If you’re thinking about installing solar panels on your roof, this is usually the first question that comes up: what size system do I actually need?
The honest answer is that it depends on your usage, not your roof. Most people start by wondering how many panels will fit on their roof, but the better starting point is how much electricity your household actually consumes. This guide walks you through how to size a system properly, without guesswork.
Start With Your Electricity Usage
The most useful number to find before doing anything else is your annual electricity consumption in kWh. You’ll find this on your electricity bill or through your online account with your supplier. If you have a smart meter, your in-home display or supplier app will show it clearly.
Electricity usage is measured in kilowatt hours (kWh). To put it in practical terms: a 3kW immersion heater running for one hour uses 3kWh of electricity. Your annual total is the sum of every appliance in your home running over the course of a year.
Typical UK household consumption falls roughly into these bands:
- Small homes (1 to 2 people): 1,800 to 2,700 kWh per year
- Medium homes (3 to 4 people): 3,000 to 4,500 kWh per year
- Larger homes or higher usage: 5,000 kWh and above
If you run an electric vehicle, an air source heat pump, an immersion heater, or a hot tub, your consumption will sit at the higher end or well above it. These loads matter a great deal when sizing a solar system and are worth accounting for from the start.
Understand What Solar Actually Produces
A solar PV system is rated in kilowatt peak (kWp), which represents its maximum electrical output under ideal conditions - full direct sunlight, optimal angle, no shading. A 5kWp system can generate up to 5kW at any given moment, but in practice it will rarely hit that peak continuously.
What matters more for sizing purposes is annual output. In the UK, a reliable rule of thumb is:
1 kWp of solar generates approximately 850 to 1,050 kWh per year
The actual figure for your home will depend on your location, roof direction, and any shading. South-facing roofs in southern England sit toward the top of that range. North-facing roofs in Scotland sit toward the bottom, but still generate meaningful electricity.
Estimated annual generation for south-facing systems at 35° pitch. Source: PVGIS (European Commission Joint Research Centre).
Using the middle of the range (950 kWh per kWp), a few common system sizes work out roughly as follows:
- 3 kWp generates around 2,700 to 3,100 kWh per year
- 4 kWp generates around 3,600 to 4,200 kWh per year
- 5 kWp generates around 4,250 to 5,250 kWh per year
- 6 kWp generates around 5,100 to 6,300 kWh per year
Match System Size to Your Usage
A straightforward starting point for sizing is:
System size in kWp ≈ your annual usage in kWh ÷ 1,000
So a household using 4,000 kWh per year would start by looking at a roughly 4kWp system. This gives you a system that broadly matches your annual demand, though how much of that generation you actually use depends on when you’re at home and how you use electricity during the day.
Decide What Coverage You’re Aiming For
Not everyone should aim for a system that matches 100% of their annual consumption. It’s worth thinking about what outcome you’re actually trying to achieve.
50 to 70% coverage is the most common target for UK homeowners. A system sized this way tends to offer a strong return on investment because you’re using most of what you generate directly, with less surplus being exported at lower rates. The upfront cost is lower and the payback period is typically shorter.
Around 100% coverage is achievable, but comes with higher upfront cost and often means generating more than you can use during summer months. A battery helps here, allowing you to shift surplus daytime generation into evening use rather than exporting it. Without a battery, a larger system that regularly oversupplies your home delivers diminishing returns.
Oversizing deliberately makes sense if you’re planning to add an electric vehicle, a heat pump, or a battery in the future. Sizing the system for your anticipated future usage rather than today’s baseline avoids having to add panels later when the cost of scaffolding and installation would otherwise be duplicated.
Consider Your Roof
Your roof may put a practical cap on system size regardless of what your usage suggests. As a rough guide, each kilowatt peak of solar requires four to five panels, and each modern panel occupies around 1.7 to 2 square metres.
Based on modern 400W panels at approximately 1.8m² each. Actual figures vary by panel model and roof layout.
South-facing roofs at a pitch of around 30 to 35 degrees get the most from their panels over the course of a year.
East and west-facing roofs generate less overall but can provide a more even spread of generation across the day, which has its own advantages for self-consumption. Their generation profiles of early mornings and late afternoons often align with household demand for electricity which could mean less of a need to import from the grid during peak periods.
North-facing roofs are the least efficient, but still worth considering on a large enough roof particularly if the rest of your usage picture makes sense.
If your roof has shading from chimneys, dormer windows, trees, or neighbouring buildings, this is a significant factor. Shading affects output more than most people expect and should be assessed properly by any installer you speak to.
Think About When You Use Electricity
This is where a lot of sizing guides fall short. Solar doesn’t just depend on how much electricity you use - it also depends on when you use it.
A household where someone is at home during the day will use a higher proportion of their solar generation directly, which is worth roughly 24 to 25p per unit saved on your electricity bill. A household where everyone is out during the day will export more of their generation to the grid, earning perhaps 10 to 15p per unit under a Smart Export Guarantee tariff. The difference in value between those two outcomes is significant over the life of a system.
If your household is largely out during the day, a battery becomes more important to the economics of solar — it allows you to capture daytime generation and use it in the evening rather than exporting it at a lower rate.
Common Mistakes to Avoid
Not making full use of available roof space. Panels themselves are relatively affordable compared to the fixed costs of scaffolding, roof preparation, and electrical work. If the roof can accommodate more panels than your current usage strictly requires, it often makes financial sense to install them particularly if you’re likely to add an EV or heat pump in the next few years.
Assuming export tariffs will always be available at current rates. Some homeowners size systems primarily to maximise export income. Smart Export Guarantee rates have been reducing over time and cannot be relied upon as a long-term income stream in the way the old Feed-in Tariff was. Self-consumption is the more reliable driver of value.
Ignoring future electricity usage. If an EV or heat pump is on the horizon, factor it in now. Adding a kilowatt or two of extra capacity at the time of installation costs far less than returning with scaffolding later.
A Worked Example
A household in the East Midlands with an annual usage of 4,000 kWh, a south-facing roof, and no significant shading would be looking at roughly a 4kWp system as a starting point. At around 950 kWh per kWp per year for that location and orientation, a 4kWp system would generate approximately 3,800 kWh annually — covering a large proportion of their usage. If the household is planning to buy an EV in the next couple of years, sizing up to 5kWp from the outset would be worth considering.
Bottom Line
There is no single perfect system size. But a sensible approach is to start with your actual electricity usage, aim for realistic coverage rather than perfection, and factor in how you live particularly when during the day you use most of your electricity.
The sizing formula (annual usage in kWh ÷ 1,000 = approximate kWp) is a useful starting point, but your location, roof, shading, and usage patterns all push the answer in different directions. A good installer will work through all of these with you. Getting at least three quotes from MCS-certified installers gives you a realistic range of options and prices.