A solar proposal often places a large annual generation number near the front. That figure is an estimate, built from weather data and assumptions about the array. It is useful when its ingredients are visible. It becomes much less useful when it is presented as a guaranteed result without explaining the site, equipment or uncertainty.
Find the inputs
Look for location, panel capacity, orientation, roof angle and shading assumptions. Ask which losses are included, such as temperature effects, wiring, conversion and soiling. An estimate should refer to the proposed layout rather than an idealized unshaded roof. If the design changes, the calculation should change with it. Save the final version alongside the installation drawings.
Ask for monthly output
Monthly figures reveal the seasonal pattern hidden inside an annual total. Compare that pattern with heating, cooling and other household demand. A home may export a large amount during one season and buy electricity during another. An annual energy balance does not mean the household is continuously self-sufficient. Any economic calculation should show how imports and exports are valued separately.
Keep uncertainty visible
Weather varies, equipment ages and household routines change. Ask whether the estimate represents a typical year and how actual results might differ. Do not confuse a panel performance warranty with a guarantee of household savings. Once the system is running, investigate substantial departures using weather and operational records. The estimate is most helpful as a transparent planning tool and a starting point for questions. It should not become a number that nobody is allowed to revisit after the sale.
