Efficiency is the number most quoted and least understood in a solar quote. It is not a measure of how good a panel is and it does not tell you how much electricity you will get. It is the share of the light falling on a panel that the panel turns into electricity, which matters only because it decides how much power a given AREA of roof can hold. On a roof with space to spare it is close to irrelevant, and on a small or awkward roof it is the whole decision.
What the percentage actually measures
A panel's efficiency is the proportion of the light energy hitting it that comes out as electricity, measured under standard test conditions in a laboratory rather than on a British roof in March. Two panels of the same wattage produce the same power; the more efficient one is simply smaller. That is the entire practical consequence, and it is why the figure matters enormously when the roof is short of space and hardly at all when it is not.
Monocrystalline against polycrystalline
Monocrystalline cells are cut from a single crystal and are more efficient, more expensive and usually black. Polycrystalline cells are cast from melted fragments, are slightly less efficient, cheaper and typically blue. The gap has narrowed a great deal and mono now dominates domestic installations in Britain, so on most quotes the choice has already been made for you. Where it still comes up, the question is the same one: is roof area the constraint, or is it the budget?
When a higher efficiency is worth paying for
On a small roof, on a roof broken up by dormers and chimneys, or where the household wants a particular output from a fixed area, paying for higher efficiency buys real capacity that would otherwise not fit. On a large unobstructed roof it buys a smaller array of the same output, which is not worth a premium. The honest test is to ask the installer how many panels of each type fit and what the total kWp is for each option, because that comparison answers the question and the percentage on its own does not.
What matters more than the efficiency figure
Shade will cost more output than any efficiency difference between two reputable panels, which is why the shading survey and the inverter or optimiser strategy deserve more attention than the spec sheet. So does orientation, so does the pitch, and so does whether the array is sized to what the house uses rather than to what fits. Degradation rate and the warranty behind it also matter more over twenty five years than a percentage point at installation, and both are on the datasheet next to the figure everybody reads instead.
Questions people ask about solar panels efficiency UK
What is a good solar panel efficiency?
Most current domestic panels sit within a narrow band of each other, and the difference between two reputable ones will be smaller than the difference shade or orientation makes. The useful question is not which percentage is good but how many panels of each option fit your roof and what total output that gives.
Is monocrystalline better than polycrystalline?
More efficient, so physically smaller for the same output, and more expensive. Better depends on whether roof area or budget is the binding constraint. Mono dominates British domestic installations now, so on most quotes the question has already been settled.
Does higher efficiency mean more electricity?
Not by itself. Two panels of the same wattage make the same power; the more efficient one takes up less room. You get more electricity from higher efficiency only when the roof area is what is limiting you, because then more capacity fits.
What reduces solar panel efficiency in practice?
Shade first, by a wide margin, then orientation and pitch, then heat, since panels are slightly less efficient when hot. Dirt matters far less than it is sold as mattering in a climate with this much rain. A survey that takes the shading seriously is worth more than a point of efficiency on the datasheet.