If you’re thinking about installing solar panels (photovoltaic ones which convert sunlight into electricity) on your roof, this is usually the first question:
“What size system do I actually need?”
The honest answer is: It depends on your usage not your roof.
This guide will walk you through how to size a system properly, without guesswork.
Step 1: Start with your electricity usage
Look at your existing consumption by checking your annual electricity usage (kWh) on your bill.
Electricity usage is measured in kWh (kilo Watt hour). For example, you might have an immersion heater inside your hot water tank rated at 3kW. If it was turned on for an hour, it would consume 3kWh of electricity.
Typical UK household usage:
- Small (1–2 people): 1,800 – 2,700 kWh
- Medium (3–4 people): 3,000 – 4,500 kWh
- Larger homes: 5,000+ kWh
If you’re running things like:
- electric air or water heating
- EV charging
- hot tubs
then your usage could be much higher.
👉 Your solar system should be sized around this number.
Step 2: Understand what solar actually produces
A group of solar photovoltaic (PV) panels will be rated with a kilo Watt peak (kWp) number. This number represents the maximum electrical output any given moment in time.
For example, a 5kWp system will generate a maximum of 5kW of electricity at any one time. If it was generating its peak of 5kW for 2 hours then it would have generated 10kWh.
In reality, there are factors to take into account which may result in a 5kWp system generating more or less than 5kW.
In the UK, a good rule of thumb is:
1 kWp of solar generates ~850–1,050 kWh per year
So:
- 3 kWp → ~2,700–3,100 kWh/year
- 5 kWp → ~4,250–5,250 kWh/year
This varies depending on:
- location (distance from the equator)
- roof direction
- shading
But it’s a good starting point for estimating.
Step 3: Match system size to your usage
A simple way to size your system:
System size (kWp) ≈ Annual usage ÷ 1,000
Examples:
- 3,000 kWh usage → ~3 kWp system
- 4,500 kWh usage → ~4.5 kWp system
- 9,000 kWh usage → ~9 kWp system
This gives you a system that roughly matches your annual demand.
Step 4: Decide on your goal
Not everyone should aim for 100% coverage.
Here’s how to think about it:
50–70% coverage (most common)
- Lower upfront cost
- Good return on investment
- Less export
~100% coverage
- Maximum self-sufficiency potential
- Higher upfront cost
- More reliance on battery to avoid export
100%+ (oversizing)
-
Useful if:
- you plan to add a battery later
- your usage will increase (EV, heat pump)
Step 5: Consider your roof limits
Your roof may cap your system size.
Rough guide:
- 1 kWp ≈ 4–5 panels
- Each panel ≈ 1.7–2 m²
So:
- 4 kWp system → ~16 panels → ~30 m²
- 6 kWp system → ~24 panels → ~45 m²
South-facing roofs perform best, but:
- East/West still works well
- North-facing can still generate meaningful amount of electricity
Step 6: Don’t ignore how you use energy
This is where most guides fall down.
Solar doesn’t just depend on how much energy you use — but when you use it.
- If you’re home during the day → you’ll use more of your solar
- If you’re out all day → more may be exported or stored or generation is limited by the inverter.
👉 This affects how much value you actually get.
Step 7: Use a calculator (this is where it clicks)
Rules of thumb are useful, but they’re still rough.
For something more tailored:
👉 Use the Solar System Size Calculator
It takes into account:
- your usage
- location
- roof setup
- desired coverage
…and gives you a more realistic answer.
Common mistakes to avoid
Not making full use of roof space
Solar panels are often relatively cheap compared to the cost of scaffolding, roofing, and wiring up etc. so it’s often advised to install as many panels as you can.
Assuming export tariffs will always be available
Installing an excessive number of panels in order to maximise export potential relies heavily on export tariff availability and the rates offered. These rates have been reducing in recent times so it’s something to be considered against the cost of installation.
Ignoring future changes
Planning an EV or heat pump? Your future usage matters.
A quick example
Let’s say:
- Usage: 4,000 kWh/year
- Location: Midlands
- South-facing roof
A good starting point would be:
- ~4 kWp system
- ~3,800–4,200 kWh annual generation
This system would make a significant difference to the household electricity bill.
Bottom line
There’s no “perfect” system size.
But a good approach is:
- Start with your usage
- Aim for realistic coverage (not perfection)
- Factor in how you actually live and when you most use electricity
If you want a more precise answer for your home:
👉 Try the Solar System Size Calculator
It’ll give you a tailored estimate in seconds.