One solar quote advertises a higher panel efficiency than another. That can be useful, especially on a small roof, but the percentage alone does not tell you how much electricity the finished installation will produce in a year.
The US Department of Energy explains photovoltaic efficiency as the share of incoming solar energy converted into electricity. It is a conversion measure, not a promise that a household will cover the same percentage of its electricity bill.
Area changes the comparison
In a simplified illustration, a one-square-metre device receiving 1,000 watts of sunlight would convert 200 watts at 20 percent efficiency or 220 watts at 22 percent. Those invented conditions demonstrate the arithmetic; real module ratings use specified test conditions, and roof operation changes with weather and temperature.
Compare the actual module dimensions and rated power, not efficiency in isolation. A physically larger panel can have more rated watts without having the higher efficiency percentage. The installer also needs to fit usable rows around the roof’s real constraints.
Ask for a layout and a generation estimate
A practical proposal should identify the usable roof area, orientation, shading assumptions, number of modules, total rated system capacity and expected annual energy. Ask what losses and weather data the estimate includes. Keep the assumptions beside the number so two quotes can be compared on the same basis.
For example, a more efficient module may allow a useful extra amount of capacity on a constrained roof. On a large uncomplicated roof, another layout may meet the same objective at a different cost. Neither situation can be settled by circling the largest percentage on a brochure.
Connect output to the household
Annual generation and the amount you use directly are different questions. Timing of demand, export arrangements and storage affect how the electricity is used. Request a site-specific assessment from a qualified installer rather than converting a laboratory efficiency figure into a savings guarantee. A clear layout and transparent assumptions make the efficiency number meaningful.