Enter a panel's rated wattage and its length and width to calculate its true efficiency percentage.
Efficiency is measured under Standard Test Conditions (1000 W/m² irradiance); real-world output varies with temperature and angle.
Efficiency measures how much of the sunlight hitting a panel's surface it converts to electricity — it's not the same as total wattage, since a bigger panel can output more watts at lower efficiency. Most residential panels today fall between 18% and 23%; anything higher usually means premium cell technology at a premium price. This matters most when roof space is limited and you need maximum watts per square foot.
Efficiency and wattage get conflated constantly in casual solar conversations, but they're measuring different things — a physically larger panel can hit a high wattage number at only mediocre efficiency, while a smaller, more efficient panel can match that wattage in less roof space. Knowing both numbers, not just one, is what actually tells you whether a panel is a good fit for a space-constrained roof.
Not necessarily — high-efficiency panels cost more per watt, so they make the most sense when roof space is the limiting factor, not budget.
A smaller panel producing 400W is more efficient per square foot than a larger panel producing the same 400W — physical size and rated output together determine the real efficiency percentage.
Not directly — efficiency measures energy conversion, not durability; check degradation rate and warranty terms separately for a sense of how a panel will hold up over its lifetime.