How Many Solar Panels Does Your Home Need?
Sizing a solar panel system correctly is crucial. Too small and you miss out on potential savings. Too large and you are paying for capacity you cannot use effectively. At ALPS Electrical, we design every system based on a detailed assessment of your electricity usage, roof space and future plans. This guide explains how we calculate the right system size.
Start With Your Electricity Usage
The single most important factor in sizing your solar system is how much electricity you use. Check your annual electricity consumption on your energy bills or smart meter data. If you cannot find this, here are typical figures for UK households:
- 1-2 bedroom flat/terrace: 2,000 - 2,500 kWh per year
- 3 bedroom semi-detached: 3,000 - 4,000 kWh per year
- 4 bedroom detached: 4,000 - 5,500 kWh per year
- Large detached with EV: 6,000 - 8,000 kWh per year
- Home with heat pump and EV: 8,000 - 12,000 kWh per year
Your target system size should generate enough electricity to cover 80-100% of your annual usage, assuming you have battery storage to maximise self-consumption.
How Much Does Each Panel Generate?
Modern solar panels typically have a rated output of 400-440W each. In the North East, each 420W panel generates approximately 380-430 kWh per year on a south-facing roof. On east or west-facing roofs, this drops to approximately 300-360 kWh per panel. North-facing roofs are generally not suitable for solar panels.
System Size Calculator
Here is a quick reference table for the North East based on south-facing roofs using 420W panels:
- 2,500 kWh annual usage: 3kW system = 7 panels
- 3,500 kWh annual usage: 4kW system = 10 panels
- 5,000 kWh annual usage: 5.5kW system = 13 panels
- 7,000 kWh annual usage: 7.5kW system = 18 panels
- 10,000 kWh annual usage: 10kW system = 24 panels
Roof Space Requirements
Each 420W panel measures approximately 1.7m x 1.1m, requiring roughly 1.9 square metres of usable roof space per panel. A 10-panel system needs approximately 19 square metres. You also need clearance around the edges of the roof and gaps around any obstacles like skylights, vents or chimneys.
If your roof space is limited, higher-efficiency panels like the Aiko range can generate more electricity per square metre, allowing you to maximise output from a smaller area.
Does Roof Orientation Matter?
Yes, significantly. South-facing roofs generate the most electricity overall. East-facing roofs generate more in the morning, west-facing roofs more in the afternoon, and both produce approximately 80-85% of what a south-facing roof achieves. A split east-west array can actually be beneficial because it spreads generation more evenly throughout the day, which can be advantageous for self-consumption.
Should You Oversize Your System?
If you are planning to add an electric vehicle, heat pump or battery storage in the near future, it makes sense to install a slightly larger system now. Adding panels later is possible but more expensive due to the additional scaffolding, labour and potential inverter upgrade. We always discuss your future plans during the survey so we can future-proof your system design.
For homes planning to add an EV within the next few years, we typically recommend adding an extra 1.5-2kW of capacity beyond what your current electricity usage alone would suggest. An electric vehicle typically adds 2,500-4,000 kWh per year to your household electricity consumption depending on your annual mileage, and having the solar capacity to cover this from day one makes excellent financial sense.
Common Sizing Mistakes to Avoid
The most common mistake homeowners make is relying on online calculators that do not account for real-world factors. A calculator might tell you that you need 10 panels based on your annual usage, but if your roof has partial shading from a chimney during morning hours, the actual output per panel will be lower than the calculator assumes. Similarly, if your roof faces east-west rather than south, you need more panels to achieve the same annual generation.
Another mistake is focusing solely on current usage without considering future changes. If you are planning renovations that might include an electric hob, underfloor heating or a garden office, your electricity usage could increase significantly. It is always better to slightly oversize your system than to wish you had installed more panels later.
The Professional Survey
While this guide gives you a useful starting point, the most accurate system sizing comes from a professional roof survey. At ALPS Electrical, our survey includes shade analysis using specialist software, precise roof measurements, assessment of your roof structure and fixings, analysis of your electricity usage patterns, and consideration of future plans like EVs and heat pumps. Book your free survey today and we will design the perfect system for your home.