4kW solar panel systems: an expert guide

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 the solar battery you choose.

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 quick questions below and we'll generate an estimate for you.

What is a 4kW solar panel system?

A 4kW solar panel system has a peak power rating of four kilowatts, meaning it would produce 4,000 kilowatt-hours (kWh) of electricity per year in standard test conditions.

You can build a 4kW system by purchasing solar panels with output ratings that add up to 4,000 watts (W) – for instance, 10 panels that are all rated at 400W.

This doesn’t mean your system will automatically produce 4,000kWh, as solar panel output depends on factors like your location, roof angle and direction, and the quality of the gear.

You may also see a 4kW system referred to as a 4kWp (kilowatt peak) system. In this context, they mean the same thing.

How many solar panels are in a 4kW system?

There are nine solar panels in a 4kW system, if you buy 430W panels.

The number of solar panels you’ll need to install a 4kW system will completely depend on your panels’ peak power ratings, though.

For instance, if your chosen installer has 350W solar panels in stock, you’ll need 11 panels.

Purchasing panels with higher peak output ratings can reduce the number of panels you need, which is a good solution for households with limited roof space.

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

You should usually add a 5-6kWh battery to a 4kW solar panel system.

This will allow you to store your excess solar energy all year round, to use on cloudy days and after the sun goes down.

You’ll hold onto more of the plentiful energy your solar panels produce in spring and summer, and make the most of the electricity they generate in autumn and winter, to minimise wasted energy.

The size of battery you need will depend on several factors though, including how much electricity your household uses per day, and whether you use more in summer or winter.

If you have air conditioning, for instance, you may consume more of your solar electricity in summer than most homes, which could mean you won't make much use of a large battery.

Your battery’s size will also depend on how long you spend at home during the daytime. For example, if your household spends most days in an office or at school, you may need a bigger battery, since you won’t be using most of the solar electricity as it’s produced.

To learn more, check out our guide to solar battery sizing.

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

Your solar panel system should be 50% bigger than your inverter, as a rule – so for a 4kW system, you’ll typically need a 3kW inverter.

This is because in the UK, your solar panels won’t usually reach their peak power rating, due to our weather generally falling short of standard test conditions.

Standard test conditions include a cell temperature of 25°C and solar irradiance of 1,000W per m², and is what companies use to check a solar panel’s capabilities.

A 4kW system in this country can still save you hundreds of pounds with the amount of electricity it produces, but won’t produce much more than 4,000kWh per year in the sunniest parts of the UK, on optimum roofs.

And it’s more efficient to pick an inverter size that matches your system’s actual output, rather than its idealised levels.

Since an inverter requires power to start working, called its 'startup voltage', it should always be at this level, at least – which means undersizing the inverter is also safer.  

With this knowledge, you can discuss and understand the reasons behind your inverter’s size with your installer, though you should ultimately leave the decision with them.

How much does a 4kW solar panel system cost?

A 4kW solar panel system costs around £9,500 to buy and install.

If you want to include a battery in the installation, this will add around £2,000 to the price, for an overall cost of £11,500.

This amount can vary significantly though, depending on factors including the installer you choose, where you live, the type of roof you have, and the current state of the industry.

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

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

A 4kW solar panel system in the UK will produce an average annual output of around 3,400kWh, if it's dealing with typical UK irradiance.

This means you’ll usually produce roughly 85% of your system’s peak power output.

Your system will generally produce lower amounts of solar electricity in places with less sunshine, like Scotland or northern England, or more if you live in relatively sunny areas, like Cornwall, Essex, Kent, and Sussex.

For instance, a 4.3kW solar panel array we designed for an Exeter home has an estimated total output of 4,811kWh, while a 3.87kW system we designed for a household in Bristol would produce approximately 3,618kWh per year.

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 9.3kWh of solar electricity per day, on average.

This amount of electricity can power all of the following devices for the stated amount of time, according to Centre for Sustainable Energy data – and still have 1kWh left over.

Is a 4kW solar panel system enough?

A 4kW solar panel system is enough if it roughly matches your annual electricity consumption.

However, you should always look to get as large a solar panel system as possible, if you can afford to.

The fixed prices of scaffolding and labour usually make up a large part of your costs, so adding more panels doesn’t push up the overall amount by much, which means it makes financial sense to maximise your roof space – unless it’d require more scaffolding.

Installing a bigger system and signing up to one of the best export tariffs will help you to save more money on your electricity bills – and could prepare you for potential increases in your usage.

If you’re thinking of buying a heat pump, getting an electric vehicle, or adding to your family at any point in the future, you may want to install a bigger solar panel system.

After all, it’ll cost more overall if you add to your solar panel system at a later date, since you’ll have to pay the additional costs – like scaffolding – all over again.

If you're interested, read our guide to a 5kW solar panel system.

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

You’ll need 28.8 square metres 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 40 cm gap between the panels and the sides of your roof, and a 3 cm space between each panel, due to Microgeneration Certification Scheme (MCS) guidelines.

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 0.81 m², for a total of 29.6 m².

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.

If it is, it’ll bring you various benefits. You’ll cut your electricity bills by 103%, on average, which means across a year you actually earn more than you spend. This might sound too good to be true, but it’s all because of the high performance of modern solar panels and storage batteries, as well as export income.

The panels will dramatically reduce the amount of electricity you buy from the grid, and you’ll also earn money by selling your unused electricity to the grid. Octopus Energy's best export tariffs have transformed the financial benefit of solar panels, and it's easy to sign up for one.

This 103% figure is based on a household experiencing average UK irradiance with a 4.4 kilowatt-peak (kWp) solar panel system and a 5.2 kilowatt-hour (kWh) battery, using 3,500kWh of electricity each year and signed up to the Intelligent Octopus Flux export tariff.

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.

However, 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.

This won’t typically be the case with a 4kW installation, as your solar panel system should be 50% bigger than your inverter – but some installers will recommend a larger inverter, which would trigger this 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.

Can you get an off-grid 4kW solar panel system?

You can definitely get an off-grid 4kW solar panel system installed, and it can supply a large chunk of the electricity you need.

As you’re not able to export excess energy to the grid, you’ll need a large battery to hold onto as much electricity as possible.

Depending on your location and energy consumption, an 8-10kWh battery will probably suit your needs.

In any case, you may need to supplement a 4kW system with other renewable sources, like wind.

A 4kW solar panel system will only supply you with enough electricity to live off-grid if you’re prepared to keep a close eye on your consumption and use a lot less energy in winter.

Summary

A 4kW solar panel system is a standard size for a household with three or four bedrooms, and can massively cut your electricity bills.

However, most homes don’t align with ‘the average’, and the size of your system should depend on your current and future electricity consumption, not industry averages.

The best way to get exactly the right size of solar panel system for your home is to hire a solar panel company with the relevant expertise and abilities, like Sunsave.

If you’re interested in how much you could save with a solar & battery system, simply answer a few quick questions below, and we’ll calculate an estimate.

4kW solar panel system: FAQs