When you’re looking to buy a solar panel system, you want to make sure it’s the right size for your home’s needs.

A 4kW solar panel system is often the right choice for a three-bedroom household, but it depends on your present and future consumption, as well as your roof size.

In this guide, we’ll explain what a 4kW solar panel system is, how much it costs, and how many devices it can power.

If you would like to see the savings you could get from a solar & battery system, just answer a few questions below, and we’ll provide an estimate.

How many solar panels are in a 4kW system?

The number of solar panels in a 4kW system depends on the size of the panels themselves.

If you have a 450W panel, it will produce 450 watt-hours in standard test conditions, which includes a cell temperature of 25°C and solar irradiance of 1,000W per m², and is how every company checks a solar panel’s capabilities.

If you have nine of these 450W panels, you'll have a 4.05kW system, which produces 4,050kWh per year under these conditions.

The higher your panels' peak output ratings, the more electricity your panels will produce, everything else being equal – and the more money you'll save.

This table shows how many panels you’d need (of different panel sizes) to create a system that is at least 4kW.

How much does a 4kW solar panel system cost?

A 4kW solar panel system costs around £6,488 to buy and install, according to the most recent government data.

The mean cost per kW for 4-10kW systems is currently £1,622.

If you added a 10kWh battery to this system, it'd cost around £4,000 more, for a total price of £10,488.

We discuss battery sizing for a 4kW system further down.

The amount you'll pay can vary significantly though, depending on factors including the installer you choose, where you live, and the type of roof you have.

If you want to learn more, check out our full guide to solar panel costs.

And if you would like to see the savings you could get from a solar & battery system, just answer a few questions below, and we’ll provide an estimate.

How much energy will a 4kW solar panel system produce?

A 4kW solar panel system in the UK will produce an annual output of around 3,600kWh, in average UK irradiance. This is based on Dunsop Bridge, which is the geographic centrepoint of the British Isles, and sees irradiance of about 900kWh per kW.

This means you’ll usually produce roughly 90% of your system’s peak power output – but as you can see above, these figures vary significantly, depending on where you live.

Output can even vary in the same area, because there are many other factors at play beyond location, including the direction and angle of your roof, the amount of shading present, and how clean your panels are.

To get an accurate idea of how much solar electricity your home can generate with a 4kW rooftop system, speak to a top solar panel installer.

What can a 4kW solar panel system run?

A 4kW solar panel system can run the average three-bedroom household, on a typical day.

It can generate 3,600kWh of electricity per year in the UK, which is more than the national average of 3,400kWh.

This adds up to 9.9kWh per day, on average.

With this much electricity, you can power all of the following devices for the stated amount of time, according to Centre for Sustainable Energy data – and still have some left over.

What size inverter do you need for a 4kW system?

Installers usually choose an inverter with 80% to 90% of the array's capacity – so for a 4kW solar panel system, your inverter will probably be between 3.2kW and 3.6kW.

You won't have to work it out yourself, though. A good installer will choose the right inverter for your home.

This is important, since inverters turn the direct current (DC) electricity produced by your panels into alternating current (AC) power which you can use in your home and sell to the grid.

Its size directly affects how much electricity it can feed to your home, which determines the amount of power you can use at any one time.

Your inverter's size also dictates whether you'll need a G98 or G99 certificate (more on this below).

What size battery should you add to a 4kW system?

You should base the size of your battery on your electricity usage, rather than the size of your solar panel system.

A 9-10kWh battery is suitable for the average UK home, which uses 3,400kWh of electricity per year – or roughly 9.3kWh per day.

This approach to sizing is better, largely because home batteries shouldn't just charge up on solar electricity anymore.

New offerings like time-of-use tariffs have made it possible to supplement this free power supply with cheap grid electricity that keeps your bills low even when the sun isn't shining.

An installer should take your household's usage into account and discuss your needs before they pick a battery size.

Generally speaking though, the more energy-intensive devices you have, the larger your battery should be.

For instance, a portable air conditioning unit typically adds 3.4kWh per day, while a heat pump usually adds 8.8kWh.

For more information, read our guide to solar battery sizing.

How much space do you need for a 4kW solar panel system?

You’ll need 28.8 square metres (m²) of roof space for a 4kW solar panel system, on average.

This takes into account the average height and width of a solar panel, which is around two square metres, as well as the extra spaces installers usually leave.

Your solar engineer will typically make sure there’s a 40cm gap between the panels and the sides of your roof, and a 3cm space between each panel, to match standards created by bodies like Flexi-Orb and MCS.

If you have a chimney, vent pipe, or skylight (also known as a Velux window), you’ll need a bit more space – but not too much.

If you have all three, you’ll usually need another 0.81m², for a total of 29.6m².

Do you need planning permission for a 4kW solar panel system?

You don’t usually need planning permission to install solar panels on a domestic property, including 4kW systems.

Solar installations on households are almost always protected by permitted development rights, which allow homeowners to make medium-sized improvements without planning permission.

You’ll normally only have to apply if you live in a flat, listed building, or a conservation area – or if you want to get a ground-mounted installation.

Do you need DNO permission for a 4kW solar panel system?

Your installer will need to submit a G99 application to your region’s Distribution Network Operator (DNO) if your inverter’s maximum capacity is over 3.68kW on a single-phase system.

If your inverter is smaller, your installer can finish the job, then send a completed G98 application to your DNO within 28 days. This is a notification, rather than a request for permission.

With a 4kW installation, your inverter will usually be slightly below the G99 limit – but some installers will recommend a larger model, which would trigger the requirement.

If it is needed, a G99 application should be a harmless formality that simply ensures the DNO is aware of your system, and able to use that information to run its grid properly.

Once your G99 form is accepted – as most are – you should be able to use your inverter at its full capacity.

Is a 4kW solar panel system worth it?

A 4kW solar panel system is absolutely worth it, as long as it’s the correct size for your household.

Whether it's right for you will depend on your usage, budget, and location, as well as your roof's angle, direction, and available space.

But if your roof has enough space for a 4kW system and no more, it’s almost certainly worth getting it.

A 4kW system can help you cut your energy bills by hundreds of pounds, and if you get a battery, you could save even more.

You could take advantage of new tariffs made for solar and battery owners, join a virtual power plant, and sign up to one of the best export tariffs.

For more information, read our guide Are solar panels worth it?

And if you'd like to see the savings you could get from a solar & battery system, just answer a few questions below, and we’ll provide an estimate.

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