK as the starting point. According to OVO Energy, the figure for a medium-use household in 2023 was 7.5kWh, or 7,500Wh, per day.

Next, you need to work out how much of your electricity needs you want your solar system to cover. You might want the system to cover only 75 per cent or even 50 per cent if you’re on a tighter budget. But for this example, assume you want to reduce your electricity bill as much as possible, so you’d opt for a solar array that will meet the entirety – that is, 100 per cent – of your home’s electricity needs.

Now you need to determine how much sunlight your home gets per day. This will vary depending on where in the UK you live, so you can explore our regional guides for more details:

• Solar panels in Wales
• Solar panels in Northern Ireland
• Solar panels in Scotland

For this example, we’ll go with the average daily sunlight hours in the UK: 4.3. However, you should also incorporate a 0.75 modifier to account for any variables that could affect the year-round performance of your solar panels – such as shade or suboptimal panel orientation – and to reflect that their real-world performance won’t ever meet that achieved under STC.

Finally, it’s time to utilise your new knowledge of solar panel wattage and pick a solar panel with a power output that’s appropriate for your home. As you know, solar panels come in various wattages, but for this example, let’s choose a robust 400W system.

Now, you have all the information you need to calculate how many solar panels your home will need. First, calculate the output of one panel:

Solar panel wattage x sunshine hours x modifier = solar panel power output

Plugging in the figures from our hypothetical example, you get:

400W x 4.3 x 0.75 = 1,290Wh

Now, all you need to do is divide your home’s daily electricity needs by this figure to find out how many solar panels you’d need to power your home. 

Electricity needs % solar panel output = how many panels requires

So, for our hypothetical household, you get: 

7,500 % 1,290 = 5.81

The household will need at least six 400W solar panels. 

Explore how the UK’s average daily electricity needs, 7,500Wh, can be generated based on different levels of solar panel wattage

Solar panel wattage	Daily electricity provided	Panels required	Panel size (m²)	Roof space required (m²)*
200W	645W	12	1.30	15.6
250W	806.25W	10	1.37	13.7
300W	967.5W	8	1.62	12.96
350W	1,128.75W	7	1.89	13.23
400W	1,290W	6	1.92	11.52
450W	1,451.25W	6	2.17	13.02
500W	1,612.5W	5	2.55	12.75

As we’ve discussed – and as the table demonstrates – the more powerful the panel, the fewer you’ll need to satisfy your home’s electricity needs. (But remember that the number of panels needed will also depend on the efficiency of the panel you choose.) Before you proceed on the basis of solar panel wattage alone, you’ll also need to factor in solar panel size – that is, the physical dimensions – and make sure there’s enough space on your home’s roof to support the appropriate solar array.

Remember, one standard-sized 350W solar panel takes up 1.89m² of precious roof real estate, with more powerful panels hogging yet more – so it’s a good idea to make sure you have between 25m² and 35m² of roof space available before scouring the market for solar panel quotes. (After all, you may want to generate more electricity than the average UK home demands and sell what you don’t use back to the grid for a profit.)

When you’re ready to start comparing, our guide to solar panel costs will help – as will our free quote-finding service.