The simple answer is only part of it
A typical solar panel used in the UK weighs somewhere in the region of 18 to 25 kilograms. On its own, that does not sound particularly significant. Spread across its surface area, the weight is fairly modest.
That is often where people stop. They picture a few panels resting lightly on a roof and assume the structure will cope without difficulty. In practice, the situation is a bit more involved.
It is the full system that matters
Solar panels are installed as part of a complete system. That includes mounting rails, brackets, fixings and sometimes ballast. Each element adds weight, and more importantly, changes how that weight is applied.
On pitched roofs, loads are transferred into rafters at specific fixing points. On flat roofs, weighted systems may spread the load across larger areas. Either way, the total system weight is what the structure needs to support, not just the panels themselves.
Weight per square metre is more useful
Rather than focusing on individual panel weight, it is more helpful to think in terms of load per square metre. Solar installations typically fall within a range that many roofs can accommodate, but that range is not universal.
The way weight is distributed makes a difference. Concentrated loads in one area can be more demanding than evenly spread loads across a larger surface.
Wind changes the picture
Panels do not just sit on a roof. They interact with the environment. Wind can create uplift forces, particularly on exposed sites or large flat roofs. Fixings and ballast must be designed to resist those forces.
This means the structure is dealing with a combination of downward weight and upward pressure. It is another reason why simple weight figures do not tell the whole story.
Different roofs respond differently
Timber roofs, often found in houses, can perform very well if they are in good condition. The spacing and size of rafters play a key role in how loads are handled.
Commercial roofs vary more. Lightweight steel frames and metal deck systems are efficient, but they may have limited spare capacity. Large open spans can behave quite differently from smaller, more closely supported structures.
Assumptions can be misleading
It is easy to assume that because solar panels are widely installed, most roofs must be suitable. In many cases they are, but not always without checks.
A roof that looks solid can still have limitations. Equally, a roof that appears lightweight may be capable once properly assessed. Visual judgement alone rarely gives a reliable answer.
Why proper assessment matters
Understanding the actual load on a roof involves more than adding up weights. It requires looking at how the structure carries those loads and how it will behave over time.
This is where a survey becomes important. Measurements are taken, materials are identified, and the way loads move through the building is considered. It turns a rough estimate into something far more dependable.
If you are trying to decide whether solar panels are suitable, knowing the true weight involved is only the starting point. Confirming how your specific roof will handle that load is what ultimately answers the question.